I'm planning a remap of my MY00 UK Turbo, nothing very drastic, just basically decating, walbro 255, 3 port, bigger tmic you know, the normal stuff.

I've currently got a 3" - 2 1/2" std downpipe, a decat 2 1/2" centre and a standard STi back box. I'm definately decating the downpipe but can't decide whether to go for a complete 3" system or just replace my existing downpipe with a decat version. What sort of difference will the extra diameter make at remap time. I'm thinking it will be minimal at the level of mods I'm talking about - what d'ya think?

Cheers

Paul

 

If you don't intend to change the TD04 turbo, no point in going FMIC as the TMIC can handle the 280+bhp potential of the turbo. The only benefit of a FMIC at that level will be reduced charge air temps... which in itself may net you a small gain in bhp, as your 440cc inj. can handle the extra fuelling demands. But no more 290-300 max! on stock turbo.

Prob not worth the expense/hassle of going FMIC if you don't intend to change turbo, etc.

I believe a 2.5 inch downpipe/system should be good for 400+bhp, so up to you what to fit i.e. a 2.5" won't limit you to the 280+bhp potential...

Obviously, all the above assumes a remap on your OEM ECU via EcuTek s/w

 

Depends on how much noise you want, most 3 " systems are loud. The system Revolution make is pretty civil, apparently. For a simple remap/stage 1 mods I don't really see the point in a 3" system.

I'd decat/sport cat the down pipe & leave it at that, it will be louder, but the STi back box should keep things under control.

 

£in system would be a waste of money for your level of tune TBh as the others have said 2.5 will be fine.

Im currently running 405bhp on a 2.5 system.

 

Very little difference in cost (if any) - the 3 inch system flows much nicer and can get a few more bhp as a result and also aid spool up a tad. Not a lot in it though until beyond 300bhp but it doesnt mean the 2.5 inch cant make the power - it just has to work harder to get there IMO.

 

if the question is decat 2.5inch downpipe, decat centre and sti back box v a 3inch after market exhaust including 3inch then the 3inch after market.

However on your spec you are likely to actually gain better spool and torque with slightly more back pressure.. ie. 2.5inch decat downpipe, 2.5inch centre and after market 2.5inch exhaust. Exhaust flow is a bit like watching water run down a window, once the first drop has run down the others just follow and flow down the quicker route getting drawn down.. the same with gas flow.. the flowing exhaust gas draws the gases out right through the system when the gas speed is lower.. ie. spool and off boost.

Simon

 

Excellent, thanks for the replies folks.

Unusually you all agree - so many post I read have so many conflicting answers you can end up more puzzled than you were before.

Have to say I'm pretty glad about the 2 1/2" being ok as I didn't really want to bin the decat centre or the STi back box thats on there already.

So, off to find a nice decat downpipe.

Simon, with what you said about back pressure will I be best to avoid a bell mouth down pipe? Is there a particular one you would recommend?

Thanks

 

open neck works the best

 

Ok, thanks - any particular one or are they all much the same for the level I'm looking at?

 

In December I went from a 3 inch cat system to a 3.5 inch de-cat system. On power you would be better off with ear plugs, but it sounds nice. I picked up a nice 40 lb/ft in torque in the miodrange and a nice 16 bhp in power. Up from 400 to 416 bhp the mapper would not push further as the car has standard bugeye STi internals and was on 97k. Before anyone comments I am sure the majority of the gain is due to the de-cat but some will be from the bigger pipes and the spool up is earlier.