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i read this on another post on this site


"As stated previously, it's not backpressure, but exhaust velocity that matters. Reducing backpressure in an exhaust without making any other changes can lean out a cylinder, but it's the lean condition that hurts the motor, not the lack of backpressure. The old adage that you need backpressure to make torque is . "



so, i just did 2.25 inch piping on friday. would that actually hurt my engine?

 
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no, i don't think it would...i think 2.25 is that max that you should go unless your boosted....do you have any other mods?

 
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K&N FIPK II with a purolator filter. thats about it for the engine.

 
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That size piping would be good for turbo. Otherwise, exhaust gases will slow down due to a loss of backpressure. You'll just lose some low end responsiveness. Our engines are not powerful enough to benefit from larger piping.

 
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I was wondering the same when I installed my pipe, so I went with 2". Is this ok?

 
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The piping is put on after the catalytic converter. All the scavenging and potntial lean conditions happens well before that, in the exhaust manifold or header. A 2.25" exhaust after the cat is fine. Anything larger is a pain to plumb. 2" may add a little backpressure, but it may not be enough to measure in our cars.

You won't lose low end in either case. The claim that we need backpressure for low end torque is BS. Your motor is an air pump. The more it pumps in and out, the more power you make. Simple fact.

 
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thanks for all your replies guys!

 
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wow someone who actually knows a thing or two!

But thanks for clearing this up!

 
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Yeah SMX knows his stuff. So is a high flow cat actually worse for performance?

 
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Most of the exhaust scavenging effect happens in the manifold and collector. After that, it's a matter of getting the gas out of the tail end. A high flow cat shouldn't lead to a lean condition that can't be compensated for. When I did my exhaust, the shop and I both looked into the stock cat and it looks like it should flow pretty well. I didn't back up obsrvation with dyno time though, so take that observation with a grain of salt. I can't truly say whether a high flow cat would produce gains or not. It's not a mod I'd do because of potential problems during the first smog check.

 
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oh, that reminds me (sorry if this is stupid), but would i have any problems passing my smog check?

btw, SMX, how do you know all this stuff? you gotta teach me

 
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Years of trial and error.

 
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No smog check in Miami, Yeah!!!!! he he...

 
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I have 2.5" back to the Cat.
Not Forced
You do lose low end exhaust velocity.. but it's a high end engine anyway.
I know 1000 people will get on here and say it's too large, but backpressure does not help your engine in anyway. and larger pipe can not harm your car.

 
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2.5 is the largest you should go with a NA motor. It won't hurt your low end that much. It makes up for it at highway speeds. Your top end is way better with a cat-back on the car. I have an old 88 maxima that has a cat back. Its way more responsive at highway speeds.