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can someone explain the need or lack of need for back pressure on a normially aspirated engine. i have heard all kinds of stories, like with no b/c you will burn your valves and such of the like. now i know this holds true in the reverse effect with the isuzu 2.6, to much and it burns valves. anyway can anyone tell me what the real deal is, and if you have facts to back it up post those also. id appreciate it.
 

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This was a huge discussion about 6 months

to a year ago. Search the archives under "backpressure" and you will answer your questions.
 

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Go ZZZZZ!

Basically the story is there for the finding.
The dyno will tell the story, too. High-flow exhausts normally sacrifice down-low torque for top end power. This has to do with reversion and proper exhausscavenging. It's not so much a quantifiable number you are looking for, but pressure differential at certian points of cam events. That is what you will find back there...
 

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You're a bad boy Zmefly....

After reading Tony D's response, I got curious and did the search myself. Guess what? YOU were a part of this thread. Here it is:

click here
 

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And here's another thread....

<a href="http://www.zcar.com/forums/read.php?f=1&i=187207&t=187207">

click here</a>
 

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Re: I was Mistaken!

There is such a thing as backpressure and I saw it being measured!

My original comments about flow volume and velocity are still true, but backpressure (or pressure in the exhaust system) does exist and is measured by good engine builders. Engines do prefer some level of exhaust sytem pressurization, but the level is very low (2 to 5 psi) and depends on head port flows, combution chamber design, overlap, valve size, and many other things. These variables are so great that its hard to come up with a generalized statement that says "X motors like 3 psi of backpressure."

So, I stand corrected.

- John
 

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John...

Unless the engine is turbocharged, very rarely will the backpressure of the exhaust measure in Psi...
Normally it is measured in inches of water column, or inches of mercury...
2" mercury is one psi, but again, that's turbo territory, measuring away from the port...
 
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