Why more torque with lower pulley?

[09Troublemaker]

No, we're saying the myth that a lower pulley adds more torque than an upper pulley is, well, a myth.

It stems from the '03 Cobras and the overspun 112 Eatons. You would add a big lower and only gain torque because they just couldn't make any more HP in the upper RPM.

Not a Myth..... did the pepsi-coke challenge on my 09. forgot all the #'s at the moment (too many beers in me now) but the upper pulley generated X power and the lower generated Y. What was obvious was the lower "by itself" picked up 50 ft lbs of Tq.... as we all know, the upper pulley doesn't come close to these results. and this was with the 10 % I-W

I agree with Van's analogy. :beer:

 

[biminiLX]

I know Kenne Bell and a few other sites give pulley calculators, the blower speed is determined by the pulley ratio and it would be interesting to calculate it based on the common sizes.
That should tell us if say the 10% crank generates more blower speed than a 2.4 upper?
I also think the crank pulley sees a bigger lower end gain as it generates a faster response from engine rpm to blower speed mechanically?
All I know is that I plan on using both in due time
-J

 

[Bad Company]

I'm not the best one to answer this question, but I believe it is simply a matter of the fact that the larger diameter lower pulley is spinning the blower to higher RPM at lower engine RPM. Which creates higher boost at lower engine RPM. Higher boost at lower engine RPM = more torque.

The stock lower is 7.1" in diameter with a 2.71 diameter blower pulley. 7.1 divided by 2.71 = 2.61 This gives you a 2.61 drive ratio. .

2.61Ratio x 4500 Engine RPM =11,745 Blower RPM

Lets say we increase the lower pulley diameter to 8"

8 divided by 2.71 =2.952 drive ratio

2.952 X 4500 Engine RPM = 13,284 blower RPM

Now lets try a stock lower 7.1" diameter pulley with a smaller blower pulley. For ease of installation and not requiring an idler pulley in the equation to keep the belt from slipping we will reduce the blower pulley down to a 2.5" diameter.

7.1 divided by 2.5 = 2.84 drive ratio

2.84 Ratio x 4500 Engine RPM = 12,780 Blower RPM

As can be seen by the math the blower is turning 504 more RPM at 4500 engine RPM with the 8" diameter lower pulley and the stock 2.71" blower pulley, versus the 2.5" upper pulley and the 7.1" stock lower pulley. That is a difference of 1,159.2 Liters of air entering the engine with the 8" lower over the 2.5" upper where peak torque would be produced. Higher blower speeds will generate more boost at the same engine RPM, so power output of the engine will increase.

Can you go down in upper pulley diameter to match the 8" lower? Yes

The 2.4" diameter upper with the 7.1" diameter lower has a slightly faster drive ratio of

7.1" divided by 2.4" = 2.958

2.958 Ratio x 4500 Engine RPM = 13,311 blower RPM

But to install this small of a pulley is now getting into other problems. The blower support may need to be modified to fit the pulley. An Idler pulley should be added to get a higher degree of belt wrap around the smaller diameter upper pulley to counter against belt slippage. You get into more supporting mods with the smaller upper pulley versus the lower for not much gain in blower RPM.

 

[2112]

^^^ Now there is a damn good explanation

Thank you
.

 

[Bad Company]

^^^ Now there is a damn good explanation

Thank you
.

You're welcome

I was surprised that nobody had done the math to see the differences in the use of different upper pulleys versus lower pulleys. So I took a few minutes to run the numbers to verify my thoughts using a stock 2013 GT500 for the baseline and posted my results for everybody else to see.

Now the next part of this question is, why doesn't the Hp increase as much at high RPM with the larger lower with a stock upper on the 2.3L TVS?

Again this has to do with the fact you are turning the blower extremely fast.

8" divided by 2.71" = 2.952

2.952 ratio x 7000 engine RPM = 20,664 blower RPM

Now we are getting to the point of the parasitic Hp loss to turn the blower this fast is getting as high as the the extra Hp generated by the higher boost. So the net Hp gain at the flywheel isn't as high as one would think for the boost being introduced into the engine.

2.3L of air per revolution of the Supercharger x 20,664 RPM = 47,527.2L of air per minute is being ingested by the engine.

Now lets look at a 3.6L SC and the number of RPMs needed for this blower to generate the same amount of air flow entering the engine.

47,527.2L divided by 3.6L = 13,202 RPM for the blower speed.

The 3.6L SC closest pulley combo to match this RPM would need to be a stock 7.1 lower with a 3.75 upper.

7.1 divided by 3.75 = 1.8933 drive ratio

1.8933 x 7000 engine RPM = 13,253.1 Blower RPM.

We are close, but not a perfect match. Unfortunately there isn't a upper/lower combo that would be a perfect match. But in doing this math we see how close we can get to the same air flow from both blowers and the RPM each is turning to achieve it.

Now for the real math I'll use our original RPM numbers to calculate the differences in blower speeds

20,664(2.3L blower RPM) - 13,202(3.6L blower RPM) = 7,462 RPM

This is the difference between these two Superchargers for the same exact amount of air flow entering into the engine. I'm not a mechanical engineer, but it would be easy to see how much more energy(Hp) is required to turn the smaller blower(2.3L) for the same air flow entering the engine at peak Hp with this big of a difference in blower speed. The 3.6L SC is turning 36% slower in RPM for the same air flow when compared to the 2.3L

Again we can also do calculations for the Peak Torque RPM of the engine, to see if the air flow is still close at these lower RPM for a solid torque gain with the larger blower.

3.6L blower math is

1.8933 drive ratio x 4500 Engine RPM = 8,519.85 blower RPM

3.6L of air per revolution x 8,519.85 blower RPM = 30,671.46L of air per minute at peak torque

2.3L blower math is


8" divided by 2.71" = 2.952 ratio

2.952 ratio x 4500 engine RPM = 13,284 blower RPM

2.3L of air per revolution x 13,284 blower RPM = 30,553.2L of air per minute at peak torque.

Both of these blowers again are very close at the peak torque in air flow. But the RPM difference is still high.

13,284 - 8,519.85 = 4,764.15 difference in blower speeds.

Now the question is, are both of these blowers working in there perspective efficiency range as far as RPM and air flow being produced? Because it will be a matter of which SC is doing the job more efficiently then the other for the highest net Hp and torque result at this engine RPM. Again there is a big RPM difference in blower speeds, but it also will be about which SC is more efficient at this target RPM for the air being moved. I'd be willing to bet the 3.6L is starting to lose some efficiency turning this slow, so the peak torque and Hp numbers are going to be closer to each other between these two SC at this RPM. But then that again there is a large difference in blower RPM, so on paper the 3.6L should be ahead by a considerable amount and a slight drop in efficiency shouldn't hurt its final dyno numbers.

Now we could also do this math again for 2500 engine RPM to see how slow we are turning the 3.6. But you've seen how it works and can do it now. The math will be very close to each other again. The difference will be in whether or not the 3.6L SC can be efficient at moving air at this slow of a blower RPM

Yes, I was bored and spent waaaaaaaayyyyyyyyyyyy too much time on this LOL

 

[Lethalchem]

Those explanations are so good even *I* could follow it! :lol:

 

[2112]

Those explanations are so good even *I* could follow it! :lol:

+1

Excellent tutorial, Thank you
.

 

[stangfreak]

Not a Myth..... did the pepsi-coke challenge on my 09. forgot all the #'s at the moment (too many beers in me now) but the upper pulley generated X power and the lower generated Y. What was obvious was the lower "by itself" picked up 50 ft lbs of Tq.... as we all know, the upper pulley doesn't come close to these results. and this was with the 10 % I-W

I agree with Van's analogy. :beer:

How do you explain the cars that have a upper only making lets say 15lbs of boost and a upper lower car making 15lbs of boost and both are making similar power. I have seen it time and time again for 10 years with 03 04 cobras. we basically started this upper pulley lower pulley stuff. Is it different for shelbys? I don't know. But I went through every single pulley combo you can think of on a supercharger.

when jay had his white cobra and 2.9 whipple he was making the same tq as me and I have a 4lb lower and he had a stock lower pulley. Granted my car is a under achiever and makes 20 less hp and tq from other cobras. lower pulley adds boost. more boost, more hp and more tq. Its nothing magical.

 

[1320 Junkie]

How do you explain the cars that have a upper only making lets say 15lbs of boost and a upper lower car making 15lbs of boost and both are making similar power. I have seen it time and time again for 10 years with 03 04 cobras. we basically started this upper pulley lower pulley stuff. Is it different for shelbys? I don't know. But I went through every single pulley combo you can think of on a supercharger.

when jay had his white cobra and 2.9 whipple he was making the same tq as me and I have a 4lb lower and he had a stock lower pulley. Granted my car is a under achiever and makes 20 less hp and tq from other cobras. lower pulley adds boost. more boost, more hp and more tq. Its nothing magical.

ill stick with my myth opinion..ive seen it first hand since 03'. As Alex stated same boost pressures no lower pulley on my car (terminator)...almost same tq...no -50ft/lb on my side...blower rpm is blower rpm.

I however run a 15% on my Shelby for the TVS to achieve higher blower rpms than upper only rpms.

 

[stangfreak]

yup. jay and I also went to the same tuner and same dyno. jays car even with the stock lower was just as violent as a lower pulley car.

we have been playing around with these pulleys for 10 years and jay blwn04svtcobra is not a new comer to this. He has so much experience and knowledge with these cars. Now he is doing it with a better refined car lol. jay, get another terminator dammit!!!! lol

 

[1320 Junkie]

yup. jay and I also went to the same tuner and same dyno. jays car even with the stock lower was just as violent as a lower pulley car.

we have been playing around with these pulleys for 10 years and jay blwn04svtcobra is not a new comer to this. He has so much experience and knowledge with these cars. Now he is doing it with a better refined car lol. jay, get another terminator dammit!!!! lol

I'm actually working on picking up another Sonic Blue Terminator coupe for myself. See what happens at tax time.

 

[Bad Company]

How do you explain the cars that have a upper only making lets say 15lbs of boost and a upper lower car making 15lbs of boost and both are making similar power. I have seen it time and time again for 10 years with 03 04 cobras. we basically started this upper pulley lower pulley stuff. Is it different for shelbys? I don't know. But I went through every single pulley combo you can think of on a supercharger.

when jay had his white cobra and 2.9 whipple he was making the same tq as me and I have a 4lb lower and he had a stock lower pulley. Granted my car is a under achiever and makes 20 less hp and tq from other cobras. lower pulley adds boost. more boost, more hp and more tq. Its nothing magical.

You have to remember nothing is equal between each car. There are plus and minus variables in every part in each car from the manufacturer.

One of the biggest differences I've seen is exhaust systems. A slightly freer flowing exhaust will allow a car to build more power with less boost pressure.

Everything I've posted is the mathematical calculations for blower speeds and the volume of air in Liters for that size blower. It is also what each blower will move into the engine at 100% volumetric efficiency between the inlet and outlet of the SC. So there can be no inlet restrictions in front of each SC.

The next part of your comment is saying to me that you had one brand of blower on your car, while your friend had a 2.9L Whipple. That doesn't take into consideration of the differences in rotor designs and there efficiency. Did you have a 2.9L SC on your car? If not then you had to spin it faster for the same boost pressure. If you were spinning it faster then you were using more Hp to turn your smaller blower for the same air flow, so the net power at the flywheel was less. Were both SC a screw compressor design? If not, then the efficiency of each is different. My posts never took into account these differences. I was only trying to compare how much air in volume you could move by each modification in pulley changes. I used the reference with the 3.6L SC to show again how much less SC RPM you had to turn that SC for the same air in volume entering the engine. So people could then see the parasitic lose of spinning the smaller SC for the same Liters in air entering the engine. I never said anything about the efficiency of rotor design in my posts and the possible effects on Hp.

The Roots Supercharger versus the Screw Compressor Supercharger will cause the inlet air temperatures of the Screw Compressor to be lower at the same boost pressure entering the engine. In this case 15 psi of boost isn't equal. The Engine Control Module will see that the air entering the engine from the Roots SC is hotter then the air from the Screw Compressor SC. The ECM of the Roots SC engine will restrict fuel by volume, because it knows there is less oxygen in that same 15 psi of boost entering the engine, because it is hotter. Hotter air is less dense and carries less oxygen. The ECM will see the lower oxygen content of the air in the O2 sensor readings of the Roots SC engine and adjust fuel accordingly. If it means that your engine was getting less fuel than your buddies at 15 psi of boost than you had less power than he did. If he had a larger SC in Liters of air displacement then he needed to turn less SC RPM to achieve the 15psi of boost then you did. Then he also has less parasitic loss in Hp used to turn the SC. This means he has more net Hp at the flywheel then you do.

Can you see that 15 psi of boost isn't equal for the same engines now?

 

[Bad Company]

+1

Excellent tutorial, Thank you
.

I'm really not the one to be giving tutorials on this subject. I saw the differences in a mathematical/mechanical form and decided to do the math to see the difference for myself. After doing the math it is very straight forward and easy to understand, so I thought I'd post it.

You're welcome.

 

[1320 Junkie]

You have to remember nothing is equal between each car. There are plus and minus variables in every part in each car from the manufacturer.

One of the biggest differences I've seen is exhaust systems. A slightly freer flowing exhaust will allow a car to build more power with less boost pressure.

Everything I've posted is the mathematical calculations for blower speeds and the volume of air in Liters for that size blower. It is also what each blower will move into the engine at 100% volumetric efficiency between the inlet and outlet of the SC. So there can be no inlet restrictions in front of each SC.

The next part of your comment is saying to me that you had one brand of blower on your car, while your friend had a 2.9L Whipple. That doesn't take into consideration of the differences in rotor designs and there efficiency. Did you have a 2.9L SC on your car? If not then you had to spin it faster for the same boost pressure. If you were spinning it faster then you were using more Hp to turn your smaller blower for the same air flow, so the net power at the flywheel was less. Were both SC a screw compressor design? If not, then the efficiency of each is different. My posts never took into account these differences. I was only trying to compare how much air in volume you could move by each modification in pulley changes. I used the reference with the 3.6L SC to show again how much less SC RPM you had to turn that SC for the same air in volume entering the engine. So people could then see the parasitic lose of spinning the smaller SC for the same Liters in air entering the engine. I never said anything about the efficiency of rotor design in my posts and the possible effects on Hp.

The Roots Supercharger versus the Screw Compressor Supercharger will cause the inlet air temperatures of the Screw Compressor to be lower at the same boost pressure entering the engine. In this case 15 psi of boost isn't equal. The Engine Control Module will see that the air entering the engine from the Roots SC is hotter then the air from the Screw Compressor SC. The ECM of the Roots SC engine will restrict fuel by volume, because it knows there is less oxygen in that same 15 psi of boost entering the engine, because it is hotter. Hotter air is less dense and carries less oxygen. The ECM will see the lower oxygen content of the air in the O2 sensor readings of the Roots SC engine and adjust fuel accordingly. If it means that your engine was getting less fuel than your buddies at 15 psi of boost than you had less power than he did. If he had a larger SC in Liters of air displacement then he needed to turn less SC RPM to achieve the 15psi of boost then you did. Then he also has less parasitic loss in Hp used to turn the SC. This means he has more net Hp at the flywheel then you do.

Can you see that 15 psi of boost isn't equal for the same engines now?

Same 2.9 blowers pretty sure Stangfreak said that....same stock 4.6L motors..both cars had free flowing exhaust. The variable are no different from 4.6 to 5.4 both utilize same pulley configuration. Rotor speed is rotor speed....if a 2.5" upper = 20 psi and a 3.0" + 4lb lower = 20 psi....both at 20k rotor rpms...the car with the lower pulley isnt out tq'ing the upper only...sorry...this is a myth I've seen the myth dispoven time and time again..since 03'

Lower pullies are a tool to achieve less belt slip or gain rotor rpm...that's it. If someone says they gained 50lb/ft tq with a lower pulley over a upper pulley only I would have to say they were expieriencing belt slip no other reason with all things being equal should a lower pulley magically gain tq.

 

[Bad Company]

Same 2.9 blowers pretty sure Stangfreak said that....same stock 4.6L motors..both cars had free flowing exhaust. The variable are no different from 4.6 to 5.4 both utilize same pulley configuration. Rotor speed is rotor speed....if a 2.5" upper = 20 psi and a 3.0" + 4lb lower = 20 psi....both at 20k rotor rpms...the car with the lower pulley isnt out tq'ing the upper only...sorry...this is a myth I've seen the myth dispoven time and time again..since 03'

Lower pullies are a tool to achieve less belt slip or gain rotor rpm...that's it. If someone says they gained 50lb/ft tq with a lower pulley over a upper pulley only I would have to say they were expieriencing belt slip no other reason with all things being equal should a lower pulley magically gain tq.

Somewhere we have had a miscommunication. If you re-read everything I've posted, you will see that I agree with you 100%. I clearly stated that the extra torque from the larger lower was because of the extra volume of air entering the engine, due to higher rotor speeds achieved over a smaller upper pulley change with a stock lower. Which couldn't achieve the same rotor speeds as a larger lower with a stock upper. The math clearly shows what it took to achieve the same volume in air versus SC speeds. Air is air and if you get the same volume of air entering the engine, the net results should be close between two engines.

I misunderstood your comments when I quoted you and posted my last answer. You didn't read my earlier posts thoroughly or I believe you would have understood that I would have agreed with you. When you say that you can achieve the similar Hp and Torque no matter what pulleys are used, as long as the same amount of air in volume is entering the engine across the complete power band of the engine and everything else with the engines is equal.

 

[19COBRA93]

!

Not a Myth..... did the pepsi-coke challenge on my 09. forgot all the #'s at the moment (too many beers in me now) but the upper pulley generated X power and the lower generated Y. What was obvious was the lower "by itself" picked up 50 ft lbs of Tq.... as we all know, the upper pulley doesn't come close to these results. and this was with the 10 % I-W

I agree with Van's analogy. :beer:

It is absolutely a myth. I think you've forgotten the wrong numbers.

All the supercharger cares about is how fast it's being spun. It doesn't matter whether it's from an upper pulley, or a lower pulley, the blower is being spun to a calculated RPM which makes X amount of HP/TQ. You can change around the pulleys all you want to, but if the calculated blower RPM is the same, the hp/tq will be the same.

 

[03SBSNAKE]

This is not a myth if you think it is then you have never had a car that you did a lower swap on. I have a posi ported blower with a 2.76 upper that made about 480 rwhp and 470rwtq. I added one thing a 4lb lower and my numbers went to 490 rwhp and 530rwtq. This works because your peak torque is in the lower rpm's and a larger lower pulley gets it spinning faster in the lower rpm so you have more boost right when you hit it.

 

[mach330]

This is not a myth if you think it is then you have never had a car that you did a lower swap on. I have a posi ported blower with a 2.76 upper that made about 480 rwhp and 470rwtq. I added one thing a 4lb lower and my numbers went to 490 rwhp and 530rwtq. This works because your peak torque is in the lower rpm's and a larger lower pulley gets it spinning faster in the lower rpm so you have more boost right when you hit it.

:rollseyes

Oversimplified, but...

A 10% larger lower pulley or a 10% smaller upper pulley will do the same thing. They both spin the blower 10% faster, regardless of rpm.

Eventually the blower can't keep up at high rpm and horsepower stops increasing or even decreases. Torque will keep rising because the blower isn't maxed out at 4,000 engine rpm.

 

[Bad Company]

This is not a myth if you think it is then you have never had a car that you did a lower swap on. I have a posi ported blower with a 2.76 upper that made about 480 rwhp and 470rwtq. I added one thing a 4lb lower and my numbers went to 490 rwhp and 530rwtq. This works because your peak torque is in the lower rpm's and a larger lower pulley gets it spinning faster in the lower rpm so you have more boost right when you hit it.

But at the same time you increased your boost at peak Hp at Max Engine RPM with faster supercharger RPM across the complete RPM of the engine. Now you can do the same thing with only changing the upper. WHY???????

Because you can match the blower RPM you achieved with either an upper and lower pulley change. It is strictly about blower RPM and the volume of air exiting the blower at any given RPM.

This whole thread is really about moving air into the engine and nothing more than that.

 

[1320 Junkie]

Somewhere we have had a miscommunication. If you re-read everything I've posted, you will see that I agree with you 100%. I clearly stated that the extra torque from the larger lower was because of the extra volume of air entering the engine, due to higher rotor speeds achieved over a smaller upper pulley change with a stock lower. Which couldn't achieve the same rotor speeds as a larger lower with a stock upper. The math clearly shows what it took to achieve the same volume in air versus SC speeds. Air is air and if you get the same volume of air entering the engine, the net results should be close between two engines.

I misunderstood your comments when I quoted you and posted my last answer. You didn't read my earlier posts thoroughly or I believe you would have understood that I would have agreed with you. When you say that you can achieve the similar Hp and Torque no matter what pulleys are used, as long as the same amount of air in volume is entering the engine across the complete power band of the engine and everything else with the engines is equal.

Wasnt directed directly at u bud...just explaining the explination of how this lower pulley = more tq misguides people :beer: