MadUsa1

What are the advantages and disadvantages of a 2.5" and a 3" exhaust?

I'm on a 2006 Spec C, looking to get the most out of the standard turbo.

Ginola

iTrader: (25)

you will gain a tiny bit of top end power with a 3" at the sacrifice of a tiny bit of low end torque over a 2.5"

If your looking to go bigger power later on the 3" is "future proofing" yourself.

MadUsa1

That's what I figured.

I've heard that the standard 2.5" exhaust is quite restrictive. Is it just the OE cat that is restrictive or is it an inherent design flaw with the entire system?

legacy_gtb

iTrader: (3)

Mainly the CAT, 2.5" systems are fine up to 450bhp ish.

Its the downpipe that needs to be 3" and catless or sports catted.

Id say 3" systems are completely overkill for most peoples build and result in a wheezy fart in a jamjar noise! 2.5" leaves it raspey and burbley, without being too nuts for track noise levels.

STI7PPP

+1

Rob

MadUsa1

450bhp on a 2.5"?

I didn't realise that was the case. So 3" is overkill for the vast majority of builds.

I'm hoping for around 340 or 350bhp, so I suppose 2.5" will be ideal.

A question on the downpipe: I beleive the standard JDM Twin Scroll downpipe is 3" or even a bit larger. With the original cat replaced by a high quality sports cat, this ought to work rather well right?

rickya

iTrader: (18)

- 1

Load of tosh, pardon my French ! Lol

rickya

iTrader: (18)

Get a 3" bellmoth downpipe, to a sports cat or decat , and you can either go 3" all the way or can taper down the down pipe to 2.5" for your power level. Changing from a std exhaust system to above options will be the main factor in achieving 320bhp instead of 340 bhp approx after remap with higher boost and timing. The twin scrolls will then howl like a banshee! Ps they will never burble!

legacy_gtb

iTrader: (3)

Really??????

TonyBurns

iTrader: (3)

A little bit

You will get restrictions on the 2.5 inch system, its good upto a point but you are right that most people wont need more than a 2.5, but for anything above the 350bhp mark then its got to be replaced and you may as well go for the 3 inch.

Tony

RICHARD J

iTrader: (4)

Mine ran 407bhp on the rollers on a 2.5 system. It was only when I wanted to go for a Meth map that my mapper advised on a 3inch. Iv also heard of cars over 450 BHP on a 2.5 system,but I think a full open 3 inch downpipe is essential for good power.

rickya

iTrader: (18)

I was referring more to the "wheezy fart in jam jar and burble" bit!! lol