Cam Science 101 and Beyond


Established Member
Aug 9, 2002
Seattle, WA
Cam Science 101 and Beyond​


How often do you finish reading a tech article on camshafts and feel like you’re more confused than when you started? Magazines and the internet are filled with hundreds of articles attempting to explain how camshafts work, and trying to make sense of the often conflicting information can leave the inquisitive reader more than just a little frustrated. Most of the advice is recycled directives like “advance the cam for more torque” and “tighten the lobe centers if you want to make more horsepower”. These things all seem simple enough on the surface but applying them to a specific combination can be hit or miss.

Why is the topic so widely misunderstood? I think I can speak for most people including myself when I say it is a very complex subject to try and tackle within the space of a magazine article or the typical hot rodders span of attention, and many people who write these articles have little more than a cursory understanding. Even some of the most successful engine builders struggle with the mysteries of cam timing. Fortunately, most cam companies know there craft so well that anyone can call up and obtain 2 or 3 different grinds and after a day on the dyno have a fully optimized, custom ground camshaft that will get the job done and leave very little horsepower on the table. As a matter of fact, according to Competition Cams lead camshaft engineer Billy Godbold, they spend more time configuring affective ramp acceleration rates than they do defining all other valve events combined. For most engine combinations camshaft manufactures have a pretty good handle on what works and what doesn’t so engine builders generally don’t have to understand all the voodoo of cam design.

Why does cam design seem so complex? Well, for starters there are several parameters that can be adjusted: Lift, duration, lobe separation angle, overlap, lobe centerline, intake opening, intake closing, exhaust opening and exhaust closing to name a few. Adjusting one of these parameters can affect the contribution from one of the others. In other words, some benefit may be derived from one adjustment but the inherent changes it makes in other aspects of cam timing could easily result in a net loss. Let me give an example. Joe Mechanic reads that delaying the exhaust opening increases bottom end torque. This sounds good so he retards the exhaust lobe on his cam (assuming DOHC) only to find that his engine runs worse at low RPM. Why? Because delaying the opening point by retarding the lobe also delays the closing point which in turn increases overlap. The engine probably picked up torque in the mid RPM range but overlap has a greater influence on how the engine idles and runs at low RPM than the exhaust valve opening point. If this is not what Joe was expecting it can be a disappointment.

The overlap would have to have been equalized in order to derive the benefits from a delayed opening without any of the consequences but then equalizing the overlap would have required moving the intake lobe, which has its own set of issues.

In addition to the complex interplay between camshaft timing points is the external elements that influence how the engine will respond to a particular grind. Things like the vehicles intended use, customer’s goals, intake and exhaust capabilities, gearing, vehicle weight and a whole host of things can play a big part in which camshafts work and which don’t.

From a big picture standpoint each camshaft parameter can only be adjusted in one direction or the other and the change associated with it are fairly well understood, but making changes to one valve event usually sets off a chain reaction that alters the way other valve events affect engine operation. As a result, a cam design is typically a holistic approach that requires a second nature understanding of all the parameters and how they work with, and sometimes against each other. You start with the most important attribute first than work your way down the hierarchy. Typically that hierarchy would look something like this; duration, lift, lobe centers, overlap/LSA, intake closing point, intake open, exhaust close, exhaust open. Each one of these can be approximated within a general range and then viewed as a whole to obtain specifics. This way the affect each one has on the other is taken into consideration when finalizing the cam specs.

I think David Vizard said it best:

“The problem is, if you are something of a novice at this engine business, just about everything to do with cams and valvetrains looks complex, and the truth is, it's that and more. If cam and valvetrain design at the top level is in your future, you had better think in terms of a Ph.D. in mechanical engineering.” David Vizard

Vizard writes the best camshaft articles available. More than anybody he gives us the nuts and bolts of how things really work and why, and it is for that reason that I will quote from him quite a bit. What I want to do with this article is give you the entire picture from beginning to end from a typical hot rodders perspective. What I want to avoid is what we get with 95% of the articles written, and that is a plethora of anecdotal statements telling us how delaying this increases torque and widening that increases horsepower without telling us exactly why. Invariably these types of statements will be accurate under certain conditions and inaccurate under others. I want to explain the inner workings of camshafts simply and with enough specifics so the reader can understand the different events and how adjusting them will affect a wide variety of engine applications. Unfortunately, to do this topic justice it takes A LOT of space and even more patience to read through all the technical jargon. You will also find a lot of repetition on some of the more complex topics but I think it will help tie things together a little better. This article is not intended to be the last word as even the experts have trouble agreeing sometimes, but in the end you should find quite a bit of valuable information here.

The Basics

The basic crux of camshaft design is timing the valve opening and closing to appropriately coincide with pressure differentiations and inertial energies in relation to the engines intended RPM range. That’s a mouthful and will take quite a few pages to explain so I’ll break it into pieces that can be read individually.

Some of the major parameters affecting camshaft timing events include the engines bore, stroke, and connecting rod length. Also, the compression, both static and dynamic, the engines intended RPM range, intake and exhaust restrictions, velocities or lack thereof, and how the general strengths and weaknesses of each of these fit together to form a complete picture. We get these things right and we have a successfully designed camshaft.

If you don’t have a pretty good handle on the basics of lift and duration, things might get a little far removed from the information you are accustomed to seeing regarding camshafts but not to worry, that is the purpose of this article, to bring the black art of camshaft timing kicking and screaming into the light of day. At least some of it.

As a pre-cursor I’ll take just a moment to quickly define duration, lift, lobe centerline (LC) and another common term, lobe separation angle (LSA) also known as lobe centerline angle (LCA).

We’ll start with duration. It is the number of degrees in crankshaft rotation that the valve is off its seat. While off the seat, air fuel mixture or exhaust gasses can escape past the valve to their intended destination. Generally the longer the valve stays off its seat, the more air can pass. Two figures are usually supplied for duration; advertised and .050. Advertised is usually derived from the duration the valve is open starting and ending somewhere between .003 - .006 off the seat, with .006 being the official SAE figure. Technically total duration is slightly more but what happens before .003 is largely considered irrelevant. The other duration figure that is most commonly used is measured starting and ending at .050 in crankshaft degrees. This is the most commonly used figure when specifying a camshaft because it comprises a large part of how the cam is going to behave. There are other measurement but these are the most common, and useful for this discussion.

Lift is probably the most well understood of the camshaft variables. Advertised lift numbers that we are accustomed to seeing come from multiplying the actual height of the highest part of the camshaft lobe from its base circle, times the rocker arm ratio. Part of the rocker arms job is to multiply or artificially increase the effective lift of the camshafts lobe in order to overcome the limitations of the lifter/lobe relationship. In a practical application we would take a typical lobe height of .3843 of an inch and multiply it by the rocker arm ratio. If we use 1.5:1 as an example and multiply it by .3843 we get a net lift of .577, or just over half an inch.

The application of lift is fairly well understood. Find the point where the cylinder head port hits maximum flow and spec the lift within that range. Too much lift places undue stress on the valve train, and not enough lift results in wasted horsepower potential and a component mismatch that has a negative effect on performance.

Lobe centerline (LC) is simply the peak lift point of the camshaft lobe in crankshaft degrees. On symmetrical cams it is the halfway point between opening and closing. On asymmetrical cams the centerline may not be dead center because the opening and closing sides of the lobe can have different durations. The difference is very small with the majority of the difference being in the ramp rates.

Getting slightly more technical, were going to look at lobe separation angle (LSA), also known as Lobe Centerline Angle (LCA). These terms identify the number of camshaft degrees (not crankshaft), that separate the intake and exhaust lobe centerlines. It is measured by taking the center or highest lift of each lobe and measuring the distance between the two points. It also gives a pretty good idea of where the lobe centerlines are placed in crankshaft degrees. A LSA spec’d at 112 may mean that the intake and exhaust lobes are both placed at 112*. But it could also represent a 110 intake and a 114 exhaust. We get LSA by finding the average of the two centerlines. 110 + 114 = 224. 224/2 = 112.

There is a lot of debate surrounding LSA but I will point out that the term “LSA” or “LCA” is just a generic label. It is a figure used to provide a general idea of how several timing points are placed. These include things like lobe centerline placement, opening and closing points, and how long both valves are open at the same time (overlap). Lobe centerline and overlap would be considered the most critical of these elements and they are what the term LSA is really all about. Much of the debate is a result of discussing the term from differing perspectives. Where one person is focusing on the affects of overlap, another is considering the intake lobe centerline. The conversation can get messy but we’ll dissect it piece by piece later on.

The 4 (5) Cycles

Before going any further we need to get a solid picture in our heads of what happens in a four stroke engine as it relates to camshaft timing. The four cycles include the Intake Cycle, Compression Cycle, Combustion, and Exhaust. Refer to the chart below by as a reference while we work through a day in the life of the engines camshaft.


Here is what would be seen looking at a pushrod V-8 cam, from the front: 1) Intake lobe lift; 2) Exhaust lobe lift; 3) Intake duration; 4) Exhaust duration; 5) Overlap; 6) Lobe Centerline Angle (LCA); 7) Cam Advance (A) and Retard (R).

When looking at the engine from the front, the camshaft spins in a clockwise motion. Of the two cam lobes pictured above it is the exhaust lobe that leads the way on the right with the intake lobe following closely behind. We are going to start the explanation of the cycles with the exhaust lobe, or the one on the right in the figure above.

Visualization is the key to understanding the process so I’m going to walk through it step by step. For the sake of this conversation I will utilize the specs of Competition Cams 106400 camshafts for the 4.6 DOHC V8.

Once again, the four basic cycles include the induction cycle, compression, combustion and exhaust. The first thing we want to suppose is that the intake valve has just closed ending the induction cycle and the combustion cycle has already started. Both intake and exhaust valves are closed and the force of the ignited air/fuel mixture is shoving the piston down towards Bottom Dead Center (BDC). As it travels down, the force acting against it from the combustion is slowly dissipating. At 69.5 degrees (on Comp Cams 106400) before the piston reaches bottom dead center (BDC) the exhaust valve starts to open. There is still considerable pressure from the exploding air fuel mixture and instead of continuing to exert force against the piston it is used to help push the burnt gasses past the exhaust valve. This is called the blowdown period and its purpose is to get a jump start on eliminating all the exhaust gasses and reducing as much cylinder pressure as possible before the piston starts back up the cylinder. Any cylinder pressure or exhaust gas remaining in the cylinder acts as resistance to the rising piston. This lost power is referred to as pumping losses.

After the piston rounds BDC it starts pushing up against the remaining gasses and shoving them out the exhaust port. As the piston nears the top the exhaust valve begins to close while at the same time the intake valve begins to open. The intake opens at 22.5* Before Top Dead Center (BTDC) and the exhaust valve doesn’t close until 17.5* After Top Dead Center (ATDC). By doing a little math we can see that both the intake valve and the exhaust valve are open for 40* of crankshaft rotation. This period where both valves are open at the same time is referred to as the overlap period. In the 1950’s Ed Iskendarian discovered that overlap could be used as a powerful camshaft design element and used it with such success that it was coined the fifth cycle. He even produced a line of cams with that namesake. More on overlap later.

At 17.5* ATDC the exhaust valve is shut and the piston is drawing in the air fuel mixture from the open intake valve. As it descends down the cylinder it continues drawing in air/fuel until it reaches BDC. At this point the intake valve is still open as the piston starts to move back up the cylinder. Even though the piston is no longer pulling the intake charge in, the inertia created during overlap and the down stroke momentarily continues to ram air into the cylinder. With 106400 intake cams it’s not until the piston reaches 66.5* ABDC that the intake valve is completely shut. It is at this point on the graph above where both intake and exhaust valves remain shut as the lifters travel around the back side of the lobes. Now that both valves are closed, the piston can begin compressing the air/fuel mixture in preparation for ignition near TDC. As the piston approaches TDC the fuel mixture is highly compressed and ignited, shoving the piston back down the cylinder, putting us back where we started.

It is important to have a good visualization of the above process in order to understand what will follow. In particular, the principles of advancing and retarding both the intake and exhaust lobes.

The Fun Stuff

Alright, so you’re probably asking what all this talk is about advancing this and closing that. First thing to look at is the primary valve events. They are Intake Open (IO), Intake Close (IC), Exhaust Open (EO), and Exhaust Close (EC).

As in many things camshaft related we have to find a balance between opposing design parameters. Any time we decide we would like to change the opening or closing point of a lobe we have to remember that whether achieving this by adding duration or advancing the lobe, we affect what is happening on the other side of the lobe equally. For example we might want a later closing exhaust valve in order to help eliminate exhaust gasses. Adding duration to achieve this would both close the valve later and open it earlier, resulting in a net benefit. Retarding the lobe would close the valve later but also open it later. The later opening results in less pressure during the blowdown period which also means fewer exhaust gasses are eliminated during this period. Since blowdown is more critical to exhaust gas elimination than the exhaust valve closing point, the net result would be negative. Every adjustment has a host of other inherent changes that also occur and none of the camshaft valve events exist in a vacuum.

How do we figure out which points to design around? As you will see from the opposing opinions of the experts below, it isn’t always a clear answer:

“Now the last timing event is the most important and the most critical THE CLOSING OF THE INTAKE VALVE. The closing of the intake valve is the governor of the RPM range and the governor of the effective compression ratio!” Dimitri Elgin

“I consider the IVO and the EVO to be of equal valve, and first in importance. If anything, the EVO occurs first, and influences what the IVO sees and does.” Harold Brookshire

The closing of the intake valve, the opening of the intake valve, or the opening of the exhaust valve. It’s interesting how these two veteran cam designers with exceedingly impressive credentials have differing opinions. Dimitri Elgin is very “inertial charging” centric while Harold Brookshire focuses on the importance of airflow in the very early stages of the intake cycle. Dimitri is more road race oriented while Harold is more drag race focused. Do these differences account for the opposing philosophies? Maybe a little but the reality is they’re both pretty much talking about the same thing. I imagine if these two men were discussing cams they would agree a lot more than disagree. Often times differing design opinions are nothing more than perspective.

Since we are not actually designing camshafts we can get away with this basic outline provided by Racer Brown:

Listed in order of importance and significance to engine operation are:

1. Intake valve closing point;
2. Intake valve opening point (probable);
3. Exhaust valve closing point (probable; could be reversed with (2) under certain conditions);
4. Exhaust valve opening point;

Racer Brown

In addition to opposing design parameters we have to think in terms of levels and priorities. Adjusting one valve event can set off a chain reaction of consequences that greatly effect how the engine performs. This line of thought can be seen in Harold Brookshire’s design philosophy which basically says that if you don’t get things right at the beginning of the intake cycle, it doesn’t matter where the all important intake closing is placed.

Some design consequences are dramatic while others are hardly noticed. However, these levels of impact can all be turned upside down based on the priorities of the individual and the engines inherent strengths and weaknesses. A late EVO on a small cubic inch engine with low compression can save the day with the additional torque it provides, yet at the same time it can be disastrous if the engine is being used in a high RPM application. The negative high RPM consequences of a late EVO are not important to an individual that is prioritizing low RPM torque, yet this inconsequential element is a big priority to the high RPM enthusiast. Bottom line is that a particular strength or weakness of a given combination can sometimes magnify what would normally be an inconsequential effect of changing a valve event.

In addition to intended usage, the myriad of cylinder heads, intake manifolds, and exhaust systems, along with all the bore/stroke and connecting rod combinations can make camshaft design very technical. I will touch on some of the more obvious challenges but in general we are going to continue with the assumption that we are dealing with a reasonably well designed and applied combination that will respond accordingly to generally accepted parameters.

We will use some of Racer Browns hierarchy above to guide us as we look at each valve event and its effects on engine performance. Keep in mind that opening and closing events can be adjusted or spec’d in a couple different ways. One is by adding or subtracting duration. When we add duration to the lobe we are simultaneously opening the valve earlier and closing the valve later because the duration is added to both sides of the lobe. The other way to change opening and closing points is to simply take an existing lobe and advance or retard it. If we advance it we end up with an earlier opening point and an earlier closing point. If we add duration and move the lobe one way or the other we can maintain the original opening or closing while concentrating the additional duration on the opposite side of the lobe. Each possibility has individual consequences that are sometimes opposing. If that sounds confusing, don’t worry, I’ll repeat it a few times throughout the article.

Intake Lobe

There are 4 major components of the intake lobe that contribute to the success of any given combination; Duration, IVC, IVO, and Lobe Centerline. Intake events are generally considered more important and less forgiving than exhaust events. Not only is the induction cycle slightly more complex but it can be dramatically affected by small changes in the engines combination. For that reason I will dedicate considerable space to the intake event.

Intake Lobe Duration
The most important place to start intake lobe design is duration. There are two main components of duration that contribute to building power (other than overlap). The first is it gives the piston access to an open valve for a longer period of time, allowing air to continue filling the cylinder efficiently at high RPM. Note that an engine will not automatically make more power when duration is added unless the engine rev’s high enough to utilize it. Putting aside inertial charging, a cylinder can only hold so much air. The volume of that cylinder when the piston is at BDC never changes and that space represents the maximum amount of air that can be drawn in. An engine makes 500 horsepower at 6500 RPM with roughly the same amount of air/fuel in the cylinder that it uses to make 50 horsepower at idle. It’s the number of revolutions per minute that produces the work known as horsepower. As engine RPM increases, the amount of time the valve remains open is reduced. Mechanically the valve is open for the same number of degrees but it is the time component that becomes limited at high RPM. At a certain point the mechanical elements are moving too fast for the air to keep up and the engine runs out of breath. Increasing the cams duration adds critical milliseconds to the amount of time the valve stays open and feeds the engine.

Adding duration also extends both the opening point and the closing point. The earlier opening places the valve at a more useful lift when the induction stroke begins, making the air/fuel mixture more responsive to the piston. Extending the closing point allows the inertia of the air/fuel to continue filling the cylinder ABDC when the piston starts moving back up the bore. This allows better cylinder fill at high RPM. As you will see later, these two power producing timing events divide the cam design industry into two different camps; those that believe the intake valve opening point is the purveyor of power production and those that think the closing point is the holy grail. I remain neutral.

Intake Valve Closing

Now we’re going to get into the nuts and bolts of things with some repetition and further explanation. The Intake Valve Closing Point is often considered the most important because of its direct effect on the engines operating range, volumetric efficiency, and dynamic compression ratio. First thing to note is that the intake valve closes after bottom dead center (ABDC). In other words, it is still open after the piston has traveled all the way to the bottom of the bore and started moving back up the cylinder.

Inertial Charging

The reason for waiting until ABDC to close the valve is that the incoming charge will keep forcing its way past the valve even after the piston has stopped drawing it in. Higher RPM means higher velocity and continued cylinder filling further into the compression stroke. On a high RPM race engine, air/fuel can continue forcing its way past the closing intake valve long after the piston has started its path up the bore because the fast moving piston has increased the speed of the air to the point that it cannot easily be stopped. To what extent the “inertial supercharging” takes place depends on many variables but the most obvious elements are velocity through the intake path and the RPM of the engine, or more specifically, piston speed. Along with pressure wave, it is inertial supercharging that is responsible for the volumetric efficiencies beyond 100% found in properly designed and built performance engines. Similar to a supercharger or turbocharger, inertia literally rams more fuel into the cylinder than would be possible using atmospheric pressure alone. More detail to come.

Dynamic Compression

If inertial supercharging is the high RPM component of Intake Valve Closing, than the Dynamic Compression Ratio (DCR) is the low RPM component. Dynamic compression is a label used to give a general idea of cylinder pressure over a limited, low RPM range of operation. DCR cannot account for non mechanical elements of the induction cycle because it is derived from a mathematical equation involving stroke, connecting rod length and intake valve closing. As a result, the actual figure never changes, even though the cylinder pressure it is supposed to represent is greatly affected by RPM. This makes it less accurate as the RPM’s increase.

To understand dynamic compression ratio it is easiest to first understand the basics of calculating static compression ratio. When an engine is rated at 10:1 compression it means that the volume of air in the cylinder when the piston is at Bottom Dead Center (swept volume) is 10 times the volume of air that the cylinder can hold when the piston is at TDC. So the piston compresses 10 parts of air into a tiny space 1/10th its original size when at TDC, or 10:1 Static Compression Ratio. That’s it, fairly straight forward.

What differentiates the Static Compression Ratio from the Dynamic Compression Ratio is that we use the pistons position in the bore at time of intake valve closing, rather than BDC. The logic behind this is that holding the intake valve open after the piston has started moving back up the bore causes some of the inducted air to be pushed back out of the cylinder and into the intake port. This means that less air is trapped and compressed. This loss of air/fuel mix into the intake port is known as intake reversion. Instead of compressing 10 parts of air into 1/10th the space (10:1 compression), we are only compressing 8 parts air into 1/8th the space, or 8:1 dynamic compression. Less air/fuel trapped, compressed and ignited translates into a loss of power and torque, at low to mid RPM.

To recap, the two components critical to IVC is Dynamic Compression and Inertial Supercharging. Dynamic compression prefers an early IVC and is critical to low RPM operation while being largely irrelevant to high RPM operation. Inertial supercharging uses a late IVC and is critical to high RPM operation while being largely irrelevant at low RPM operation.

Time, Not Distance

To understand the RPM component of IVC we have to first understand that valve events are largely a function of time (milliseconds). In the case of the intake event, the air/fuel mass is supposed to fill the cylinder between the time the intake valve opens and when the piston reaches just past BDC when the valve closes. With low to mid RPM operation this is easily accomplished but as RPM increases, the air/fuel mix has less and less time to fill the cylinder before the piston reaches BDC. This is one of the major power and RPM limiting components of airflow.

Here is why: At low RPM the air is able to follow closely behind the piston on the intake stroke so that at BDC the piston has pulled in a full cylinder of air before the intake valve closes. At this point the air/fuel is trapped for compression and ignition.

As RPM increases, the intake charge or column of air/fuel mixture begins to lag behind the faster moving piston and closing intake valve. It continues to fall farther and farther behind as the engine revs higher. As a result, less of that column of air makes it past the valve before it slams shut. When this happens we end up with only a partially filled cylinder when the valve closes. This limits the RPM capability of the engine because it basically runs out of breath.

The remedy is to close the intake valve later. The high speed of the mechanical parts has two components. One is that the piston and valve outrun the intake charge and the second is positive in that the high speed of the piston also creates a more powerful pressure differential and a faster moving intake charge. At high RPM the piston outruns the intake charge at the beginning of the cycle but eventually the intake charge gets up to speed and when it does it is really moving. If we keep the intake valve open a little longer ABDC we can use the inertia of the high speed charge to make up for the slow starting air’s inability to fill the cylinder by the time the piston hits BDC. Not only that but as stated earlier, if the air/fuel is moving fast enough than the inertia can actually pack the cylinder with more air than the piston could ever pull in by itself.

One More Time

The bottom line from an IVC standpoint is that when the engines intended RPM range is low we tune around the concept of dynamic compression by closing the intake valve early to trap and compress as much air as possible. With low RPM combinations inertial charging is insignificant and the piston movement is slow enough so that the air can keep up with it. As a result the induction cycle has enough time to utilize an early IVC and capture a full cylinder of air near BDC. However, exceeding 100% cylinder fill is unlikely.

When the engines intended RPM range is high we tune around a late intake valve closing and inertial supercharging. Technically the late IVC combination has low dynamic compression but the reality is that only applies to low and mid RPM operation. As the RPM’s increase, the incoming column of air begins packing the cylinder beyond what the piston could draw in on its own. If the dynamic compression equation could account for inertial charging, the number it produced could actually be higher than the static compression. It’s not that dynamic compression is entirely irrelevant for high RPM combinations but it’s correlation to cylinder pressure becomes less accurate.

That sums up IVC so we will move on to the intake valve opening event.

Intake Valve Opening

Determining where to open the intake valve is critical to the success of the entire induction cycle for the same reason advancing the intake lobe centerline is beneficial to power production. Much like an advanced intake lobe centerline, an earlier intake valve opening is preferred in performance applications because it gets the valve further off the seat and into a more functional range when the piston begins to draw on the intake port. In addition to the valve being further off the seat earlier in the induction phase, the earlier opening increases overlap. Both elements enhance flow in the critical early stages of the induction cycle. Of course the benefits are not without cost and there is such thing as too much of a good thing. As in most everything camshaft related the added benefits of an earlier intake opening can create issues elsewhere in the RPM range.

The Downside

The earlier the intake valve is opened on the exhaust stroke the more exhaust is pushed up into the intake tract by the piston coming up the bore (intake reversion). This dilutes the intake charge and contributes to rough running at low RPM and if excessive enough, the narrowing of the RPM band and loss of power at low to mid RPM. Also contributing to poor low speed operation is increased overlap. At low RPM, overlap can over scavenge the cylinder and pull the intake charge out the exhaust, or it can allow exhaust to flow backwards into the cylinder (exhaust reversion) and pollute the intake charge.

Another potential issue with overlap is that in common plenum intakes a cylinder can pull exhaust in from the next cylinder in the firing sequence as it begins its induction stroke and overlap period. This is known as “cross talk” and it occurs when the throttle is in the closed or near closed position causing high manifold vacuum. When this happens it is easier for cylinder 1 to pull air from cylinder 2 than to try to pull it past the throttle blade. The result is lowered signal response to the throttle body or carburetor and a diluted intake charge causing rough low speed operation.

The Debate

According to Harold Brookshire, IVO is one of the top two most important events in cam timing because the amount of exhaust pressure that exists at intake opening impacts how well the port flows in the early stages of the induction cycle. But not all the experts agree. Ron Iskendarian also has an opinion on early IVO:

“When the intake valve opens some 40 or more degrees before T.D.C. at the end of the exhaust stroke, very little (virtually no) exhaust gases remain in the cylinder. It is a "myth" that an earlier opening of the intake valve (even by a mere 2 or 3 degrees) causes the phenomenon known as "reversion". Nothing could be further from the truth! This misconception not only defies common sense, it also establishes a false premise from which other, incorrect conclusions can be drawn. Simply put, those who focus on overlap are on the wrong end of the cam-timing diagram!” Ron Iskendarian

On the other hand, Racer Brown has this to say:

“When the intake valve opens early, the engine will usually immediately respond by being rough and balky at low engine speeds. This occurs because of the greater dilution effect the exhaust gases have on the air/fuel mixture charge as the mixture attempts to enter the cylinder. As engine speed increases, the velocity and inertia of the mixture charge overcomes most (not all) exhaust gas dilution and helps power output at higher engine speeds. Very early intake valve opening points will kill performance in the low and mid-range speeds, making engine power and response acceptable only in the highest engine speed ranges.” Racer Brown

Once again we have some of the industry’s most knowledgeable cam designers disagreeing. Racer Brown is known by many as the Father of the Camshaft because of his pioneering work in the field. Harold Brookshire’s track record of success is indisputable, and Ron is the son of famous cam grinder Ed Iskendarian. Each of these individuals has vast knowledge of camshaft design and should be studied a little further for a complete understanding of their perception. For now we’ll go with the prevailing opinion that opening the intake valve early can create some level of reversion.

IVO Nuts and Bolts

Opening the valve earlier can be accomplished by either adding duration to the lobe or advancing it. As discussed earlier, the primary purpose for adding duration to a lobe is to extend the RPM range for additional power by allowing the valve to stay open for a longer period of time and to close the valve later after bottom dead center. Adding duration for the purpose of an earlier intake valve opening can only be beneficial up to a point because it has the potential to extend the cams functional range beyond the engines capability.

If RPM limitations of certain components of the engine are preventing the builder from adding enough duration to achieve the desired IVO point we can simply advance the entire lobe. The earlier opening is good for getting the valve further off the seat earlier in the induction stroke and aligning the lobe center closer to maximum piston draw. As an added benefit, the earlier closing (not earlier opening) of the valve actually brings the RPM range back down a bit. Some engine builders subscribe to the philosophy that adding as much duration as the combination needs to get the job done at the appropriate RPM is the first priority followed by advancing the intake lobe as far as possible within the constraints dictated by early IVO, IVC, and overlap. But don’t forget, changes on the opening side create changes on the closing side.

Intake Lobe Centerline

In a perfect world the ideal intake lobe for any engine would be full valve lift coinciding with the point in the induction cycle where the piston is creating the most suction on the intake port. At low RPM this occurs between the point of maximum piston speed and peak valve lift, but it changes with the RPM of the engine. This is one of the reasons we can only tune for a limited RPM range. At low RPM the greatest pressure differential occurs around 30* ATDC. By mid RPM it is somewhere between 70 – 80 degrees ATDC, and at high RPM it occurs around 100 degrees ATDC. According to David Vizard, once maximum pressure disparity occurs past 115* ATDC the engine has reached its maximum potential and will not produce any additional power no matter how high the engine is spun. This number is not exact for every engine but it represents a good approximation.

To get a better understanding of the critical placement of the ICL we have to re-visit the relationship between the movement of mechanical parts and air mass. Simply stated; as RPM increases, the air mass becomes less cooperative. At low engine speeds the air can move fast enough to keep up with the piston but air and fuel mixture carries mass that takes time to get moving. Where mechanical parts can be forced to move instantaneously, air will take its time no matter how we try to shove it around. Consequently, as RPM increases, the piston makes its way further down the bore before the air begins to follow it. As this delayed reaction increases, the useful distance in the pistons downward stroke that becomes shorter. At a certain point (around 115 ATDC) the piston can no longer pull in enough air to properly feed the engine. Picture the piston only pulling on the air/fuel for 35* of the downward stroke and you can see how the engine would run out of breath. The RPM at which this occurs is largely dependent on how restrictive the intake path is. A more restrictive path causes this to happen at a lower RPM.

It’s amazing an engine can run at all when the intake charge doesn’t begin moving until the piston is almost to the bottom of the bore. Where 35* is not enough for the engine to continue making additional power, 55* of piston travel is enough to make tremendous power. How is that possible? It is initially made possible through an effective overlap period that can jump start the air entering the cylinder without the help of the piston. As the air front makes its way into the bore it encounters a powerful suction created by the large distance between itself and the fast moving piston. Up to a point this more powerful pressure differential and subsequent faster moving air mass can compensate for the reduction in piston draw. Think of the air as a ball with a rubber band attached. The piston is like the hand pulling the ball (column of air) by the rubber band (pressure differential) between the two. If we use the rubber band to slowly pick the ball up off the floor, it will begin to move almost instantaneously with the hand movement and it will follow the hand at roughly the same speed. Now picture jerking the ball off the floor with the rubber band. Obviously the hand will travel some distance stretching the rubber band before the ball begins moving. However, once the ball starts to react to the force acting against it by the stretched rubber band, it begins moving very quickly and with a lot of inertia. So much so that even though the hand stops just as the piston does at the bottom of the bore, the ball will utilize the kinetic energy created by the rubber band to continue moving. It is the same with the piston, the further it moves away from the air mass, the greater the pressure differential is. When the air finally starts moving, it is playing serious catch up and literally acts like a ball being pulled by an outstretched rubber band. It moves fast and makes up for lost time. Between the time the delayed air front starts moving and the piston travels the final 55* to the bottom of the bore, the column of air has picked up enough speed to not only catch up to the piston but to overcome the force of the piston pushing back up against it and to overfill the cylinder through inertial ramming.

Advancing and Retarding the Intake Lobe Centerline

You might notice that vehicles that compete in stock class racing where cylinder head ports cannot be altered from their original casting usually have extremely advanced intake lobes and a lot of overlap. This is done to make up for the engines inability to RPM. The advanced intake centerline reduces power at the top of the RPM scale and concentrates it lower in the RPM range. Unrestricted classes like pro-stock don’t require near as much advance and overlap because they utilize monster ports and valves relative to the cubic inch dimensions of the engine.

Advancing ICL

Advancing the intake lobe opening point can help performance in a couple ways. Opening the valve sooner increases overlap and allows the exhaust to have a greater influence on the intake charge as the intake valve is opening. Advancing the entire lobe also moves the centerline and effective lift of the opening ramp closer to the pistons highest point of velocity where it produces the most suction or pressure differential. Both of these attributes help kick start airflow allowing a weaker inlet path to appear bigger to the cylinder. As David Vizard said; what happens at the beginning of the intake valve opening determines the success of the entire intake event.

As noted earlier, the biggest problem with creating the perfect lobe is that maximum pressure differentials occur too late in the induction stroke to align with the lobes centerline. As we advance the lobe further to attempt parity we encounter some negative side effects that limit how far we can go. The first is limited RPM from the intake valve closing sooner and hindering inertial ramming at the end of the induction cycle. The second is intake reversion gets worse as the intake valve is opened earlier in the exhaust cycle causing rough running at low RPM and requiring more engine speed to get into the cams comfort zone. These two elements combined can narrow the power band. The final problem we have with advancing the intake lobe is excessive overlap. This can over scavenge the cylinder, pull intake charge out the exhaust and allow exhaust reversion back into the cylinder. Excessive overlap can also create cross scavenging between adjacent cylinders in the firing order. Advancing the intake lobe can be beneficial but care must be taken not to overdo it.

Retarding ICL

At this point it is probably somewhat obvious what happens when retarding the intake lobe. We lose the benefit of a more efficient opening event but it delays the closing point and provides improved inertial charging for high RPM operation. It also reduces overlap which improves low RPM drivability but can hinder high RPM operation. Around in circles we go but as usual each one of these conditions occurs to more or less extremes based on the engine combination. If the engine is seriously starving for more RPM then retarding the lobe up to a certain point will help remedy the problem. If the engine cannot tolerate much RPM than advancing the lobe would be the preferred adjustment.

Again a balance must be struck between opposing benefits and consequences and the extent to which each attribute contributes to or hinders power production depends on several other factors in the combination.

Exhaust Lobe

The theoretically perfect exhaust lobe wouldn’t open the valve until BDC where it would instantaneously open to full valve lift. This way every last ounce of power could be extracted from the ignited air/fuel mixture by the piston while at the same time pumping losses would be minimized. Similar to a perfect intake event, the perfect exhaust lobe would require an instantaneous full opening of the valve. This way the exhaust gasses could escape easily and efficiently without a lot of backpressure. Unfortunately we are again bound by current technology which requires a compromise in low end performance in the pursuit of high RPM horsepower. To find out more, let’s look at the individual valve events.

Exhaust Valve Closing

When we are speaking about EVC the topic will revolve around overlap, intake scavenging, exhaust scavenging, combustion chamber heat and reversion.

When closed at just the right moment the exhaust valve allows the piston to push out all the exhaust, it gives the escaping gasses time to pull exhaust from the combustion chamber where the piston cannot reach, and it delays closing just long enough so the inertia of the escaping exhaust gas can initiate the intake charge, but close soon enough to prevent it from pulling the air/fuel mix out the exhaust pipe.

Most exhaust gasses are evacuated during the blow down period at exhaust valve open but it is critical that the valve is not closed to early or we end up with a contaminated intake charge and no cross flow to the intake. This is a fairly straight forward consequence but an exhaust valve closed too late is a little more complicated. It can over scavenge the exhaust, cause reversion back into the cylinder, or prevent the piston from pulling in enough air/fuel to produce sufficient power. To what extent any of these conditions occur or do not occur has as much to do with how well the exhaust is matched to the combination as the actual specifications of the camshaft.

Ignoring the exhaust system component of this equation for a moment, we’ll address each of the potential problems of closing the exhaust valve too late. The problem with over scavenging the cylinder is that the air fuel mix will be pulled out the exhaust pipe instead of down into the cylinder, resulting in less combustible material to act against the piston and poor fuel mileage. In the case of an excessively late closing valve, not only is too much air/fuel mixture pulled out the exhaust but enough heat can be pulled from the combustion chamber to result in reduced combustion efficiency. Finally, the ultimate mismatch occurs when the valve is closed so late that the piston has traveled far enough down the bore to negatively affect the amount of air it can draw in. Remember that the piston cannot start pulling in air/fuel until the exhaust valve is completely closed. A late closing works with high RPM applications because the speed of the piston creates a more powerful pressure differential between the cylinder and the intake track so even though the piston starts to pull charge in later in the process, it is a more powerful pull that gets the job. If the exhaust valve is closed to late in relation to the engines maximum RPM capability than the piston never reaches high enough speed to create the dramatic pressure differential needed to make up for the loss of time it has to pull on the intake charge.

EVC and the Exhaust System

In addition to over scavenging, the other possible condition that can occur with a late closing exhaust valve is reversion. If the exhaust system doesn’t have either enough escape velocity or it has too much resistance, exhaust can reverse direction and come back into the cylinder, polluting the intake charge.

Back pressure usually rears its head at higher RPM when the piston cannot overcome the resistance of the restrictive exhaust, and reversion is caused by poor escape velocity in the exhaust system. Both are RPM dependent and require an exhaust system that matches the RPM range of the camshaft.

The exhaust system is a critical component to a successful exhaust lobe and has the ability to render a perfect lobe ineffective, and help a poorly spec’d lobe perform better. An engine cammed properly for high RPM with a late closing exhaust valve needs an appropriately sized exhaust to complement it. A small primary tube header combined with the late exhaust close will over scavenge the cylinder at low RPM and fail to rid exhaust gasses at high RPM.

Conversely, a primary tube that is too large for a given camshafts closing point and intended RPM range may not properly scavenge the combustion chamber until the RPM’s are beyond the combinations capability. In which case the exhaust may never be fully evacuated from the cylinder and the intake charge will experience little to know initiation during overlap.

A certain amount of reversion and inadequate scavenging is inevitable for properly designed, high RPM applications because an exhaust big enough to handle 7500 RPM will carry little to no escape velocity at very low RPM. Whether “too big” or “too small”, every possible level of mismatch can occur but there is some consolation in that it is only the extremes that amount to sizeable losses.

EVC and Overlap

When addressing the exhaust and intake scavenging component of the exhaust valve closing point, were actually talking about overlap. It is this period where both the intake and exhaust valve are open at the same time that determines how well exhaust is evacuated from the space in the combustion chamber where the piston cannot reach (clearance volume) and also how much the escaping exhaust helps initiate the intake charge.

When speaking in terms of positioning the intake and exhaust lobes to obtain proper overlap, it is the exhaust lobe that usually bears the brunt of the responsibility because in general, an intake lobe will have an appropriate lobe center for a given combination that is decided on in terms of dynamic compression, pressure differential and inertial charging near the end of the cycle. These components make the intake lobe more sensitive than the exhaust lobe to ideal placement and it is for that reason that the more forgiving exhaust lobe can often be moved around without jeopardizing its primary purpose.

Even though the exhaust lobe is typically less sensitive to movement, it is important to note that whether we are adding duration or moving the lobe, both the opening and closing events could be altered and the consequences need to be considered.

1) Adding duration to delay the closing and increase overlap will also result in an earlier opening point. Opening the exhaust valve further into the combustion period provides additional blowdown pressure for removing exhaust gasses but can have a negative effect on low to mid RPM power because it is sent out the exhaust pipe.

2) Retarding the exhaust lobe as opposed to adding duration to increase overlap will also delay the point at which the exhaust valve opens. This helps low to mid RPM power but may result in greater pumping losses and incomplete exhaust removal.

3) If we want to maintain the same opening point but alter the closing point we can add duration and retard the lobe so all of the added duration takes place on the closing side while the opening point remains the same.
Exhaust valve closing is no different from any other camshaft event. It needs to be considered from a broad perspective, spec’d properly and supported with matching components throughout the engine.

Exhaust Valve Opening

The exhaust valve opening event can make for interesting conversation because some engine combinations can make it very forgiving to improper placement while other combinations will make it a critical component to a successful build. Harold Brookshire considers it the most important valve event while others consider it the least important. Who is right? They all are, given the right context. The argument is slightly different for every engine. Let’s find out why.

As we saw earlier, the exhaust lobe closing point can be critical to the success of the intake cycle. But it is the opening event of the exhaust lobe that is most critical for expelling the burnt gasses. The end of the combustion cycle marks the beginning of the exhaust cycle. This is where the piston starts moving back up the bore shoving the exhaust out past the valve. When the piston reaches top dead center and has pushed the exhaust out, the cycle is nearly finished. This simplistic view gives a good idea of how the exhaust cycle works but as we know it isn’t quite that simple.

In order to effectively remove all the exhaust and avoid pumping losses (horsepower used by the piston to push exhaust out) the exhaust cycle actually begins around 70 degrees BBDC as the valve begins to open during the power stroke (blowdown). By cracking the valve open under high pressure, much of the exhaust can escape early in the cycle and create good velocity for scavenging later in the cycle. The downside is the energy used to rid the cylinder of exhaust will not be able to act against the piston. That energy is lost and can never be recovered, so the compromise is a balance between how much energy is lost out the exhaust pipe VS how much is used to push the piston down VS how much horsepower is used by the piston to push the remaining exhaust out (pumping losses).

Each combination has a point where opening the valve any sooner loses more horsepower out the exhaust than is gained by the pumping losses it eliminates. The opposite is also true; each combination has a point where delaying the exhaust valve opening to extract extra power from the still expanding combustion costs more power to pump the leftover exhaust gasses out than is gained by the force it exerts on the piston. In typical fashion, this is largely RPM dependent.

A low RPM combination can utilize a later closing and benefit from the extra force applied against the piston. Exhaust is still adequately eliminated with the low RPM, later closing combination because of another recurring theme in camshaft events; time. Most camshaft events are a function of time which makes them RPM centric. Exhaust opening is no different, and at low RPM there is enough time for the exhaust to escape. Additionally, the remaining exhaust gasses do not act as forcefully against a slower moving piston. A high speed piston will slam into the remaining gasses creating a strong opposing force as it tries to push the exhaust past the valve, but a slow speed piston has a much gentler encounter with the remaining exhaust, placing less demand on the valve while allowing more time to move past it with less corresponding resistance.

As RPM increases the exhaust valve needs to be opened earlier because the amount of time it has to rid the cylinder of exhaust gasses becomes shorter. Opening the valve sooner exposes the exhaust port to progressively higher cylinder pressure. As the amount of time decreases to get rid of the exhaust gas, more pressure is needed to jump start the cycle and increase exit velocity. Finding the correct pressure needed to adequately blow enough exhaust past the valve (blow down) without wasting power can be a delicate balance.

There is no hard and fast rule that defines a specific RPM but to simplify the concept I will simply note that if the exhaust valve is opened a little too early it may only require a few hundred more RPM to move the cam into its power band. From a big picture standpoint we can see that a few degrees off may not be a big deal. If we have a combination that can easily handle a few hundred more RPM in terms of adequate intake flow, intake pressure wave tuning, exhaust system capability, short block durability, valvetrain stability, etc. than it won’t be a problem. Then again if we don’t have the wiggle room, horsepower is left on the table.

When attempting to properly define the timing of the exhaust opening event, engine builders like to point out that 80% of the power is extracted from the combustion by the time the piston has reached 90 degrees past TDC. Often people will infer from this statement that the last 90 degrees is useless. For example, a popular magazine author recently wrote that; “The bottom half of the power stroke actually provides very little in terms of engine power”.
Racer Brown, a veteran cam designer states that; “Very late exhaust valve opening points, when properly done will improve engine performance through most of the engine speed range, if not all of it.”

You might think I would be more inclined to agree with a veteran cam designer rather than a writer for an automobile magazine but the fact is they are both right under the right circumstances. Racer Brown further stated the importance of considering the RPM range of the engine when determining the opening event.

There is more to the RPM element than just time. We also have to consider piston speed during the blowdown period. As the piston moves farther down the cylinder it is moving away from an ever weakening combustion charge. The piston and the combustion are moving in the same direction. As a result, force acting on the piston is going to be greater on a slower moving piston than a faster moving piston. Think about running with the wind at your back, the faster you run the less that force pushes against you. As the RPM increases, the piston simply runs away from any meaningful combustion earlier in the power stroke.

Crankshaft stroke and connecting rod length also need to be considered in terms of piston speed and position during the blowdown period but an entire chapter can be written on that subject alone so I’m going to take a mulligan on it for now.

Another related element is static and dynamic compression, or more appropriately, cylinder pressure. It affects both when the valve should be opened and the influence the combustion has on the piston throughout the power stroke. High cylinder pressure environments will exact more force against the piston further into the exhaust stroke so that a later opening event becomes more relevant in terms of the torque it provides and the ability to affectively evacuate the exhaust when the piston is further down the bore. For example, higher cylinder pressure can be just as effective at pushing exhaust out at 50* BBDC as a lower pressured cylinder at 60* BBDC. Not only that but the pressure being exerted against the piston at 50* BBDC in a high compression combination can be just as forceful as the pressure being exerted at 60* BBDC in a lower compression combination. Compression and cylinder pressure support the best of both worlds, low speed torque and high RPM horsepower.

A consideration related to cylinder pressure is volumetric efficiency. An engine operating at 75% VE will be generating less useful force against the piston at 50* before BDC than an engine running at 100%. Think of this in terms of cylinder fill or simply, more air/fuel causing a bigger explosion and more cylinder pressure. This also has an effect on when the exhaust valve should be opened.

As you can see, there are no simple valve events. Each is unique in its own way and has the potential to make or break an engine combination.


Overlap is the period when both the intake and exhaust valves are open at the same time. It starts when the piston is approaching TDC on the exhaust stroke and lasts until just after TDC. In the 1950’s racer and camshaft designer Ed Iskendarian was blowing away the competition with his new 5 cycle cams and as a result the term 5th cycle is now common vernacular in camshaft terminology. It refers to the addition of a new cycle to the traditional induction, compression, combustion and exhaust grouping.

The power producing potential of overlap is considered second only to duration. Under the right conditions a cam utilizing the proper amount of overlap can easily pull 20 additional horsepower over an almost identical cam on wider lobe centers. If you look at the difference between cams of progressively more aggressive performance levels you will see that the additional duration is primarily used to increase overlap by opening the intake valve sooner and closing the exhaust valve later. In other words, the intake valve opening point is the most aggressively altered timing event when designing performance camshafts and it is followed closely by EVC whose primary function is dialing in the proper amount of overlap.

Of course, the extra power doesn’t come without a cost. Additional overlap often results in a rough idle, increased emissions, low vacuum and poor throttle response until the RPM’s are sufficient enough to overcome intake and exhaust reversion.

Let’s take an in depth look at overlap. In a performance application, the primary purpose of overlap is to allow the velocity of the escaping exhaust gasses to pull on the intake port while both valves are open. As the piston is rising in the bore and pushing exhaust out of the engine it will encounter a slowly opening intake valve about a half inch from TDC. Some race applications will start opening the intake valve earlier than 50* BTDC while some mild street applications can be as late as 3* BTDC. The exhaust valve closing ATDC has a similar range, it can be very early or quite late. If we add the number of degrees before TDC that the intake valve opens and the number of degrees ATDC that the exhaust valve closes, we get the number of degrees of overlap. Typical street engines have between 30* - 50* of overlap and race oriented engines can have more than 100*.

In performance applications the intake valve will start to open early enough that the piston will push some exhaust gas into the intake port (intake reversion) and some will continue to flow out the exhaust pipe. To the extent this reversion occurs depends primarily on how early the intake valve opens and how much back pressure exists at the valve and in the exhaust. As the piston proceeds up the bore and reaches TDC, the inertia created by the velocity of the escaping exhaust gasses pulls exhaust from the combustion chamber where the piston can’t reach. This tiny pocket of air between the top of the piston and the roof of the combustion chamber is referred to as “clearance volume”. Removing this pocket of left over exhaust gas is the minimum expectation of the escaping exhaust gasses even for the mildest of combinations. If a slightly more aggressive combination with an earlier opening intake valve is being built than the goal for the escaping exhaust gasses during the overlap period is to not only rid the chamber of exhaust, but to pull out the exhaust that was pushed up into the intake port so the induction stroke gets a clean, un-diluted intake charge. When we start talking about high performance applications, overlaps responsibilities become even bigger because the inertia of the escaping exhaust gasses is expected to initiate the intake charge and kick off the induction cycle.

The goal is to run enough overlap so that the exhaust has time to not only pull spent gasses from the combustion chamber where the piston cannot reach, to not only clear the intake port of reversion, but to also give the fresh intake charge a yank. This phenomenon where the exhaust is used to pull the intake charge into the cylinder is also known as flow through and it is the holy grail of overlap tuning.

Here is how David Vizard regards this aspect of overlap:
“With a well-tuned exhaust, we find that the strongest draw on the intake port is brought about by the negative pressure created by the exhaust - not, as is so often supposed, the piston going down the bore.”

You might want to read that twice. I know I did when I first read it.

And this is why it is so important:
“We find from port and cylinder pressure measurements, that success in the first half of the induction, dictates the success in the second half. If the first half is not optimal, there is nothing that can be done in the second half to rectify the problem.”

When building for maximum power the challenge is to leave the exhaust valve open long enough so that the escaping gasses can pull the fresh intake charge into the cylinder but snap shut just before it pulls the charge out the exhaust pipe. Some considerations when tuning overlap are the intended RPM range, the exhaust system, and drivability. Since this is a discussion built around performance applications we won’t spend time on drivability except to say that if rough idle and poor throttle response at low RPM are a concern, don’t run a lot of overlap.

RPM is the primary consideration when tuning overlap because it imparts the same time constraints that have to be considered when specifying other aspects of cam timing. As RPM increases, the overlap period has less time to evacuate exhaust and pull intake into the cylinder. Mechanically the amount of overlap remains the same but the amount of time the two valves have to create effective cross-flow is reduced. Consequently overlap has to be increased to keep both valves open long enough to be effective. If both valves aren’t held open long enough the intake won’t be jump started and exhaust gasses will not be effectively evacuated.

When overlap is spec’d for high RPM, the time quotient will be more than enough at low RPM. The most notable consequences being that either the intake charge is pulled out the exhaust or in the case of big primary tube headers that lack velocity, exhaust gasses will reverse direction and be sucked back in by the descending piston (exhaust reversion). Both conditions result in lower torque and a rough idle.

Aggressive overlap requires a good exhaust system. With the exhaust valve being held open for so long, any backpressure can reduce the effectiveness of the overlap period and result in reversion and polluting of the intake charge. The primary tube header needs to be sized big enough so there is no back pressure in the intended RPM range but not so big that it doesn’t have enough velocity to scavenge the combustion chamber and intake port. As stated in the EVC section, any primary tube big enough to handle 6800 RPM operation is going to be lazy enough at low RPM to fail at scavenging and likely have some exhaust reversion with a big cam. But when the RPM’s come up, hold on.

The theme continues; what works for high RPM doesn’t work for low RPM. We must choose.

There are a few things beside RPM that will have an influence on how much overlap we should run: Compression, combustion chamber size, crankshaft stroke, rod length, rod to stroke ratio, low lift port flow, rocker arm ratio and engine displacement.

1) Higher compression equals higher cylinder pressure which causes higher exit speeds for escaping gasses. This causes a more powerful scavenging ability requiring a shorter overlap period.

2) A smaller combustion chamber is evacuated more quickly. This equals less need for overlap.

3) Crankshaft stroke, rod length and rod to stroke ratio all affect how long the piston dwells at TDC and how many feet per second the piston travels before and after. This in turn affects the amount of time overlap has to get the job done.

4) Low lift port flow and rocker arm ratio both affect how well the intake port moves air at the beginning of the induction stroke. Improved low lift flow resulting from a well designed port and valve layout or higher ratio rockers reduce required overlap.

5) As engine displacement goes up, the same size intake port will need more help to feed the engine so overlap must be increased.
Now that we have a pretty good understanding of overlap we can move on to lobe separation angle.

Now that we have a pretty good understanding of overlap we can move on to lobe separation angle.

Lobe Separation Angle or Lobe Centerline Angle (LSA/LCA)

Lobe separation angle is just a generic label used to provide a general idea of how several different cam timing events are working together. The two components these terms intend to represent are overlap and intake/exhaust centerlines, but other implications include all cam events. As a result, LSA is probably the most confusing element of camshaft timing. When someone says to “tighten up the lobe centerline angle”, they could be referring to increasing the overlap, advancing the intake lobe, retarding the exhaust lobe, or any combination. Each action carries unique characteristics and should be looked at individually, and as a whole.

Even when you have a good understanding of the basics, several things can make lobe separation angle difficult to understand. Two cams with identical LSA, durations, lift and opening/closing points can run very different because differences in ramp acceleration rates can change the area under the overlap curve. The timing points may be the same but the overlap triangle will look completely different. It’s very similar with duration. Two cams can have the same LSA but have different degrees of overlap because they differ in duration. A 240* cam will have a lot more overlap with a 112* LSA than a 200* cam on the same 112* LSA.

Lobe Separation Angle can be used to give a general comparison between similar cams in the same family but it is also important to look at all the critical timing factors to get a true picture of a camshafts capabilities.

We’ve covered each of the cam events so now it’s time to look at some practical application.

Unique Ford DOHC Characteristics

Every engine combination is more or less unique in its cam requirements but I’ll focus on the Ford DOHC engine and some of the major design elements that make specifying their camshafts unique from the brand X alternatives.

The most obvious departure from traditional domestic architectures is the DOHC 4 valve layout. When Ford designed the new modular engine family they knew the small cubic inch blocks were at a disadvantage and that the tiny bore size would never be big enough to support enough valve for reasonable performance applications. As a result they employed a 4 valve per cylinder, dual overhead camshaft combination. Compared to typical two valve wedge heads it was very expensive to engineer, produce, assemble and package but it is far superior to conventional pushrod and 2 valve variants in terms of horsepower per cubic inch. I was always surprised that the 4.6 didn’t get more press for producing almost identical power numbers to the LS-1’s of the same era and they did it with 50 fewer cubic inches, smaller camshafts and better fuel economy. Here’s why:

The biggest restriction to getting air into and out of an engine is the valve. This is how David Vizard puts it:

“The most important factor in making hp is the circumference of the intake valve in relation to the cylinder's displacement. All two-valve V-8s are under-valved for the cubes those valves have to feed.” David Vizard

In other words, almost all domestic V-8’s from the inception of the internal combustion engine has been under valved. It’s not hard to imagine when you look at this nice straight pathway the port provides, only to plug the end up with a 2” hole and a valve that only lifts a half inch off the seat to allow air to pass. Valve limitation is the nature of the beast because a cylinder can only be so big before the dimension of the block will no longer fit within a reasonably sized engine bay. Four valve architecture, expensive as it is, does away with this problem by allowing a lot more valve area to be applied to a given combustion chamber/cylinder bore size. To put this into perspective we can compare the valve size of the 281 cubic inch 4.6L (1.456”) with those of a traditional 350 cubic inch Chevrolet small blocks (1.94” – 2.02”), the LS1 346 cubic inch small blocks (2.00” valve size), the current model LSX 380 cubic inch 6.2L small block (2.165”) and finally, the venerable Z06 427 cubic inch animal at 2.2”.

To accurately compare valve sizes we have to convert the diameter of both the DOHC valves to their area, add them and then convert back to the diameter for a single valve. When we do this we get 2.06” equivalency. That’s a 2.06” valve to feed a paltry 281 cubic inches, or 4.6L. With larger and larger cubic inch engines coming on line, Detroit is producing heads with massive ports and large valves to go with them. But even with modern day technology it’s impossible to match the area of a four valve head. To bring more perspective to the table let’s look at what size valves the Chevrolet’s would need to equal the valve area per cubic inch of the 4.6.

The 347ci LS1 (2.00”) would need a 2.28” valve.
The 380ci LSX (2.165”) would need a 2.4” valve.
The 427ci LS7 (2.2”) would need a 2.54” valve!

It is safe to say the DOHC 4.6 is not “under valved”, which accounts for the superior horsepower per cubic inch figures they deliver, especially with mild cam timing. In addition to superior valving over their domestic 2 valve brethren, the 4 valve layout utilizes a semi-hemispherical combustion chamber that places the intake and exhaust valves directly across from each other instead of side by side. As a result, the incoming air doesn’t have to make a 90* left hand turn followed by a 90* right hand turn in order to exit the cylinder. Airflow direction remains unaltered traveling straight in and straight out during the overlap period. That is a huge advantage.

With the 4 valve layout, maximum airflow potential is impressive but more important is the low lift flow figures which are nothing short of excellent. Remember that what happens in the beginning of the valve event determines the success of the entire induction cycle.

Here are a few things you may or may not have known about the 4 valve layout:

More valve area allows the use of smaller ports for increased velocity while achieving the same peak airflow numbers as a larger port. The increased velocity also provides even better low lift flow and improved fuel atomization and inertial ramming.

The smaller valves way less and don’t need to be lifted as far off the seat to move an equivalent amount of air so the valvetrain utilizes lower spring rates and lighter components for improved high RPM operation.

Superior low lift flow allows a smaller cam to perform as well as a larger cam. More specifically, a 260 degree cam in the 4 valve engine has the equivalent cylinder fill capacity of a 280 degree cam in a two valve engine.

The same is true for overlap and lobe centers. Superior low lift flow and opposing valves instead of side by side allows wider lobe centers and a minimum of 20 fewer degrees of overlap.

Compare 4v cam specs to 2v cam specs and you will see dramatic differences between the two. Camshaft engineers didn’t suddenly forget how to design cams when they went to work on the Ford DOHC. They have wider lobe separation angle (less overlap), later intake centerlines, less duration and less lift for very specific reasons.

The characteristics that improve the function of the intake side carry over to the exhaust as well. The exhaust of the four valve design is so efficient that the valves can be held closed deep into the blowdown period and still efficiently rid the chamber of spent gasses. This results in more low end torque even with very aggressive cams. And coincidentally, better fuel mileage.

Without the 4 valve layout, a streetable 450+ flywheel horsepower, naturally aspirated, pump gas, 281 cubic inch engine wouldn’t be possible. That’s 1.6 horsepower per cubic inch. Comparable horsepower per cubic inch figures for a 6.2L LSX Chevrolet would be 608 horsepower. How streetable do you think that would be? There is no comparison. Of course, cubic inches rule so the little DOHC leaves a lot to be desired in that department. If Ford pulls out all the stops and gives us some DOHC cylinder heads for the new 440 cubic inch capable Boss block, the blue oval will be untouchable. Did someone say Boss 429?

Choose a Cam

Obviously, designing a camshaft isn’t for the week of heart but most cam grinders have done all the hard work to get you in the ball park. The area they usually fudge is LSA in terms of less overlap and a later intake centerline because they are willing to sacrifice some power for drivability. It typically makes for a happier customer. As a result, advancing the intake lobe a bit and increasing overlap can usually pick up a little power but don’t expect miracles with the DOHC.

That leaves the builder with choosing the appropriate duration for the RPM the combination is designed to support, then adjust LSA/overlap appropriate to the RPM level and within the confines of the individual drivability tolerance.

To simplify (blasphemy) things we can assume that most cam manufacturers have figured out the hard stuff. As mentioned above, pick an off the shelf cam with the appropriate duration for your RPM range and if you want the last bit of horsepower, consult with the manufacturer for a custom grind which will probably give it a little more overlap and advance in the intake lobe centerline.

How do we choose duration? Well, if we’re lucky we have dyno sheets to refer to for the purpose of identifying how our particular combination responds to duration. Doesn’t matter if you’re a Chevrolet guy, Mopar or Ford, the internet forums are filled with dyno sheets from like minded enthusiasts.

If we’re talking about the 4.6L DOHC Ford we have to be careful because the restricted factory intake skews the results. It’s easy to look at the cylinder head architecture and start drooling but reality is they need a good intake to support them.

Cams with anywhere between 218* - 234* duration at .050 will peak at 6300 - 6400 RPM with the stock intake while stock and mild performance cams in the 209* - 212* range (FR500’s, GT’s Cobra R’s) will peak around 6100 RPM. Shorten the runners in the 5” – 7” range and a little 209* cam wont peak until at least 6800 RPM and it will carry the curve flat well beyond that. Once again, cylinder head airflow is the least of our worries. Prepare to sacrifice up to 75 horsepower in the 5000 RPM range and not make as much power as the stock intake until 6000 RPM if running a 5” runner. For most people, somewhere between 7” - 10” runners are ideal.

Here are some comparisons between different intake and camshaft combinations. Use it as a general guideline to give an idea of each ones characteristics.


It is OK to over cam the DOHC as long as your power curve within your racing RPM range is better than the next mildest cam down. If it’s not than you can pull the cams RPM range down some by advancing the intake lobe if you don’t mind even rougher low speed operation.

For a basic 4.6 DOHC that runs to 6800 RPM in stock configuration we can figure an RPM drop of roughly 2000 between gear shifts, we would like the most aggressive cam that begins to overpower the next milder cam starting at 4800 RPM. Doesn’t matter how it performs below that if we are pursuing a combination that we want to accelerate as quickly as possible. Notice were not talking about drivability.

Now we must determine a reference point. We know that stock Mach 1’s peak horsepower around 5200 RPM with 184* of duration and 99/01 Cobra’s with basically the same engine and 200* of intake duration peak at around 6100 RPM. In this particular case, 15 degrees of duration add almost 1000 RPM to the point at which peak power occurs. We must maintain perspective of runner length and airflow restriction which are both elements lowering the RPM characteristics of the above combinations. Adding another 15* of duration will not move the horsepower peak to 7000 RPM with the stock intake manifold but with short enough runners and unrestricted airflow, it will.

The stock intake starts restricting airflow around 5200 RPM and by 6100 RPM it is pretty much finished. This means that no matter how large the camshaft is, it will not peak power much higher. Maximum is about 6400 with a very aggressive cam. As a result, we have to shift focus from peak numbers to average numbers and what a bigger cam will do is hold power at the peak level longer and fall off slower after peak has been reached. This is why it isn’t futile to “over cam” this particular engine. A Comp Cams 106400 with 234* duration will peak around 6300 – 6400 and hold power longer than a smaller cam. However, if this cam is given enough air and shorter runners it will make considerably more power and peak well into the mid 7000 RPM range. When camming this engine we are typically working within the constraints of the shortblock and the intake manifold.

I would say the 106400 at 234* duration is about maximum for a stock or ported stock intake equipped engine. It does sacrifice low RPM operation but has decent mid range and gives excellent power within the limitations of the stock short block and intake. Also, it offers plenty of potential for any aftermarket or modified intake. On the other hand, if a person is willing to sacrifice racing performance for some mid range torque and smoother low speed operation, cams in the 218* - 226* range provide similar power with a little more docile behavior.


Choosing a camshaft is not too difficult given the fact that most manufacturers produce well designed products. Understanding how all the different elements work together is the hard part and I hope you come away from this article with a little more knowledge than when you started. There is still quite a bit more to this subject but that may be addressed some other day. For now, I imagine most people will be challenged enough to find the time to read what has been provided her.

Todd Hutchinson
Last edited:


Established Member
Aug 9, 2002
Seattle, WA
Yeh, that's a good one. The guy's a genius when it comes to engines. Check out his "pollyquad" cylinder head he patented.

na svt

say no to power adders
Established Member
Sep 30, 2005
Beavercreek, Ohio
Last edited:


Established Member
Aug 9, 2002
Seattle, WA
Check out the forums at "", he post over there quite a bit. There's a lot of good cam info between him and the articles/books by Holdener and Mihovitz.

Yeh, I cant believe he finds time to post on a forum. The other guy that posts a lot of good cam info is Harold Brookshire of the now defunct Ultradyne. He's the godfather of advanced intake centerlines.

I think that book is out of print. Bummer.

Users who are viewing this thread