Right gents i'm toying with the idea of fitting a sports cat to my hawk wrx, at the moment it's got the standard PPP exhaust system (so 2.5" dia. i presume). Question is do i replace the whole lot with a 3" system or just go for the 2.5" sports cat (bearing in mind the main expense either way is the cat itself)?

I should just say I've got no particular end bhp figure in mind, it's just a case of changing bits and seeing where it ends up. Just i'm conscious of the fact the 2.5" system will become a limiting factor quite early on (obviously that and the OE turbo).

 

i think i have seen a few systems where you can have a 2.5" cat back and have the sports cat 3" reducing to 2.5" if you know what i mean? i've been thinking about doing that with my blob wrx

 

I have a full 3" system and as soon as I fitting it you could feel the power increase.

Saying that the first 3" system I had was decat, then shortly after bought a 3" sports cat system.

Don't know what increase the 3" system made, as it could just of been due to decat or sports cat.

Don't see the point of having a 3" into a 2.5 as the exhaust gases will just be bottle necking after the cat.

 

yeah thats what i thought about the 3" to 2.5" idea, thats whats been putting me off it

does a 3" rather than a 2.5" turbo back make alot of difference? power and volume wise

 

I spoke to Hayward and scott the other day as I have a 2.5 inch system from my bugeye and was thinking about fitting it to my twin scroll. They said it will be fine up to around 370 bhp.

 

This thread might add something to this one.

https://www.scoobynet.com/scoobynet-...rformance.html

FYI, im getting 440/440 out of a H&S 2.5 full system and that's capped by the map rather than any restriction.

 

This is my problem, different people/tuners reccomend changing exchaust diameters at different point. Iirc engine tuners guide even says you want 3.5" for upwards of 400bhp, yet the RaceDynamix guide doesn't mention it at all and it covers a mod path up to 400bhp.

 

your PPP pack should already have a sports cat in it which is good for lots of power/torque. I would say stick with the 2.5 from cat back for a good bump in sound quality.

 

I phoned where I'm getting my car mapped and they said pretty much exactly the same as H&S. For me personally I wont be doing any mods other than a decat so I'm sticking with my old 2.5 inch system. Apparently I should be getting in the region of 360 with that on and just say in the future I want more power I'll have to buy a 3 inch system.

 

I had a h and s 2.5 at 407bhp. Ran perfect

 

Hi there

You can buy 3" Downpipe tapered to 2.5" which will be best from both worlds,but agree 2.5" is OK up to 400-450bhp and with 2.5" you should have quiet exhaust too,but this depends on the backbox too

About the PPP,this backbox is pretty restrictive on anything beyond 350bhp(but I've seen few guys making on them 360bhp),due this I would replace for something which is more free flowing

Thanks,Jura