Not enought backpressure in the exhaust = bad for daylidriving?
Jan 28, 2010 | 07:55 PM
#11
You hit the nail on the head, perfect explanation 
Unfortunately not enough folks are educated about exhaust velocity and go with too big of pipes thinking that this will help their performance throughout the RPM range.
Unfortunately not enough folks are educated about exhaust velocity and go with too big of pipes thinking that this will help their performance throughout the RPM range.What more evidence do you need other than physics? If you would like to see for yourself, go to Home Depot and conduct an exaggerated experiment. Grab a 1" pipe and a 3" pipe. Your breath is the same temp in both tests and the temp of the pipe will remain the same, thus negating the effects of a change in temperature between the tests skewing the results.
Blow through the smaller pipe with a constant (or as close as you can) force. Now, remembering how hard your blew, do the same for the 3" pipe. Notice how much easier the 3" pipe was? The larger pipe is super easy to blow through but the air velocity (the speed of your breath) is really lazy inside the pipe. Don't believe me? Put your hand near the end of the pipes and repeat the test. You'll see that the smaller pipe moves air with a lot more velocity but the volume of air is less. Vice versa for the larger pipe: less velocity but a larger amount of air.
In an engine, at lower rpm levels, the exhaust gas isn't moving as fast so we try to promote the effective evacuation of exhaust gas from the cylinder by fitting a suitably sized pipe to the port to help keep the gas velocity up so we can get a clean incoming charge of air/gas into the cylinder. Now at higher rpm levels, the piston speed is much higher so as to help "push" the gas out, along with being "pulled" in a sense from exhaust pulses but that is another discussion, so we don't need the aid of the smaller pipe as much. Hence where a larger pipe is handy to help move the larger volume of air in this circumstance.
An engine is just a big air pump. We try to size the exhaust according to the rpm level we want the engine to thrive. Smaller pipes are going to help lower rpm cylinder filling but might (keyword might) hinder upper rpm performance. It all depends on other things as well. The same goes for upper rpm performance: a larger pipe helps move the larger volume of exhaust gas but the gas velocity suffers at low rpm as a result. It's a trade-off in a sense, but again, a lot more can effect proper exhaust performance too.
Blow through the smaller pipe with a constant (or as close as you can) force. Now, remembering how hard your blew, do the same for the 3" pipe. Notice how much easier the 3" pipe was? The larger pipe is super easy to blow through but the air velocity (the speed of your breath) is really lazy inside the pipe. Don't believe me? Put your hand near the end of the pipes and repeat the test. You'll see that the smaller pipe moves air with a lot more velocity but the volume of air is less. Vice versa for the larger pipe: less velocity but a larger amount of air.
In an engine, at lower rpm levels, the exhaust gas isn't moving as fast so we try to promote the effective evacuation of exhaust gas from the cylinder by fitting a suitably sized pipe to the port to help keep the gas velocity up so we can get a clean incoming charge of air/gas into the cylinder. Now at higher rpm levels, the piston speed is much higher so as to help "push" the gas out, along with being "pulled" in a sense from exhaust pulses but that is another discussion, so we don't need the aid of the smaller pipe as much. Hence where a larger pipe is handy to help move the larger volume of air in this circumstance.
An engine is just a big air pump. We try to size the exhaust according to the rpm level we want the engine to thrive. Smaller pipes are going to help lower rpm cylinder filling but might (keyword might) hinder upper rpm performance. It all depends on other things as well. The same goes for upper rpm performance: a larger pipe helps move the larger volume of exhaust gas but the gas velocity suffers at low rpm as a result. It's a trade-off in a sense, but again, a lot more can effect proper exhaust performance too.
Feb 2, 2010 | 09:07 PM
#15
On The Tree
Joined: Jul 2005
Posts: 130
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From: Cherry Valley, Ca
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