well iv recently got my self an st205 to replace my mr2 rev 3 turbo which had a 3ince system and i put a berk 3 inch down pipe and im planing to do the same with the gt4 but what this car has a mongoose 2.5 inch system and there seems to be alot of back presure from it when on boost now im just wondering if it would be a bit of a restriction being 2.5 and not 3 inch and would it be worth changing to a 3 inch system ?

What other modifications do you have?

Highly unlikely that the 2.5 is causing any restriction unless your running significantly higher power than standard.

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GT4DC Events Co-ordinator
1996 ST205 UK

i have a k@n panel filter and the boost up tp 1.1 thats pritty much it !

From experience if you replace the cat and front exhaust section (including the flexi) with a 3inch downpipe and front pipe then mate it to a 2.5inch centre and back section this would handle up to 330bhp @ 1.2-1.5bar. The larger diameter at the turbo exit has a greater beneficial effect on engine response than a large diameter centre and back section.

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Don
GT4DC Chairman
1994 Toyota Celica GT-Four ST205WRC JDM 269bhp @ 0.9bar
1994 Toyota Celica GT-Four Special GT 590bhp @ 1.8bar
1989 Van Diemen RF88/89 Formula Ford 1600
2008 Nissan Patrol GU 3.0L ZD30DDTi 154bhp

cheers lads il just stick with what i have so and get the berk down pipe.cheers for the inputs dudes !