Could anyone please tell me what power gains are achieved when replacing the standard exhaust and how much of that is down to the cat as i dont realy want the extra noise of a new exhaust and would prefer to just replace the cat.
The cat is about the narrowest part of the exhaust (I think). The exhaust will only be as good as the narrowest part. You will have too much backpressure when only replacing the cat. Get a full exhaust with extension kit and give it some pressure, I'm sure you'll agree it was necessary to get rid of all the gas.
The standard exhaust has flattened sections and 90 deg bends , therefore is quite restrictive.
Lots of owners have replaced just the exhaust from cat back with good gains , however the best way is replace exhaust and decat.
Can't help u with replacement exhausts as quite as the factory system as the market is for noisier exhausts.
Another benefit of a replacement exhaust is that they are almost all stainless and much lighter.
I have the HKS Super Dragger muffler with 3 pipe from the cat back, on my 3, and this is about as quiet as the stock exhaust, so there are quiet options there if you want. But yeah evoboy is right in saying that most are loud as hell
A decat with the OEM exhaust doesn't give that much gains in my experience. The only gain I felt was a marginal increase towards the upper-middle portion of the rev-range all the way to the redline. The engine feels less strained too at full-song but I think this might be more psychological and actual. http://www.ltregister.f9.co.uk/graphics/happy1.gif border|EQU| 0 align|EQU| absmiddle >
My mechanic says the most crucial portion of the exhaust is the first 1 meter from the exhaust ports down just past the CAT on Evo's. Hence I suggest you also replace your downpipe when getting a full exhaust system (unless it already comes with one.)
A friend with an E5 installed an exhaust (Apex'i) and was quite dissappointed with it, as he felt he lost too much torque low-down. But after he installed the downpipe and removed the CAT, he says he is very happy with the gains in power, torque and over-all driveability. [img]http://www.ltregister.f9.co.uk/graphics/happy1.gif border|EQU| 0 align|EQU| absmiddle >