Exhaust width for 550hp?

[A80Avant]

As im in search for a bigger turbo,my thinking is that the 2.5inch(70mm) catback i have is not enough to keep if i want to reach that figure.My 2 mufflers are 76mm internally(3inch) so i was thinking that the cheapest modification would be to go for 76mm tubing and be done with it.

So the complete exhaust would be:

1)RS2 EVO exhaust manifold with T3 flange,
2)76mm downpipe
3)2x60mm tubes to 2x200cpi cats
4)76mm monotube catback
5)twin 70mm exhaust tips.

Im trying to figure out how to minimse the backpressure but also keep the noise levels as bearable as possible...The 70mm catback has made a big difference along with these mufflers,so if i can get away with just changing the tubes i think it will be ok!

i know that the biggest backpressure problem is the turbo hotside.For that im looking into either the K26 8 (with K26 flange on the manifold obviously) or a K27 7 ( with the T3 flange).Turbowise im thinking of a holset HX40,which i want to make a hybrid with the above hotsides.
If i get down to it at some point it will mean that the GT3071R+RS2 manifold will be up for sale.

[andyg plym]

ive got a 3" exhaust from the turbo to backbox,no cats just the one silencer. it is very loud but inside the car sounds like a V8 not a 5pot. on the outside it does though
was it you who did the video "thanks to audi?"
cos that exhaust note inside the car sounded PERFECT!

[A80Avant]

no that wasnt me..

[Rusty]

2.5" won't be enough for that. You really want 3" turbo back for that level of power and possibly even a larger down pipe. THis has been discussed before on a thread I started. Basically you need to minimise losses (from a narrow pipe) whilst maintaining gas velocity through the pipes.

Best give a few places a call. I'm looking at Pro Speed but they're in Cardiff so about 300 miles away from you . I think a few people have used Ascar Exhausts in Sheffield with good results but best ask those who live up your way, there's bound to be somewhere a lot closer than that.

[A80Avant]

yes ill see what i can dig up...Since i will be using parts that will keep everything in the same position in the engine bay,im hoping i can get away with the 3inch downpipe at least.Im also not sure if changing the 2 cats for 1 of 100cpi will improve or make things worse...

[Rusty]

1 pipe has less losses than 2 pipes of the same total area so I'd go for a single one. Your 3" DP will not be a major issue but you might not get as much out of it as if you went for a 3.5", I'm sure there are plenty on here who have or know of a 500bhp ish car with 3" DP but it's the 550 bit that makes me wonder if it's enough. What turbo? GT35 or 40 variant?

[Jamo]

Single straight through system 3" has to be a minimum with single cat. Downpipe can be anywhere fro 3" to 4"

Dual centre pipes are used to reduce noise as twin pipes are quieter than a single pipe system.

Regarding downpipe size, it's not a science what works for one car make is not always best for another.

3" will give you better overall performance with say a 3071r.

It really depends on if your planning to upgrade to a GT35r if so get a 4" downpipe now.

I'm going for a 3" system for 500 bhp range on a 3076r

[Rusty]

Yeah, I was asking about a single silencer 3" system with sports cat and I was told that it wwould more tahn likely be above the 98dB maximum for track use, might have to drop down to 2.75" or 2.5" for the rear box to keep it all within the limits, have to wait and see.

[A80Avant]

well turbowise i think im going to make a hybrid out of a Holset HX40 with a 56mm or 60mm compressor wheel and then bolt it on to K26 8 or K27 7 hotside.As nervas made 600hp with a similar hybrid(but lots of differences,such as tubular EM,cams,ported cyl.head etc) im betting that i could get away with around 500ish and be happy with it.Im usually looking into whp figures,so if im at about 350whp on a rototest now,if i can go to 420-430whp on a rototest again,then my mark is done,cause on rototest you have less than 25% losses as you dont have wheels,its a hub dyno.

In any case,the turbo will be capable of around 550max so since inittially i may not have cams or ported head to support that,AND the EVO manifold is at about it's limit of 500hp,it will be quite an experience to see what it will do!