back pressure
01-22-2009, 09:10 PM
#1
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back pressure https://ls1tech.com/forums/lt1-lt4-m...-4-cutout.html
Brought this up. I was always told that back pressure is a must and I've been looking online about the subject since, and I get mixed answers. Does it make or lose power without back pressure or gain if you are NA? and why? Thanks.
Brought this up. I was always told that back pressure is a must and I've been looking online about the subject since, and I get mixed answers. Does it make or lose power without back pressure or gain if you are NA? and why? Thanks.
01-22-2009, 11:22 PM
#6
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http://www.ls1.com.au/forum/showthread.php?t=94260
is the best I could find to support that but if you have a cutout instead of larger pipes would the exhaust fumes expand?
is the best I could find to support that but if you have a cutout instead of larger pipes would the exhaust fumes expand?
01-23-2009, 01:59 AM
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I'd hate to squash your thread/backpressure ideas without actually giving you viable answers, but the truth is you aren't in the position for them.
Backpressure, as you and many uninformed car folk understand it, is a term that is horrendously misused. It's an incorrect oversimplification to try and attempt an answer about exhaust. Luckily for many (including exhaust sales reps) it sounds great and seems self explanatory.
The truth is that the internal combustion engine is very complex and even something as seemingly simple as exhaust is quite complicated. There are NO simple answers, and if you want them then you've got a lot of study ahead of you.
BTW, the internet is a terrible place to find real information.
Backpressure, as you and many uninformed car folk understand it, is a term that is horrendously misused. It's an incorrect oversimplification to try and attempt an answer about exhaust. Luckily for many (including exhaust sales reps) it sounds great and seems self explanatory.
The truth is that the internal combustion engine is very complex and even something as seemingly simple as exhaust is quite complicated. There are NO simple answers, and if you want them then you've got a lot of study ahead of you.
BTW, the internet is a terrible place to find real information.
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01-23-2009, 07:37 AM
#8
+1
You want scavenging - which tends to introduce some backpressure to the system, which is where the myth comes from. Scavenging is a product of header shape and primary length though. Any post-collector restriction is a bad thing, period.
Of course your tune will need to be changed if you ditch a catback for open longtubes, but when tuned backpressure is always a bad thing.
Drag cars would run flowmasters instead of open headers if backpressure was a good thing
. Here is some good reading that explains it in simple terms.
Please kill this myth and stop telling everyone not to run 3"TD's or cutouts because it will "kill their torque"!!
You want scavenging - which tends to introduce some backpressure to the system, which is where the myth comes from. Scavenging is a product of header shape and primary length though. Any post-collector restriction is a bad thing, period.
Of course your tune will need to be changed if you ditch a catback for open longtubes, but when tuned backpressure is always a bad thing.
Drag cars would run flowmasters instead of open headers if backpressure was a good thing
. Here is some good reading that explains it in simple terms.
Originally Posted by http://autospeed.drive.com.au/cms/article.html?&A=0046
Another furphy that has widespread currency is the concept that engines need back-pressure. Simply, there is no properly tuned engine where increasing exhaust back-pressure causes an improvement - in power, torque or fuel economy. One of the reasons that this idea has gained support is because when people change their exhaust they seldom check the air/fuel ratio or re-map the ignition timing to once again give optimal performance. For example, some MAP sensed cars drop substantially in power with a large exhaust fitted because they are then running lean.
Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.
Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!
The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.
One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!
And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!
Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.
Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!
The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.
One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!
And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!
