Long tube headers, which Primary size is 'best'?

[Shaun@AED]

I get this question all the time, or more accurately I repeatedly talk customers out of going 'too large' with primary size.

Figured perhaps a thread is a better way to educate a little bit.

When it comes to headers, bigger is not always better. The easiest way to visualize this when it comes flow potential we can use math to see which primary size is best for a given exhaust diameter.
For the purpose of standardizing the math we will assume all headers have .050" wall thickness

Calculating the volume of exhaust flow is not easy, so we will focus on the area of the pipes for ease of comparison as volume is directly related.

Factory 2.75" OD exhaust has an area of 5.51 square inches
3.0" OD exhaust has an area of 6.60 square inches

Now it's time to calculate the area of the primaries.
Since these are V8 engines with dual exhaust the calculation for total area of the primaries would be 4x the primary size.

4 - 1 & 1/2" Primaries have a toal area of 6.15 square inches (Yes, larger than factory 2.75" exhaust)
4 - 1 & 5/8th" Primaries have an area of 7.30 square inches (A perfect size for dual 3" exhaust IMO)
4 - 1 & 3/4" Primaries have an area of 8.55 square inches
4 - 1 & 7/8th" Primaries have an area of 9.89 square inches

Given this information you can see unless you are running 3.5" + exhaust, there is no reason to run large primary headers.

However, there are good reasons to run smaller primary headers, not the least of which is heat and volume of exhaust in the system. The more volume you have in the primaries, the longer it takes the exhaust to reach the O2 sensors and the colder it will be when it gets there. Velocity will be lower with larger primaries, O2 data will not be as accurate, the exhaust will have lost more heat energy, and the blow-down pulse will have less energy to pull exhaust out of the next cylinder. The result is lower power output until RPM's are high along with poor O2 readings which causes driveability and idle issues.

Food for thought. You CAN go 'Too Big'.

 

[TrackpackGT]

It's funny I read so many posts saying these 5.0's love 1 7/8ths. I always thought that was overkill. I don't even think my LS1 header primaries were even close to that. Maybe in a non-computer monitored car, the effect would be different? I guess there would need to be a significant change in exhaust volume/velocity change if you had a max effort n/a set up like nasty cams, custom intake, larger displacement, or a big turbo set up. I definitely thought a stock engine with bolt ons wasn't going to realize real "gains" from 1 7/8th's primaries. I'm sure there is someone with their own "science" to disagree.

 

[stags]

Damn Shaun. If you're ever randomly in Nashville PM me and I will buy you all the beer you can drink.

Seriously, I learn more from your posts than I do anywhere else.

So I am guessing that the key takeaway here is unless you run a blower and 3" catback the 1.5" primaries are the way to go?

 

[Shaun@AED]

I do not know of any header manufactures that do 1.5" primaries, but I do know Kooks makes 1 & 5/8th" for the coyote.

FWIW CPR uses 1.5" primaries with his turbo kits and we recently made over 1000RWHP SAE corrected with his kit at 16Psi. The 1.5" primaries we not a restriction even at that high power level.

The more important aspect of headers is the collector design along with the primary length as a proper setup will take full advantage of the blowdown pulse for a power boost at the appropriate RPM. And of course the unique firing order of the coyote means special attention with primary lengths.
But that would be a different discussion/thread.

 

[Angry50]

i have looked at headers before this sheds some light on things.i could never be sure what was best. even at the ideal size i havent seen large enough gains from LTs to justify the cost. i went just O/R X and looking at doing tire and converter next as those seem to be best bang for the buck. but i love the explanation gives us some clarity awesome as usual Shaun!

 

[Jabooh1]

So I have always read that a cat delete and tune will gain about 20hp while LT cat delete and tune is 30. Is this about correct? If the LT's only contribute 10ish HP and primary sizes come into play, could you get a scenario where the LT's actually don't add anything?

 

[will2115]

Alright so headers are not really worth it? Even with the cobra jet set up?. Had a tuner that told me my cobra jet wasn't running that good because I didn't had headers.(don't want to say who ) Because it was breathing too much air in (CJ mani, oval TB, and rev auto CAI) and that my car needed headers to let it breath out.

 

[Aaron@JPCRacing]

We've made over 1200rwhp with 1 3/4 headers.

Just food for thought

 

[AKDMB]

About to do a cobra kit car with a coyote in it, longtubes straight into sidepipes :-D. Would I benefit from a 1 3/4" or 1 7/8" primary since there's hardly any exhaust to flow through? Cold air , the longtubes I mentioned and a tune will be all that's on it power wise.

 

[CPRsm]

Some I agree with, some I don't. That's not to say I know which is best here for N/A.

I don't think you can compare turbo headers, only because they are pressurized. You can make anything flow with enough pressure. That's why log manifolds "work."

But I don't think you would want to limit header size by exhaust size or volume. Makes sense at first glance, but there is no account for drop in EGT and there for less need for volume. By the time it gets from head to exhuast it has dropped enough temp the volume has dropped. We've seen it said our 5in down pipe is overkill especially when routing back to dual 3in exhuast, when dual 3.5 is closer to 5in in area. We have the benefit of not needing the right size for scavenging, but going into 5in exhuast drastically cools the exhaust, and from there doesn't need 3.5 all the way out the back. This is why we can clear 1000rwhp thru full exhuast and no cutouts.

I do think 1 7/8 is going to be pretty damn hard to get the volume high enough for the exhuast pulse to seal and create scavenge. It's still a small engine in the end.

 

[Dave_6]

I went with 1 7/8 Kooks mid lengths; oh well.

 

[JonnyW]

I went with 1 7/8 Kooks mid lengths; oh well.

I wouldn't sweat it. The more air in, the more air out. If your primary size is really too large for optimal performance, get more air in to get more air out and take advantage of the size. Boss or CJM, throttle body, cams, boost, etc. Plenty of people run a 1 7/8. I don't see how there could be a huge difference over a 1 3/4.

 

[Dave_6]

I'm happy with my decision as my plans involve a Cobra Jet setup at some point, possibly a blower instead if I can swing it. A lot of my decision was also based on this thread as well: http://www.svtperformance.com/forum...hs-Catted-H-Airaid-and-AED-91-Tune&highlight=

I've also seen some hard running NA setups running 1 7/8 headers...

 

[JonnyW]

I'm happy with my decision as my plans involve a Cobra Jet setup at some point, possibly a blower instead if I can swing it. A lot of my decision was also based on this thread as well: http://www.svtperformance.com/forum...hs-Catted-H-Airaid-and-AED-91-Tune&highlight=

I've also seen some hard running NA setups running 1 7/8 headers...

I think you'll be pleased to know that after doing my research based off of this thread, I called up Kooks and gave Trevor the rundown on my plans. CJM, monoblade TB, 5in RevAuto CAI, Kooks headers, offroad midpipe, Kooks catback, cams, and eventually a procharger. He said 1 3/4 until cams and/or a supercharger is added, then 1 7/8 is ideal. Keep in mind that our cars can make 600hp at the crank with the CJM setup and full bolt-ons/cams. If you're going to be among the few who rock that setup, stick with the 1 7/8, especially since you'll be seeing an 8,000+RPM redline depending on internal mods.
He also mentioned the green cats, but recommended the offroad X pipe. He said he prefers that sound over the H, but others prefer the H over the X.. so it is all based on personal preference. In terms of what they sell, its about a 50/50 split.

 

[Ultrakla$$ic]

The more important aspect of headers is the collector design along with the primary length as a proper setup will take full advantage of the blowdown pulse for a power boost at the appropriate RPM. And of course the unique firing order of the coyote means special attention with primary lengths.
But that would be a different discussion/thread.

Which headers do this the best?????

 

[Shaun@AED]

Which headers do this the best?????

I've been eyeballing the Doug Thorley headers as they have a Tri-Y design, but based on the pics I've seen online of their Coyote headers it seems they got the driver side correct but FUBAR'd the Passenger side. Looks like they designed the headers based on the old firing order, not the Coyote specific.

 

[JonnyW]

Which headers do this the best?????

I'm partial to Kook's equal length headers and would think these would be the most effective design as well as the most efficient. They are indeed mid length, but give the best of both worlds in low end torque and high end power. The headers also feature merge collectors to center the exhaust and reduce turbulence. StainlessWorks claims that equal length headers changes flow characteristics, but primaries will have bends even without being equal length. What isn't mentioned is that theoretically there would be more drastic flow change if the exhaust from the primaries t-boned eachother in the collector. Thus came the invention of equal length headers. I still agree with 1 3/4 for full bolt-ons, and 1 7/8 for full bolt-ons + cam or supercharger. 2" sounds like total overkill unless serious power is being made ei. 1500+hp

 

[Voltwings]

I went with 1 7/8 Kooks mid lengths; oh well.

I did too, but i think it might not be the end of the world. I never thought about the larger primary making the scavenging less effective, but Kooks shorties (or mid lengths) are still that, shorties. A true long tube still has like another 1 + foot of primary before it gets to the collector, so that effect may be softened some in a shorty design. My CJ made no shortage of power with the kooks mid lengths, but i'm not beyond saying it may very well have made more with a proper long tube... i just didnt want to give up my lethal H-pipe, liked the sound too much.

 

[Dave_6]

I'm happy with the ones I went with. I'll never need another header again, unless I ever go with a turbo (obviously).

 

[Turd Fergusen]

I was thinking of going with the 1 3/4 BBK tuned length shorties. i'm probably going single turbo, Figured I can still use them when I do.