backpressure = Torque
Tims_240
03-08-2006, 10:55 PM
just thought id pop in and share some info...
Backpressure = Torque?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast moving but free flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of the scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking, power-robbing backpressure. In direct contrast, a well designed, high performance street exhaust system typically has about 2-6 psi of backpressure. For an interesting comparison, an un-muffled straight piped on a real racecar usually has 1-3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance mufflers available today have what is called a straight through design. These mufflers quiet the exhaust by absorbing high frequency vibrations in the heat resistant packing, usually consisting of stainless steel mesh and heat resistant ceramic fibers.
They typically have one inner core that is straight through with no baffling at all, much like a straight pipe with many small holes in it. The pipe is louvered or perforated when it passes inside the muffler's shell, allowing sound energy to pass through the holes but leaving the exhaust gas flow unimpeded. You can see straight through these types of mufflers. The louvered or perforated core is usually wrapped in either fiberglass wadding (hence the old school term, Glass Pack) or, in the better mufflers, stainless steel mesh backed by ceramic fiber to help further absorb the sound.
On straight through mufflers, the longer the muffler the bigger the can, the quieter it is. The length usually has no effect on backpressure, just noise output. These absorption type mufflers work in the same manner as the silencers used on guns. If a silencer had baffles that impeded bullet travel, you would definitely have problems! The same is for a perforated core absorption muffler, straight through, no baffles, no restriction, and no backpressure.
It is best to avoid straight-through mufflers that have a louvered core. Many old school glass packs suffer from this design. Some spiffy polished stainless and big tip mufflers on the market also have these. The louvers generate quite a bit of backpressure because they stick into the exhaust stream and create considerable turbulence. Even though these mufflers are a straight through design, they have more backpressure than a stock muffler.
When buying a straight-through muffler, look for one with a perforated core if you are interested in producing more power. A good, properly sized, perforated core straight-through muffler will add only about 1-2 psi of backpressure to your exhaust system. Mufflers like the Walker Ultra Flow. Thermal A`pexi, , Borla, Edelbrock, or Magnaflow are examples of good, low-backpressure mufflers with an absorption design. Many Pre-made exhausts like A`pexi, Tanabe, Greddy, Borla, Magnaflow, Thermal or HKS also have mufflers of the free-flowing absorption design.
An old school type performance muffler, which is till very common in sped shops, that has seen better days is the Turbo Muffler. This is a less restrictive version of a stock-like reverse-flow muffler. In the old days, these were well flowing mufflers, but now the new jack perforated core, straight through absorption types, has superseded them. Many old school domestic shops will try to sell you one of these as a hot setup, but they should be avoided, just like the louvered core glass pack.
A disadvantage to the straight-through muffler is that it is often louder than a reverse-flow muffler. Usually a straight through muffler needs a small sub muffler or a resonator to keep the exhaust quiet. A resonator is usually a small-perforated core glass pack placed somewhere in between the catalytic converter and the main muffler. Like the main muffler, the longer the resonator, the better a sub muffler will be good for noise reduction. A Walker Magnum Glass-Pack is a good muffler to use as a resonator.
Almost all of the pre-made performance exhausts feature resonators. Some good performance mufflers are only available with a semi-universal 2.5-inch inner diameter. If you have a smaller engine that requires a 2-inch pipe, it is still OK to use a main muffler with a slightly bigger inside diameter. This larger step up in diameter at the very end of the exhaust system setup won't hurt performance and sometimes can help it slightly.
When designing you own custom exhaust, it is important to remember the make it as quiet as possible. Loud might be cool to you, but remember that a too loud exhaust is perhaps the number one harassment ticket given to performance enthusiasts by your friendly law enforcement officer. Don't ask how we know this.
Backpressure = Torque?
An old hot-rodder's tall tale: Engines need some backpressure to work properly and make torque. That is not true. What engines need is low backpressure, but high exhaust stream velocity. A fast moving but free flowing gas column in the exhaust helps create a rarefaction or a negative pressure wave behind the exhaust valve as it opens. This vacuum helps scavenge the cylinder of exhaust gas faster and more thoroughly with less pumping losses. An exhaust pipe that is too big in diameter has low backpressure but lower velocity. The low velocity reduces the effectiveness of the scavenging effect, which has the greatest impact on low-end torque.
Low backpressure and high exhaust stream velocity can be achieved by running straight through free-flowing mufflers and small pipe diameters. The only two exceptions to this are turbocharged engines and engines optimized for large amounts of nitrous oxide. Both of these devices vastly increase the exhaust gas volume and simply need larger pipes to get rid of it all.
Some stock mufflers and exhaust systems have up to 18psi of choking, power-robbing backpressure. In direct contrast, a well designed, high performance street exhaust system typically has about 2-6 psi of backpressure. For an interesting comparison, an un-muffled straight piped on a real racecar usually has 1-3 psi of backpressure.
To get the least amount of backpressure, most of the good, high-performance mufflers available today have what is called a straight through design. These mufflers quiet the exhaust by absorbing high frequency vibrations in the heat resistant packing, usually consisting of stainless steel mesh and heat resistant ceramic fibers.
They typically have one inner core that is straight through with no baffling at all, much like a straight pipe with many small holes in it. The pipe is louvered or perforated when it passes inside the muffler's shell, allowing sound energy to pass through the holes but leaving the exhaust gas flow unimpeded. You can see straight through these types of mufflers. The louvered or perforated core is usually wrapped in either fiberglass wadding (hence the old school term, Glass Pack) or, in the better mufflers, stainless steel mesh backed by ceramic fiber to help further absorb the sound.
On straight through mufflers, the longer the muffler the bigger the can, the quieter it is. The length usually has no effect on backpressure, just noise output. These absorption type mufflers work in the same manner as the silencers used on guns. If a silencer had baffles that impeded bullet travel, you would definitely have problems! The same is for a perforated core absorption muffler, straight through, no baffles, no restriction, and no backpressure.
It is best to avoid straight-through mufflers that have a louvered core. Many old school glass packs suffer from this design. Some spiffy polished stainless and big tip mufflers on the market also have these. The louvers generate quite a bit of backpressure because they stick into the exhaust stream and create considerable turbulence. Even though these mufflers are a straight through design, they have more backpressure than a stock muffler.
When buying a straight-through muffler, look for one with a perforated core if you are interested in producing more power. A good, properly sized, perforated core straight-through muffler will add only about 1-2 psi of backpressure to your exhaust system. Mufflers like the Walker Ultra Flow. Thermal A`pexi, , Borla, Edelbrock, or Magnaflow are examples of good, low-backpressure mufflers with an absorption design. Many Pre-made exhausts like A`pexi, Tanabe, Greddy, Borla, Magnaflow, Thermal or HKS also have mufflers of the free-flowing absorption design.
An old school type performance muffler, which is till very common in sped shops, that has seen better days is the Turbo Muffler. This is a less restrictive version of a stock-like reverse-flow muffler. In the old days, these were well flowing mufflers, but now the new jack perforated core, straight through absorption types, has superseded them. Many old school domestic shops will try to sell you one of these as a hot setup, but they should be avoided, just like the louvered core glass pack.
A disadvantage to the straight-through muffler is that it is often louder than a reverse-flow muffler. Usually a straight through muffler needs a small sub muffler or a resonator to keep the exhaust quiet. A resonator is usually a small-perforated core glass pack placed somewhere in between the catalytic converter and the main muffler. Like the main muffler, the longer the resonator, the better a sub muffler will be good for noise reduction. A Walker Magnum Glass-Pack is a good muffler to use as a resonator.
Almost all of the pre-made performance exhausts feature resonators. Some good performance mufflers are only available with a semi-universal 2.5-inch inner diameter. If you have a smaller engine that requires a 2-inch pipe, it is still OK to use a main muffler with a slightly bigger inside diameter. This larger step up in diameter at the very end of the exhaust system setup won't hurt performance and sometimes can help it slightly.
When designing you own custom exhaust, it is important to remember the make it as quiet as possible. Loud might be cool to you, but remember that a too loud exhaust is perhaps the number one harassment ticket given to performance enthusiasts by your friendly law enforcement officer. Don't ask how we know this.
slideways...
03-08-2006, 10:57 PM
hmmmm...copied from SCC, i presume?
still its correct in every way.
still its correct in every way.
nissan_240sx
03-08-2006, 11:45 PM
^
lol yea it does look alittle familar
lol yea it does look alittle familar
Tims_240
03-09-2006, 11:57 AM
hmmmm...copied from SCC, i presume?
no
still its correct in every way.
thats why
no
still its correct in every way.
thats why
Skylindrftr
03-09-2006, 05:06 PM
I coulda swore ssc had a similar article at the very least, and most of their tech articles tend to be on target...
David_NISMO
03-09-2006, 05:22 PM
More than likely he has read the SCC article too, and with his previous knowledge, some of it is going to sound like the article.
Cheers, thanks for the info
Cheers, thanks for the info
TerminalVelocity
03-09-2006, 10:22 PM
very well written!
should be a sticky!
should be a sticky!
Pavlo
03-09-2006, 10:48 PM
/\ +1
slideways...
03-09-2006, 10:50 PM
post it as a sticky in the chevy or ford forum and youll get lit on fire.
Skylindrftr
03-10-2006, 12:00 AM
lol they really do stick to their old hot rodders myths huh...
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