Overdrive Balancer

[musclemustangcb]

Looking for the factual reason of why an overdrive balancer provides more low end torque than a stock lower, with both at the same boost level. For example... If I had a 2.4 upper/stock lower and achieved 18psi...VS a 10% lower and a 2.7 upper @18 psi same engine...the dyno results show higher torque on the o/d setup.

 

[Bad Company]

This is an interesting question. First things first. Boost isn't a measurement of CFM of airflow. Boost is strictly a measurement of restriction placed upon the air as it moves from the Supercharger into the engine.

If you sit down and do the math to match CFM air flow or the more common way is Revolutions of the Supercharger to match Liters Per Revolution. With all things being equal in airflow exiting the supercharger throughout the complete RPM range of the engine, there shouldn't be any difference. Most people don't do the math to see if they are comparing apples to apples when it comes to the airflow in CFM or LPM exiting the supercharger with different pulley combinations when they change pulleys. So they may be moving more CFM of air with the larger diameter lower, yet they only see a small boost change and don't realize the gain was about gaining airflow, which intern caused the PCM to add more fuel. Which made more power and torque.

 

[fullboogie]

How is a larger diameter lower, which spins the blower faster at all RPM, going to provide the same boost/airflow as the stock diameter lower? That seems to be your assumption, so the question doesn't make sense to me.

 

[Joewee500]

So in theory, would a 15% lower and 2.5 upper make more tq than a 10% lower and 2.4 upper? Similar boost but different size lower. I'm just feeding the thread, I assume tuning can add tq/hp to whichever combo at different rpms.

 

[musclemustangcb]

This is an interesting question. First things first. Boost isn't a measurement of CFM of airflow. Boost is strictly a measurement of restriction placed upon the air as it moves from the Supercharger into the engine.

If you sit down and do the math to match CFM air flow or the more common way is Revolutions of the Supercharger to match Liters Per Revolution. With all things being equal in airflow exiting the supercharger throughout the complete RPM range of the engine, there shouldn't be any difference. Most people don't do the math to see if they are comparing apples to apples when it comes to the airflow in CFM or LPM exiting the supercharger with different pulley combinations when they change pulleys. So they may be moving more CFM of air with the larger diameter lower, yet they only see a small boost change and don't realize the gain was about gaining airflow, which intern caused the PCM to add more fuel. Which made more power and torque.

I'm assuming that the even though the peak boost numbers are close to the same, the over drive will bring the max cfm in earlier which will reflect in the torque curve?

 

[musclemustangcb]

say a 10% lower/2.7 upper VS stock lower/2.4 upper for example?

 

[musclemustangcb]

How is a larger diameter lower, which spins the blower faster at all RPM, going to provide the same boost/airflow as the stock diameter lower? That seems to be your assumption, so the question doesn't make sense to me.

Say a 10% lower/2.7 upper VS a stock lower/2.4 upper

 

[C.Stanley]

Keep in mind the stock lower weighs more compared to say an 10% iw.

 

[fullboogie]

Say a 10% lower/2.7 upper VS a stock lower/2.4 upper

But that's just guessing that those two combos have the same drive ratio. Regardless of what the upper/lower combo is between two setups, if the drive ratio between the crank and blower are the same, blower speed would be identical at all rpm's. Someone correct me if I'm wrong.

 

[einehund]

I don't know why, but i'm not complaining. I've got the 2.6 upper and 10%IW lower and i only made 526 hp but 566 tq (on a stingy ass mustang dyno). The torque down low was immediately noticable vs when i put the smaller upper on.

 

[Bad Company]

But that's just guessing that those two combos have the same drive ratio. Regardless of what the upper/lower combo is between two setups, if the drive ratio between the crank and blower are the same, blower speed would be identical at all rpm's. Someone correct me if I'm wrong.

You're 100% correct.

Now here is the only thing that I can think of that would make a difference if SC RPM stay the same for both pulley combinations, with the exact same drive ratios on the same engine.

1) both the lower and upper pulleys would be larger in diameter with an OD lower and a larger diameter upper for the same Supercharger pulley drive ratio

2)with both pulleys being larger in diameter for the same drive ratio. Each pulley has more of the SC belt surface area to contact around the circumference of both pulleys.

3)with each pulley having more surface contact around the circumference of each pulley with the drive belt. The chance of belt slippage goes down significantly. Especially while trying to accelerate the engine at full throttle in the lower gears of the transmission. Where the engine's RPM rate of acceleration per second increases significantly. The less SC belt slippage as the engine RPM sweeps through the engine RPM range, the more Hp and torque produced for the same exact SC pulley drive ratio.

 

[Black Cobra '99]

You're 100% correct.

Now here is the only thing that I can think of that would make a difference if SC RPM stay the same for both pulley combinations, with the exact same drive ratios on the same engine.

1) both the lower and upper pulleys would be larger in diameter with an OD lower and a larger diameter upper for the same Supercharger pulley drive ratio

2)with both pulleys being larger in diameter for the same drive ratio. Each pulley has more of the SC belt surface area to contact around the circumference of both pulleys.

3)with each pulley having more surface contact around the circumference of each pulley with the drive belt. The chance of belt slippage goes down significantly. Especially while trying to accelerate the engine at full throttle in the lower gears of the transmission. Where the engine's RPM rate of acceleration per second increases significantly. The less SC belt slippage as the engine RPM sweeps through the engine RPM range, the more Hp and torque produced for the same exact SC pulley drive ratio.

Interesting point. However, thats only if the belt slips with the first combination.
If you do have belt slippage, changing the lower and upper pulley to solve it isn't a smart move.

 

[HillbillyHotRod]

Interesting point. However, thats only if the belt slips with the first combination.
If you do have belt slippage, changing the lower and upper pulley to solve it isn't a smart move.

Why?

 

[Black Cobra '99]

Why?

There is less expensive and easier ways to reduce belt slip.

 

[Bad Company]

Interesting point. However, thats only if the belt slips with the first combination.
If you do have belt slippage, changing the lower and upper pulley to solve it isn't a smart move.

Yes it is an issue. Yes it would be better to change the belt tensionor. But the real problem comes into play with people trying to use a 2.4" or smaller upper on a TVS, versus adding a larger diameter lower

But here is what the OP is truly trying to understand. Why does the larger diameter lower add torque? Well then another poster asked if pulley drive ratios stayed exactly the same why would there be a difference? My answer never deviled in the OP question

The only difference in this second question is truly the amount of circumference area on the belt as it travels around each components pulleys. The drive ratio determines SC RPM, which determines airflow.

If drive ratios are equal regardless of pulley size then SC speeds are equal throughout the complete RPM range of the engine. If SC speeds are equal then ariflow exiting the SC at all RPM of engine speed are equal. The only variable left is belt slippage.

As the engine's RPM rate of acceleration increases, so does the chance for the belt to slip.

An engine accelerating at a rate of 1000 RPM per second versus and engine accelerating at 5000 RPM per second. Which would be more prone to slip the belt of a supercharger?

Remember the SC has mass that must be accelerated in the exact same drive ratio versus engine RPM for the belt not to slip. I did a math calculation based off of a Kenne Bell 3.6L LC SC using their own parasitic Hp losses to determine Hp usage. If my memory serves me correctly, the Hp draw to turn that SC was 271Hp at 6500 engine RPM. That is a huge amount of power for the 10 rib SC belt to transfer from the crankshaft to the SC input shaft without slippage. The bigger the pulley diameter, the less chance of belt slippage. No matter what SC you're trying to spin up, as engine RPM increases at a fast rate of speed.

 

[Bad Company]

So in theory, would a 15% lower and 2.5 upper make more tq than a 10% lower and 2.4 upper? Similar boost but different size lower. I'm just feeding the thread, I assume tuning can add tq/hp to whichever combo at different rpms.

Joewee

You have to do a drive ratio calculation to answer this question

Now the big question is what manufacturer's lower are you using in the calculation?

IW and STI diameter of their products. For some unknown reason each has different diameters listed for dampers for the GT500 engines.

Stock Diameter

IW = 7.225" diameter

STI = 7.5" diameter

IW and STI list 2 different damper diameters for the 10% OD lower.

IW = 7.95" diameter

STI = 7.81" diameter

Yet when it comes to the 15% OD dampers, both companies have very close to the same diameters listed for their products.

ATI = 8.13" diameter

STI = 8.12" diameter


So we have quite a few different math calculations to perform to see what each pulley combination does to the drive ratio of the SC versus the engine, depending on which brand of lower pulley that you'd purchase.

Lets do the math using IW as your choice for the lowers in this first calculations

15% lower with a 2.5" upper is

8.13 divided by 2.5 = 3.252 drive ratio

10% lower with a 2.4" upper is

7.95 divided by 2.4 =3.3125 drive ratio

So the larger diameters between these two would actually being spinning the SC slower

Always multiply the drive ratio number by engine RPM to achieve SC RPM for the airflow exiting the SC, if everything was at 100% efficiency as far as airflow was concerned

Now lets do this with the STI dampers and the different diameters listed by STI for each

15% lower with a 2.5" upper

8.12 divided by 2.5 = 3.248 drive ratio

10% lower with a 2.4" upper

7.81 divided by 2.4 = 3.25 drive ratio

These two STI lower and upper pulley combinations have nearly identical drive ratios. The 10% lower with the 2.4" upper would yield a very slight advantage in airflow due to the drive ratio of 3.25 versus 3.248. Yet the STI 15% lower with the 2.5 upper pulley with its much larger diameter pulley combination would result in a lot less chance of belt slippage as the engine is being accelerated at a fast rate of speed per second.

Drive ratio is how you determine exactly how fast you're trying to spin the SC. It is something everybody should learn how to calculate as you modify these engines. It is the only way to determine if you've achieved maximum RPM for the SC with a given pulley combination versus what engine RPM you are spinning the engine.

A good example is this.

A STI 10% lower with a 2.4" upper has a drive ratio of 3.25

To determine SC RPM you multiply engine RPM by the drive ratio to determine SC RPM

So you've modified the engine and are turning it 7000 RPM. At 7000 RPM what is the SC RPM?


7000 x 3.25 = 22,750

This is how fast the SC is turning with that pulley combination at 7000 engine RPM

 

[sflrainmaker01]

This kinda reminds me of riding a 10 speed bicycle. Gears 1-5 use the smaller "crank" (or pedal) cogs (kinda like the stock balancer on a car engine) and 5 gears in the rear (like the SC pulley of choice). Of course, gears 6-10 use the bigger pedal crank or cog but the same rear gears (cogs). First gear is the SMALLER crank cog where the pedals are and the biggest rear cog (like the stock balancer and the stock or other pulley of choice that is consistant). Shift the lever to change the smaller cog on the pedal crank to the bigger pedal cog (like a 10 or 15 % overdrive balancer) and now it becomes 6th gear. A little harder to spin at first but it can give more power at a higher speed. I'm not sure if this analogy makes sense to any of you all, but it helps me to understand the concept of the upper and lower pulley combinations out there.