Ideal Exhaust Pipe Diameter on S4

[karmavore]

If I have some custom pipes made for my car (cat back) what diameter should I use: 2.5", 3", 3.5"...?

Thanks!

Luke.

[rlearp]

I'd use a 3" pipe, as I've done the 2.5" pipe before. It won't hurt you and might even help a little. Besides, if you upgrade your turbo to a T04E the 2.5 won't fit anymore anyhow.

Oops, just read cat back. Being as how the cat is going to be the largest restriction in the system then it might not matter a whole lot. I just did a complete 3" system, with a hi flow 3" cat, etc. You might want to do the same since you can do it for under $150 in parts, plus a little labor.

Ron

>> Edited by rlearp on Wednesday 11th August 00:07

[njgsx96]

Your choice. 2.5" is good as long as the piping is mandrel bent and not press bent. If you want that extra 1 HP, 3" is good too (again, andrel bent), but will be a touch louder. Just replacing that stock muffler with something better and removing the cat bypass valve will give some gains in performance but as Ron mentioned, the cat is another big restriction. To get the max out of your setup, a full turboback exhaust would be ideal, with a good, hi-flow cat or just a simple straight pipe if you don't care for the the emissions laws.

[karmavore]

Where's a good source for hi-flo cats?

Thanks guys.

[rlearp]

Check JEGs or Summitt, pick a few brands, then go on Ebay and get it. In my case I got a MagnaFlo 3" High Flow (Don't quote me on brand, it has been too long) for 1/3 the price of the list in the catalog. New in the box too. Cats are quite high from Summitt etc., but seriously high at the dealer!

[karmavore]

Nice tip, they are much cheaper on Ebay! Thanks.

I assume that I don't need an "air tube", but what are they for?

Luke.