[ScorpionT]

I decided to make a thread relating to AR Designs, regarding the backpressure myths and misinformation spreading on forums.

I have been working with performance cars for quite awhile and I have a huge amount of experience with both N/A and forced induction motors. Back pressure for torque is 100% myth, there is no solid foundation for the idea to exist.

Exhaust [B]velocity[B] is what helps torque. The quicker a motor can evacuate the cylinders after the valves open, the quicker torque will be produced. A smaller exhaust runner diameter will give higher velocity, but also be restrictive at high RPMs. The same principle applies to turbo manifold runners, but I will leave that out for now.

One simple way think of velocity is with a pipe and water source. If 1 gallon per second is moving through a 2" pipe, the velocity and pressure will be high. Step up to a 4" pipe and velocity and pressure will drop.

Naturally aspirated motors have exhaust manifolds designed to be a compromise for many operating ranges. Runners must be small enough to provide sufficient velocity at low RPMs, but not be too small that they become a restriction at high RPMs.

Turbochargers do not have the same principles applied to the downpipes. A turbo should essentially have a path with the least amount of restriction approx 1" after the outlet of the turbine housing. The quicker a turbocharger can spool, the sooner torque will be made, and more torque can be potentially made. Any restriction on the path of the exhaust flow will impede on potential torque.

On way this can be understood is with a pinwheel. If you blow at a pinwheel from one direction, it should spin rather well. If you face into a strong breeze while blowing at this same pinwheel, you wont have the same luck. It will require more energy to get it spinning. The same goes for turbochargers. The more the air outlet path is blocked, the more energy and time will be required to get it spooled. This means torque later in the powerband, and less of it.

I hope this helps clear up things for everyone.

[FedeM]

Well done.

[boostd92]

Back-pressure is useful to make torque in both NA and FI cars that have valve overlap at low rpm... but ultimately restrictive on the top end.

What the OP has completely missed in a generic sense (as the N54 has VANOS and may or may not have overlap at lower rpm), is that traditionally, performance cars have a period of valve overlap where the exhaust valve is closing (but not closed) while the intake valve is opening and filling the cylinder with the new fresh air charge.

At low rpm, there is nothing keeping (some of) the fresh air charge from escaping out the closing exhaust valve. When this happens you have less than ideal cylinder fill, and less torque at low rpm. Having smaller pipes or restrictive catalytic converters can create a backpressure that keeps the fresh air charge from exiting the closing exhaust valve as easily. This can be felt as more low-end torque as the cylinder is filled more fully with fresh air.

At high rpm all you would feel is the restriction of torque by the cats/small pipes as the engine is working efficiently enough to fill the cylinders and then some (intake manifold positive pressure).

This works two ways on a turbocharged car.... the low rpm backpressure helps create engine "load" as better cylinder fill makes for higher energy exhaust gases, which would theoretically feel like faster spool up. However, the backpressure works against spool as the turbocharge can not spin up as freely as its pushing against this backpressure.

This conversation may or may not be moot depending on how restrictive a turbo(s) exhaust side is to begin with....

[Clap135]

On na engines it all depends where you want to make the tq. Going with a less restrictive exhaust will make more tq higher up but will loose tq down low and vice versa. On turbo car there's a saying no exhaust is the best exhaust however depending on turbo size there is a point where it simply won't make any more power going bigger....however it won't loose power either. Going with huge pipes also messes with the way the ecu controls boost in some cars and results in boost creep. However that is a tuning issue.

[mycoupe]

Originally Posted by boostd92

Back-pressure is useful to make torque in both NA and FI cars that have valve overlap at low rpm... but ultimately restrictive on the top end.

What the OP has completely missed in a generic sense (as the N54 has VANOS and may or may not have overlap at lower rpm), is that traditionally, performance cars have a period of valve overlap where the exhaust valve is closing (but not closed) while the intake valve is opening and filling the cylinder with the new fresh air charge.

At low rpm, there is nothing keeping (some of) the fresh air charge from escaping out the closing exhaust valve. When this happens you have less than ideal cylinder fill, and less torque at low rpm. Having smaller pipes or restrictive catalytic converters can create a backpressure that keeps the fresh air charge from exiting the closing exhaust valve as easily. This can be felt as more low-end torque as the cylinder is filled more fully with fresh air.

At high rpm all you would feel is the restriction of torque by the cats/small pipes as the engine is working efficiently enough to fill the cylinders and then some (intake manifold positive pressure).

This works two ways on a turbocharged car.... the low rpm backpressure helps create engine "load" as better cylinder fill makes for higher energy exhaust gases, which would theoretically feel like faster spool up. However, the backpressure works against spool as the turbocharge can not spin up as freely as its pushing against this backpressure.

This conversation may or may not be moot depending on how restrictive a turbo(s) exhaust side is to begin with....

You should stop...the bolded sections are inaccurate. Valve overlap is desirable specifically for the purpose of some charge air leaving through the exhaust valves. It's called scavenging. By inducing that small bit of overlap, it speeds up the intake air flow and actually makes the cylinders fill more fully. Obviously this can be a negative effect if the valve overlap occurs for too long a duration, but that would come down to reindexing your cams or choosing a different set of cams. As for the back pressure being useful...that is absolutely false of FI cars, and what you would be more likely to make power off of in an NA car would be anti reversion pipes...which are basically larger diameter sections of pipe that are specifically placed in the exhaust at tuned lengths and distances in order to keep the exhaust gas velocity up. Hope this helps.

And your last paragraph...turbo cars have less overlap because they have turbos smashing air into the cylinder head already. And because of this, the faster you can get the exhaust gases out of the turbo and into the dp's, the faster you can spin them, and the faster you can cram more air into the cylinders for the proceeding combustion. Hence, back pressure = BAD for FI!!!!

[andrew20195]

Originally Posted by boostd92

Back-pressure is useful to make torque in both NA and FI cars that have valve overlap at low rpm... but ultimately restrictive on the top end.

What the OP has completely missed in a generic sense (as the N54 has VANOS and may or may not have overlap at lower rpm), is that traditionally, performance cars have a period of valve overlap where the exhaust valve is closing (but not closed) while the intake valve is opening and filling the cylinder with the new fresh air charge.

At low rpm, there is nothing keeping (some of) the fresh air charge from escaping out the closing exhaust valve. When this happens you have less than ideal cylinder fill, and less torque at low rpm. Having smaller pipes or restrictive catalytic converters can create a backpressure that keeps the fresh air charge from exiting the closing exhaust valve as easily. This can be felt as more low-end torque as the cylinder is filled more fully with fresh air.

At high rpm all you would feel is the restriction of torque by the cats/small pipes as the engine is working efficiently enough to fill the cylinders and then some (intake manifold positive pressure).

This works two ways on a turbocharged car.... the low rpm backpressure helps create engine "load" as better cylinder fill makes for higher energy exhaust gases, which would theoretically feel like faster spool up. However, the backpressure works against spool as the turbocharge can not spin up as freely as its pushing against this backpressure.

This conversation may or may not be moot depending on how restrictive a turbo(s) exhaust side is to begin with....

You are completely wrong concerning turbo cars, and somewhat right for the wrong reasons concerning NA cars. Turbo spool up with regard to exhaust plumbing is primarily determined by the pressure differential across the turbine. If you want better spool at lower rpm, you can either decrease pressure in the downpipe, or increase pressure in the exhaust manifold. Increasing pressure in the manifold hurts higher RPM performance, while reducing pressure in the downpipe continues to help at higher RPM.

On NA engines, the effect is almost entirely due to pressure wave tuning. Smaller/longer header primaries/secondaries line up low pressure pulses with valve events at low RPM, while larger/shorter pipes time the low pressure pulses better at higher RPM. During the valve overlap period, a well timed low pressure pulse in the header primary will cause the cylinder pressure to drop further below what piston action would naturally cause, which will allow more air from the intake to fill the cylinder. This pressure wave tuning also works for the intake manifold. The intake is not inherently higher pressure at high RPM as you suggest, rather at the higher flow rates, the pressure is lower.

[boostd92]

Originally Posted by mycoupe

By inducing that small bit of overlap, it speeds up the intake air flow and actually makes the cylinders fill more fully. Obviously this can be a negative effect if the valve overlap occurs for too long a duration, but that would come down to reindexing your cams or choosing a different set of cams. As for the back pressure being useful...that is absolutely false of FI cars

I think we're focusing on two different parts of the powerband. I'm talking about the time period BEFORE you make positive pressure (low rpm, no boost). Certainly "no exhaust" will spool the turbo quicker, but the torque before you go into positive pressure can be felt more with a functioning exhaust, at least in my experience (I have had at least a dozen turbo cars), as well as multiple people on these forums and others.

And your last paragraph...turbo cars have less overlap because they have turbos smashing air into the cylinder head already. And because of this, the faster you can get the exhaust gases out of the turbo and into the dp's, the faster you can spin them, and the faster you can cram more air into the cylinders for the proceeding combustion. Hence, back pressure = BAD for FI!!!!

I understand this.. and it certainly makes sense in positive pressure situations. But just to be argumentative .. check out some big Supra spec sheets, the 1000hp+ guys run plenty of overlap on their cams and don't make significant torque until 5,000rpm... yes the vast majority of that is due to turbo A/R, but simple cam swaps will show the torque curve move left as overlap decreases.

[Joshboody]

Originally Posted by mycoupe

You should stop...the bolded sections are inaccurate. Valve overlap is desirable specifically for the purpose of some charge air leaving through the exhaust valves. It's called scavenging. By inducing that small bit of overlap, it speeds up the intake air flow and actually makes the cylinders fill more fully. Obviously this can be a negative effect if the valve overlap occurs for too long a duration, but that would come down to reindexing your cams or choosing a different set of cams. As for the back pressure being useful...that is absolutely false of FI cars, and what you would be more likely to make power off of in an NA car would be anti reversion pipes...which are basically larger diameter sections of pipe that are specifically placed in the exhaust at tuned lengths and distances in order to keep the exhaust gas velocity up. Hope this helps.

And your last paragraph...turbo cars have less overlap because they have turbos smashing air into the cylinder head already. And because of this, the faster you can get the exhaust gases out of the turbo and into the dp's, the faster you can spin them, and the faster you can cram more air into the cylinders for the proceeding combustion. Hence, back pressure = BAD for FI!!!!

During light throttle the overlap will pull in exhaust gas for an EGR effect. But I guess you are mainly discussing WOT.

[Brey335i]

Wow. A very technical thread to confirm what we already know. 3" pipes make a little more power than 2.5" pipes....

[ar design]

http://www.modified.com/tech/modp-11...ech/index.html

[andrew20195]

Originally Posted by boostd92

I think we're focusing on two different parts of the powerband. I'm talking about the time period BEFORE you make positive pressure (low rpm, no boost). Certainly "no exhaust" will spool the turbo quicker, but the torque before you go into positive pressure can be felt more with a functioning exhaust, at least in my experience (I have had at least a dozen turbo cars), as well as multiple people on these forums and others.

So you're talking about normally aspirated engines with poorly designed exhaust systems, which is basically what a turbo engine is before it makes boost. There is some debate as to whether exhaust pulse tuning is possible or effective with turbo systems. It's something that would be very difficult to model, and most people with turbo street cars aren't really interested in making them perform better before boost. If it is effective, a simple open pipe of the proper length will have virtually zero backpressure, but still allow the exhaust pulse effect to help fill the cylinders at low rpm.

I understand this.. and it certainly makes sense in positive pressure situations. But just to be argumentative .. check out some big Supra spec sheets, the 1000hp+ guys run plenty of overlap on their cams and don't make significant torque until 5,000rpm... yes the vast majority of that is due to turbo A/R, but simple cam swaps will show the torque curve move left as overlap decreases.

Yes, of course running less overlap will lower the RPM at which the NA torque peak occurs. That's why most people building turbo engines for the street will use short duration cams and rely upon boost to fill the cylinders.

[ScorpionT]

Originally Posted by boostd92

Back-pressure is useful to make torque in both NA and FI cars that have valve overlap at low rpm... but ultimately restrictive on the top end.

What the OP has completely missed in a generic sense (as the N54 has VANOS and may or may not have overlap at lower rpm), is that traditionally, performance cars have a period of valve overlap where the exhaust valve is closing (but not closed) while the intake valve is opening and filling the cylinder with the new fresh air charge.

At low rpm, there is nothing keeping (some of) the fresh air charge from escaping out the closing exhaust valve. When this happens you have less than ideal cylinder fill, and less torque at low rpm. Having smaller pipes or restrictive catalytic converters can create a backpressure that keeps the fresh air charge from exiting the closing exhaust valve as easily. This can be felt as more low-end torque as the cylinder is filled more fully with fresh air.

At high rpm all you would feel is the restriction of torque by the cats/small pipes as the engine is working efficiently enough to fill the cylinders and then some (intake manifold positive pressure).

This works two ways on a turbocharged car.... the low rpm backpressure helps create engine "load" as better cylinder fill makes for higher energy exhaust gases, which would theoretically feel like faster spool up. However, the backpressure works against spool as the turbocharge can not spin up as freely as its pushing against this backpressure.

This conversation may or may not be moot depending on how restrictive a turbo(s) exhaust side is to begin with....

Some level of overlap is found on every motor, even turbocharged cars, because positive intake manifold pressure isnt high at low RPMs. At low RPMs the intake pressure and exhaust velocity are low, so losing some of the fresh air through the exhaust valve is not an issue. Backpressure will only hinder spool, and as I said the quicker a turbo spools the sooner torque can be made.

The bolded part is completely correct, and exactly why the lowest backpressure possible is ideal.

Originally Posted by Clap135

On na engines it all depends where you want to make the tq. Going with a less restrictive exhaust will make more tq higher up but will loose tq down low and vice versa. On turbo car there's a saying no exhaust is the best exhaust however depending on turbo size there is a point where it simply won't make any more power going bigger....however it won't loose power either. Going with huge pipes also messes with the way the ecu controls boost in some cars and results in boost creep. However that is a tuning issue.

Correct all but for one part. You dont want a restrictive exhaust, but rather one that will keep exhaust gas velocity high. Large primaries and a very dense catalyst could be seen as restrictive, but exhaust gas velocity would be low and therefor torque would suffer.

Originally Posted by boostd92

I think we're focusing on two different parts of the powerband. I'm talking about the time period BEFORE you make positive pressure (low rpm, no boost). Certainly "no exhaust" will spool the turbo quicker, but the torque before you go into positive pressure can be felt more with a functioning exhaust, at least in my experience (I have had at least a dozen turbo cars), as well as multiple people on these forums and others.



I understand this.. and it certainly makes sense in positive pressure situations. But just to be argumentative .. check out some big Supra spec sheets, the 1000hp+ guys run plenty of overlap on their cams and don't make significant torque until 5,000rpm... yes the vast majority of that is due to turbo A/R, but simple cam swaps will show the torque curve move left as overlap decreases.

On a turbocharged car you want positive torque as soon as possible. Compare any N/A motor with a turbocharged variant. Torque on the N/A motor might happen sooner, but it you wont have nearly as much. With the turbocharged motor, you arent relying on exhaust velocity to be high to get torque, you are relying on the turbocharger to spin up as quickly as possible. Remember, turbo spool raises torque at an exponential rate (until a certain point) and this is exactly what you want.

What you are talking about is sizing the turbo manifold runners properly to achieve good spool while maintaining the ability to make good peak power. This is similar to sizing the primaries on an N/A header.

The cam swap you are talking about isnt really related to overlap. Single cam motors need a cam swap to change the overlap. A dual cam motor overlap is not so much a factor of swapping cams, but rather degreeing them in relation to each other. Cams are swapped to change the lift and duration, this is whats going to have the most effect.

The idea of modifying a turbocharged motor like an N/A motor would be modified is completely backwards. You want the least restriction post turbine to get the turbo spooled as quickly as possible. Something to think about: you are trying to achieve a slight bump in torque a matter of a few hundred RPM before the turbo spools, and in doing so you are placing a greater restriction on overall torque and horsepower potential. In your thinking, you might gain 20ft-lb for 200rpm, but you lose 50ft-lb and 50hp up top for 500-1000rpm. So are you really gaining anything by that low end torque, or is it really a big loss?

[Forcefed3]

Originally Posted by boostd92

Back-pressure is useful to make torque in both NA and FI cars that have valve overlap at low rpm... but ultimately restrictive on the top end.

What the OP has completely missed in a generic sense (as the N54 has VANOS and may or may not have overlap at lower rpm), is that traditionally, performance cars have a period of valve overlap where the exhaust valve is closing (but not closed) while the intake valve is opening and filling the cylinder with the new fresh air charge.

At low rpm, there is nothing keeping (some of) the fresh air charge from escaping out the closing exhaust valve. When this happens you have less than ideal cylinder fill, and less torque at low rpm. Having smaller pipes or restrictive catalytic converters can create a backpressure that keeps the fresh air charge from exiting the closing exhaust valve as easily. This can be felt as more low-end torque as the cylinder is filled more fully with fresh air.

At high rpm all you would feel is the restriction of torque by the cats/small pipes as the engine is working efficiently enough to fill the cylinders and then some (intake manifold positive pressure).

This works two ways on a turbocharged car.... the low rpm backpressure helps create engine "load" as better cylinder fill makes for higher energy exhaust gases, which would theoretically feel like faster spool up. However, the backpressure works against spool as the turbocharge can not spin up as freely as its pushing against this backpressure.

This conversation may or may not be moot depending on how restrictive a turbo(s) exhaust side is to begin with....

Incorrect, you dont need back pressure, just velocity.

[BerkTechnology]

Originally Posted by gbreeE90

Incorrect, you dont need back pressure, just velocity.

DING DING! We have a winner!

[boostd92]

Originally Posted by ScorpionT

In your thinking, you might gain 20ft-lb for 200rpm, but you lose 50ft-lb and 50hp up top for 500-1000rpm. So are you really gaining anything by that low end torque, or is it really a big loss?

Gah.. I wasn't RECOMMENDING restriction, I was only trying to explain why some people feel a loss of torque before the turbo starts spooling when going to a huge exhaust (or no exhaust). Of course the gains from 2000rpm+ outweigh the measly amount of torque lost below that.

I'm sorry some people read my explanation and immediately went to "ZOMG! No exhaust is best! Don't you read the internetz???!" That was never my intention.

[marcvtec]

Originally Posted by ar design

http://www.modified.com/tech/modp-11...ech/index.html

out of this www address:

While many turbo cars out there use 3-inch exhausts, we've proven that a hefty amount of horses are left in the stable when doing so. In fact, with just the 3.5-inch rear section, 20-whp gains started as low as at the 300-whp level and kept climbing. With PHR's downpipe, those gains mattered more at 500 whp.

Seems when more power more bigger pipes, to each thier own on their application