View Full Version : Cat or Decat Downpipe ?

22nd October 2003, 18:46
OK so the time is rapidly approaching to bolt on new bits, one of which is a downpipe, before getting the first remap.

Would like to go for a replacement downpipe with a cat to stay legal and MOT hassle free.

But how much BHP less is the same car likely to generate using a cat downpipe instead of a decat ?

5 ? 10? 20 bhp less ?

22nd October 2003, 20:39
More of a problem is spool time. Better to just run an OE catted centre during emissions checks, you'll pass no worries.

May get more complicated if you live outside of UK...but hi-flow cats are readily available. Best keep it as far away from the turbo as poss. though.


Adam M
23rd October 2003, 11:18
I have a BPM 3 inch centre section with a hi flow sports cat.

never used it but apparently it works, and only costs a few hp.

There is a chance that if it is hot it will pass a roadside emissions test, but more importantly, because it isnt in the downpipe, it doesn't affect spool up time, which as stated above is the most important reason for decatting.

23rd October 2003, 17:27
Thanks guys.

So as a suplementary question http://bbs.22b.com/ubb/smile.gif

Open neck (eg H&S) or twin downpipe (eg Magnex) ?

I've heard twin is better top end, open neck better low down torque - any one offer any fact/experience based advice rather than the hearsay I've read ?


23rd October 2003, 18:19
Open neck with a splitter, which attempts to offer the best of both. Do a search on here and snet for posts from Michael South (I think? someone correct me if I'm worng) for the details.

Adam - centre cat still affects spool up, but less so. On an STi7 the difference between stock and highflow centre cats an a map with a 1.35 bar target in mid-range, mapped for high flow, only achieves 1.32 bar with the OE centre.

Never tried without cat....that would be illegal here in CH http://bbs.22b.com/ubb/smile.gif