Why does an engine require backpressure to develop torque/hp properly.
Thread Starter
Drifting
Joined: May 2001
Posts: 1,726
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From: COCOA Florida
Why does an engine require backpressure to develop torque/hp properly.
Can someone explain the process of the exhaust backpressure and its advantage/adverse effect on the engine/torque/horsepower. Overheard a discussion at work today and there does not seem to be a consensus of how much backpressure is needed or at all. I believe that some is needed for a variety of reasons but don't know what they are...thanks in advance for the education. Tom
Resident moron
Joined: Jun 2003
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From: Assachusetts
Re: Why does an engine require backpressure to develop torque/hp properly. (C4TOMCAT)
You pretty much will always make more power with open exhaust (no backpressure). I think backpressure == power is a huge myth. Backpressure is a restriction. Restriction == loss of power.
Just look at any sort of race car. Open exhaust, all of them (unless noise regulations affect them).
Dope
Just look at any sort of race car. Open exhaust, all of them (unless noise regulations affect them).
Dope
Team Owner
Joined: Mar 2001
Posts: 25,889
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From: Virginia Beach
CI 3-5-6-7-8 Veteran
Re: Why does an engine require backpressure to develop torque/hp properly. (Dope)
Backpressure helps the air/fuel charge "swirl" better and will develop more low-end torque. Adding straight pipes to most anything will increase horsepower, but at the cost of low end grunt.
For example a 96-98 Cobra had a 4.6 DOHC w/ "imrc" plactes (Intake Manifold Restriction Cover). Basically below 3200 rpms they blocked one of the intake runners effectively making it a 2v motor and after 3200 it opened up and ran as a full 4v motor. In 99 they ditched this method but developed "tumble port" heads to preservce the low end.
For example a 96-98 Cobra had a 4.6 DOHC w/ "imrc" plactes (Intake Manifold Restriction Cover). Basically below 3200 rpms they blocked one of the intake runners effectively making it a 2v motor and after 3200 it opened up and ran as a full 4v motor. In 99 they ditched this method but developed "tumble port" heads to preservce the low end.
Racer
Joined: Jul 2001
Posts: 386
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From: Round Rock, Texas, USA
Re: Why does an engine require backpressure to develop torque/hp properly. (C4TOMCAT)
I am certainly no expert, but I know that Judson Massingill had the following to say in an interview with GMHighTech
"anybody that tells you an engine requires backpressure from the muffler to make power is probably trying to pedal an inefficient muffler. There is no such thing as too much muffler flow but there sure is a problem with too little."
I installed some cutouts the next day.
"anybody that tells you an engine requires backpressure from the muffler to make power is probably trying to pedal an inefficient muffler. There is no such thing as too much muffler flow but there sure is a problem with too little."
I installed some cutouts the next day.
Race Director
Joined: Oct 2002
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From: Gone
CI 6-7-8-9-10 Veteran
St. Jude Donor '03
Re: Why does an engine require backpressure to develop torque/hp properly. (C4TOMCAT)
It's a myth.
If backpressure was needed, headers wouldn't provide the susbtantial gains they do. Headers work on the principle of scavenging, or actually using and tuning the expelled gasses to create a vacuum effect and help suck the exhaust gasses out of the cylinder instead of it just being forced out by the upward piston movement alone...pushing that exhaust into a restriction after the headers isn't at all beneficial in any application.
As far as loss of low end torque goes...this has more to do with the header tube dia and design and too large of tubing can lessen the scavenge effect at lower rpm's and thus cause less of a power increase. This is not to be confused with backpressure though.
If backpressure was needed, headers wouldn't provide the susbtantial gains they do. Headers work on the principle of scavenging, or actually using and tuning the expelled gasses to create a vacuum effect and help suck the exhaust gasses out of the cylinder instead of it just being forced out by the upward piston movement alone...pushing that exhaust into a restriction after the headers isn't at all beneficial in any application.
As far as loss of low end torque goes...this has more to do with the header tube dia and design and too large of tubing can lessen the scavenge effect at lower rpm's and thus cause less of a power increase. This is not to be confused with backpressure though.
Drifting
Joined: Jun 2004
Posts: 1,627
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From: Goodyear Az
Re: Why does an engine require backpressure to develop torque/hp properly. (Cobra4B)
Backpressure helps the air/fuel charge "swirl" better and will develop more low-end torque.
For example a 96-98 Cobra had a 4.6 DOHC w/ "imrc" plactes (Intake Manifold Restriction Cover).
Racer
Joined: Sep 2002
Posts: 424
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From: Va, Beach VA
Re: Why does an engine require backpressure to develop torque/hp properly. (Bink)
Then look at it this way.....most vets stock seem to have higher torque than hp.but after some mods in the exhaust area specifically hp goes up and torque lags behind. Granted most mods increase are in the higher than stock
rpm range and all of this may tie into where the torque and hp intersect(5200rpm) where torque has peaked and hp is still climbing. Anymore input on this???.....interesting
[Modified by Rays Vet, 12:40 AM 7/1/2004]
rpm range and all of this may tie into where the torque and hp intersect(5200rpm) where torque has peaked and hp is still climbing. Anymore input on this???.....interesting
[Modified by Rays Vet, 12:40 AM 7/1/2004]
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7th Gear
Joined: Jun 2004
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Re: Why does an engine require backpressure to develop torque/hp properly. (C4TOMCAT)
Need for backpressure is a myth. Any engine will have higher volumetric efficiency at any rpm with less backpressure assuming spark lead and mixture are properly optimized. Less backpressure can result in better cylinder scavenging which can lean out the mixture and result in power losses if not corrected for. In the old carburetor days people would add headers, not rejet the carb, lean out the mixture, and burn exhaust valves. The myth then was that headers burn exhaust valves. That wasn't the root cause of the problem.
Speaking of headers, adding headers adds exhaust negative pressure tuning to better scavenge at higher rpms; usually somewhere between torque and power peaks. At lower rpm ranges, the headers are out of tune and can increase backpressure there resulting in torque losses. This may also be part of the backpressure is good myth when you replace stock manifolds with headers to 'reduce backpressure'. You're not just reducing backpressure, your adding tuning optimized for higher rpms so you see a loss at lower rpms.
Speaking of headers, adding headers adds exhaust negative pressure tuning to better scavenge at higher rpms; usually somewhere between torque and power peaks. At lower rpm ranges, the headers are out of tune and can increase backpressure there resulting in torque losses. This may also be part of the backpressure is good myth when you replace stock manifolds with headers to 'reduce backpressure'. You're not just reducing backpressure, your adding tuning optimized for higher rpms so you see a loss at lower rpms.
Racer
Joined: Sep 2002
Posts: 424
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From: Va, Beach VA
Re: Why does an engine require backpressure to develop torque/hp properly. (rbrown7)
(Quote)Need for backpressure is a myth. Any engine will have higher volumetric efficiency at any rpm with less backpressure assuming spark lead and mixture are properly optimized. Less backpressure can result in better cylinder scavenging which can lean out the mixture and result in power losses if not corrected for. In the old carburetor days people would add headers, not rejet the carb, lean out the mixture, and burn exhaust valves. The myth then was that headers burn exhaust valves. That wasn't the root cause of the problem.
Speaking of headers, adding headers adds exhaust negative pressure tuning to better scavenge at higher rpms; usually somewhere between torque and power peaks. At lower rpm ranges, the headers are out of tune and can increase backpressure there resulting in torque losses. This may also be part of the backpressure is good myth when you replace stock manifolds with headers to 'reduce backpressure'. You're not just reducing backpressure, your adding tuning optimized for higher rpms so you see a loss at lower rpms.
I agree to a point... than why do "stock" ls1/ls6 cars have higher torque #s than hp???
Speaking of headers, adding headers adds exhaust negative pressure tuning to better scavenge at higher rpms; usually somewhere between torque and power peaks. At lower rpm ranges, the headers are out of tune and can increase backpressure there resulting in torque losses. This may also be part of the backpressure is good myth when you replace stock manifolds with headers to 'reduce backpressure'. You're not just reducing backpressure, your adding tuning optimized for higher rpms so you see a loss at lower rpms.
I agree to a point... than why do "stock" ls1/ls6 cars have higher torque #s than hp???
Intermediate
Joined: Jun 2002
Posts: 40
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From: Phoenix Az
Re: Why does an engine require backpressure to develop torque/hp properly. (C4TOMCAT)
This my understanding.....
The exhaust system can be more than a just place to dump hot gases. With proper "tuning" a scavenging effect can be created. The entry of pulses of hot gases will create cyclic high a low pressure waves. The diameter and length of the header runners can be tuned, usually near or at a specific RPM, so that pulses are somewhat in sync in one direction.
Although you may have a small increase in back pressure compared to the measurements of a prior system, which I'm not sure would ever happen, the overall improved performance will more than compensate.
Could use some help from an engineer who has experience tuning exhaust systems on racing engines
LEC
[Modified by 1964VETTE, 8:03 AM 7/1/2004]
The exhaust system can be more than a just place to dump hot gases. With proper "tuning" a scavenging effect can be created. The entry of pulses of hot gases will create cyclic high a low pressure waves. The diameter and length of the header runners can be tuned, usually near or at a specific RPM, so that pulses are somewhat in sync in one direction.
Although you may have a small increase in back pressure compared to the measurements of a prior system, which I'm not sure would ever happen, the overall improved performance will more than compensate.
Could use some help from an engineer who has experience tuning exhaust systems on racing engines
LEC
[Modified by 1964VETTE, 8:03 AM 7/1/2004]
