I'm sure this has been asked before but I can't seem to find anything on it, so would appreciate some opinions.

On my 05 wrx wagon I have a full PPP turbo back exhaust which has an OD of 64mm (2.5") and ID of 61.5mm (2.4").

I've bought a Ninja backbox as I fancied a change, it was previously fitted to a bugeye STi but I'm hesitant to fit it as the ID is only 55mm (2.2") which would make it the narrowest section of the system. It seems like a pretty big difference to me, so wondered whether I'm missing something or just worrying over nothing and it won't make a difference?

 

That's odd, sure is off an STI?

Non turbo exhausts are 2.25"

 

That's exactly what I thought!
I cant be completely sure but it came as part of a full system, the front half of which is a prodrive downpipe and 2nd decat, so they were definitely from an sti

 

I've seen incorrect exhausts on cars before.

My mate bought a classic wrx, owner before was old guy. While doing work on the car, noticed full exhaust was tiny. Turned out to be 2.25" I think or less.

 

To close the loop on this, it seems like the standard wrx and sti back boxes have a 2.5" flange bit then quickly reduce down to something like the 2.4" I'm seeing on the aftermarket backbox I have. So it should have at least the equivalent flow to a standard back box but not as much as the prodrive one I have fitted!

 

Really 2.4" will have check, think I have one in garage. Must have been off a non turbo model. Maybe he fitted it to an STI not realising. There's loads of people that don't know anything about cars.

Think we just get use to Subaru owners having some sort of car knowledge.