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My friend has a 2002 Si and wants an exhaust upgrades.
There are no other mods on the car.

What size exhaust is suitable?

 
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If everything else is still standard, theres no real reason to get a bigger exhaust, but if he does want that then I'd go 2.5" max, but that could even be too big (and feedback from anyone else on that?).

For a nice sounding car, get a good muffler on the end and he's set.

 
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I agree with vtech, I really don't see the point in buying tuner exhausts if his car is stock. It may sound harsh, but if the car is all noise and no speed, it'll look ridiculous.

 
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It wouldnt be all noise, but if the exhaust is too wide it will allow too much excess air flow which actually lowers your hp.
Keep it at 2" and just put a high flow muffler on the end for a nice sound.

 
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I thought less back pressure was actually a good thing on an NA car, I know on cars with turbos you have to have some back pressure or else the turbo won't function properly. I could be wrong , any body with any info that could clear this up?

 
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The turbo uses exhaust gases to spool up, thats not back pressure but just the exhaust cycle.
Backpressure in a turbo is controlled by a wastegate, but there is no such thing in an NA car obviously.

I dont believe exhaust and back pressure are directly related.

 
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One of those 10" fart cannons should work fine.

 
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Umm yeah ok speedaddict.
You mean the milo tins huh?

They are just mufflers anyway. he is asking about exhaust systems.

 
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Like I said.

 
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Thanks people... some silly answers but thanks to those who replied with serious answers, much appreciated.