jimbo1111

Has anyone went from a 2.5 cat bypass and free flow catback to a full 3" exhaust ? I am interested in this upgrade and would like to hear if anyone has dynoed a differents in the two systems. My car has a T04b #8 turbo. I have heard that the upgrade my be good for 10 to 20 rwhp on a bolt on parts car. Can anybody confirm this? Thanks

turbo944

I don't have a turbo upgrade nor a dyno close enough to test, but I do have one road nearby that I test on starting at X mph and accelerate hard to another point (no driveways, clear visibility, etc.) and see what my speed is at the end. Do that a couple of times to get an average and to remove any stupid things I might do during the run and it can tell you whether you're doing better or not. I am about to go from the stock cat pipe with a straight pipe welded in place of the car and the stock catback to 1) same straight pipe with SFR 3" catback and then 2) in another week or so to the SFR testpipe with no cat and I'll be seeing if there are any real changes. Not specific horsepowerwise, but it tells me whether things are worth the change. Part 1 should be tonite.....

IPSC

Let us know as I have that same question

IPSC

porshhhh951

Anytime you eliminate the car you are going to pick up Hp. 20rwhp gain on a upgraded turbo with mild boost is very attainable. Unfortunately when I had my full exhaust system installed(no cat) I also did many other thing's so I don't know how much help I can be in giving exact #'s. I would think 20-25rwhp on average for a upgraded car with no-cat sounds about right. Only one way to really know for sure and that's to dyno the car before and after.

Danno

The primary benefit is in removing the cat. There doesn't appear to be any difference in HP ratings between the US-spec cars with catalytic converters and the Euro ones without. But that may be due to a brand-new cat being free-flowing. Over time, it gets clogged and melted, thus decreasing your HP levels. Depending upon how clogged your cat is, you'll gain 0-20hp back to stock HP levels by removing it. So a 2.5" cat-bypass will yield the same performance as a 3.0" one really. And there's absolutely zero performance gains to be had with an aftermarket cat-back exhaust. You'll only need a 3.0" cat-bypass and 3.0" cat-back once you cross over the 350rwhp mark, but the benefits won't really show until you're over 400rwhp.

Here's a dyno-chart of a car with a 2.5" Bursch cat-bypass and stock cat-back with stock muffer (K27/8@20psi, APE-MAF, 50# injectors & custom GURU chips):

toddk911

I went from bypass to full 3" cat back. I hear the spool up much better, but not sure of any hp gains. Like Danno said, the most gain is getting the cat out.

I only did it becauase I had a problem with my test pipe I had clamped in and got a guy to build it for a good price (full 3" cat back parts and labor, $350) , other wise I wouldn't not have bothered with 3".

azmi951

I have a bursch cat bypass and want to fab up a new after cat system (rusty muffler) what is the stock pipe size and the size of the bursch?
How does the stock pipe flow?
What type of aftermarket muffler does not sound raspy or loud?