How does back pressure affect low-end torque?
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How does back pressure affect low-end torque?
I was reading somewhere that the back pressure from the exhaust on an engine helps create torque.
Is this true?
Why does that work? Does it only add to the low-end and take away from high-end?
Does anyone have headers? Any numbers on gains?
thanks
Is this true?
Why does that work? Does it only add to the low-end and take away from high-end?
Does anyone have headers? Any numbers on gains?
thanks
Elder Member
Joined: Mar 2000
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definitely a myth on turbocharged cars
Perhaps it still has some validity on NA cars because companies have been able to make exhausts with the torque hump at different RPMs. Whether that is done by increasing backpressure I don't know.
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Amen. Groundless myth...
Valve overlap is why NA cars need some back pressure for low-end torque.
While it is theoretically possible to have too little backpressure, I have yet to see it on either NA or turbo cars. You simply cannot build a streetable exhaust with a legal cat that doesn't make backpressure.
While it is theoretically possible to have too little backpressure, I have yet to see it on either NA or turbo cars. You simply cannot build a streetable exhaust with a legal cat that doesn't make backpressure.
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Ok, so it's a myth, then couldn't you ....
stick a fan or something in the exhaust to move the air, and effectively reduce as much back pressure as you want?
How much power would you get if you had zero back pressure, even negative back pressure? It would only help the exhaust stroke - but still, that power wasted on the stroke could go to the wheels.
How much power would you get if you had zero back pressure, even negative back pressure? It would only help the exhaust stroke - but still, that power wasted on the stroke could go to the wheels.
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Nope...
No fan is going to pull exhaust faster than expanding hot gas with sonic pressure waves. You may be describing the function of a turbocharger 
You actually don't gain anthing but efficiency. Low backpressure means you get more flow at high rpm and less waste heat buildup at the head. It is just one part of improving flow from the intake to the exhaust tip.
You actually don't gain anthing but efficiency. Low backpressure means you get more flow at high rpm and less waste heat buildup at the head. It is just one part of improving flow from the intake to the exhaust tip.
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it takes lateral thinkers like you to come up with new things
I appreciate putting thought into solving the problem, unlike some people who just go by what they read.
In my opinion though you would have to precisely match the flow of the fan to the natural rate of the exhaust as the RPMs change throttle opening change to make it work. Otherwise you won't flow enough at high RPMs and will draw out unburned charge at low RPMs.
In addition, backpressure will still be caused by restrictions in the exhaust. Drawing from the end will reduce it, because it will reduce pooling of air in spots, but it won't overcome the fact that the air naturally encounters resistance when the exhaust diameter narrows from the exhaust ports to the pipes and then slams into the converters and mufflers.
It might help some though.
In my opinion though you would have to precisely match the flow of the fan to the natural rate of the exhaust as the RPMs change throttle opening change to make it work. Otherwise you won't flow enough at high RPMs and will draw out unburned charge at low RPMs.
In addition, backpressure will still be caused by restrictions in the exhaust. Drawing from the end will reduce it, because it will reduce pooling of air in spots, but it won't overcome the fact that the air naturally encounters resistance when the exhaust diameter narrows from the exhaust ports to the pipes and then slams into the converters and mufflers.
It might help some though.
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thanks, i have plenty more ideas if you want to hear them...
you guys are right.. the exhaust has way too much flow.
I do think it would help the engine a lot. It would reduce the amount of energy spent on the exhaust stroke, which would instead go directly to the crank shaft.
I do think it would help the engine a lot. It would reduce the amount of energy spent on the exhaust stroke, which would instead go directly to the crank shaft.
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Here's the theory.
The piston after TDC is traveling down from the explosion of the gas and air mixture. As it is heading down, the exhaust cylinder must open early to insure that the exhuast gases are exiting before the piston gets to the bottom - or the expanding gases will work to halt the upward movement.
If you reduce back pressure then the exhaust gases will come out quicker and reduce the amount of force on the downward stroke (torque).
Your question about high rpm is valid - having the exhaust gases come out quickly is important as the piston is now traveling at higher velocity and thus getting the exhaust out quickly is important since the piston will be changing direction sooner.
All these things are based on the mfr having designed the car to be close to best performace.
Yes turbros do not apply to this factor since the turbo is between the exhaust system and the exhaust valve. A more open exhuast here is helpful since it allows the turbo to spool faster.
Reggie
98.5 2.8QMS
Hope this clears the matter up.
If you reduce back pressure then the exhaust gases will come out quicker and reduce the amount of force on the downward stroke (torque).
Your question about high rpm is valid - having the exhaust gases come out quickly is important as the piston is now traveling at higher velocity and thus getting the exhaust out quickly is important since the piston will be changing direction sooner.
All these things are based on the mfr having designed the car to be close to best performace.
Yes turbros do not apply to this factor since the turbo is between the exhaust system and the exhaust valve. A more open exhuast here is helpful since it allows the turbo to spool faster.
Reggie
98.5 2.8QMS
Hope this clears the matter up.
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It also has to do with Valve Overlap....
Normally aspirated cars sometimes use a lot of valve overlap - when the intake and exhaust are open at the same time. If there was no backpressure, then the intake charge could fly right out the exhaust valve. Backpressure helps keep the intake charge where it belongs by slowing the escaping exhaust.
It is, of course, a trade-off. Less backpressure means less low-end torque, but better high-rpm hp. You can go too big or too small though.
In general, turbo motors do not run a lot of overlap, and the turbo itself presents more than enough backpressure for the cylinders.
Usually a loss in backpressure helps power accross the whole RPM band in a turbo motor.
It is, of course, a trade-off. Less backpressure means less low-end torque, but better high-rpm hp. You can go too big or too small though.
In general, turbo motors do not run a lot of overlap, and the turbo itself presents more than enough backpressure for the cylinders.
Usually a loss in backpressure helps power accross the whole RPM band in a turbo motor.