andyseston

Hi everyone

i am looking to change my exhaust system from turbo back as not happy with the one i have.

what i would like to know is i have a 3in full system on the car but looking at 2.5in full system and wanted to know will i gain/lose anything performace etc if i go down in bore size?

cheers

NXG

What car, engine, state of tune?

andyseston

standard sti classic apart from a panel filter and the exhaust running around 300bhp.

NXG

Yes, you'll be better off with a 2.5, as you don't have sufficient back pressure with the 3, though you could put a resonator box in the 3 to quieten it down and do more or less the same thing. I had a full 3" on my 2.0, with only a sport cat in the downpipe, but it had a 3 to 2.5 constrictor section at one of the joints to balance the pressures.

andyseston

its not that its to loud it just dont give the sound i want. i thought of going to a 2.5 all the way though but what about just putting a 2.5in rear box on with a reducer to make it fit, will this help.

NXG

Should do, you just need to balance the flow a bit

andyseston

how do i need to balance the flow?

cheers

NXG

With the 2.5" restriction somewhere in the pipework. In the downpipe would be best.

andyseston

why in the downpipe?

NXG

Closer to the source

andyseston

so doing it at the rear box would not be good then.

look like i may just get a full system.

bob r

iTrader: (46)

I have a 3" decat downpipe that tapers to a 2.5" mid and backbox.

With the usual mods it made 330/310 and I doubt I could have got any more by using a different configuration of exhaust peices.HTH

Adam K

On a turbo car you don't want any backpressure.
But most people seem to agree that on a moderately modded car a 2,5" system performs just as good as a 3".
Just equip it with a sports cat or decat.
If you make a good deal out of selling it and buying a 2,5" do it, but you could just as well just swap the back box.