Hi im running over 400 bhp on my 03sti jdm should i change to 3"system when i go fmic

 

im pretty sure its advised to go 3" at the 400bhp mark to see some benefit.

 

mainly depends on the brand and how restrictive the backbox is really

 

System just now is full decat with group n back box
Md321h twinscroll
650 injectors
Rcm big trumpet

 

If your going for it Peter I'd probably take the back box if you choose to sell it mate

 

At that power level a 3 inch is recommended, I have a brand new 3 inch H&S with sports cat gathering dust at home if you are interested?

 

A friend got a 20bhp increase when he went from a 2.5 to 3" exhaust at over 400bhp

 

I had a 3" to 2.5 decat and 2.5 to 3" system on mine and was advised to look for a 3" system @ 400+

Still cant understand why the system has a step down to 2.5 where the cat pipe meets the centre, about 4" long then straight back up to 3" again, unless its designed to mate to a stock system.

I know how restrictive an exhaust can be, my last type r had a nurspec and during mapping we tried it with the bung in/out of the backbox and it made 20bhp difference due to the restriction

 

Thanks tony but it will be full decat if i do it

 

So what 3" exhaust system gives the best results?
Is full decat the way to go?

 

I'm a full 3" with decat running 507bhp with cobra back box! 400+ you'll see a Diffrance with a full 3" system

 

Does going from 2.5" to 3" affect torque

 

It did with mine, not really for the better. I was running a full decat 2.5" H & S with a Harvey up pipe and manifold at 414ft lbs. When the manifold cracked I changed it for an RCM GT Spec Gen 2 and up pipe and 3" H & S system. The torque increased just 11ft lbs but moved the spool point up 500 revs. Not the difference I was expecting or wanted. Those in the know reckon a Harvey up pipe would improve things considerably. Since Simon Roe sadly passed away I have not been inclined to try this option as it would require a map adjustment, so it remains unproved.