Exhaust Dia??? 2 or 2.5 or 3 or 4 inch?????

[e30_Turbo]

Time to bite the bullet and get an exhaust made, it's yet more money but should be worth it in the end.

I've found a nice little hot-rod type fabrication shop and they're giddy at teh prospect of fitting me a nice phat chrome straight through exhaust....but....... what internal diameter should I go for?

Now I'm running a external wastegate screamer pipe, so I don't need the exhaust to flow everything, just what I'm using and the rest gets lost under the front of the car, so I "think" I should be ok with 2.5inch straight through....but.....I'm no exhaust expert.

Anyone got any facts, figures, things to work out flow rates etc???

Otherwise it's 2.5inch straight through and a 3inch pipe out back!!

Bring on the chrome baby

[Jhonno]

approx power? 300bhp? I'd go for 3"

[lentec]

Otherwise it's 2.5inch straight through and a 3inch pipe out back!!

With a turbo to make it a bit less painfull to the ears at low rpm Will be fine

[Jon_Bmw]

Even before I saw Jhonno's answer I was thinking along the same lines as him. I think you'll be aiming at even more than 300bhp mark, so i think anything to reduce back pressure would definatly be a good idea.

I hear what you are saying about the screamer pipe, but whilst you are on full boost, my understanding is that that wastegate will be firmly shut so you will still need a good flowing exhaust.

I would say about 3 inch + maybe up to 3.5 at the top end of the scale, I don't think there would be any need to go much bigger than that, and not up to 4" thats just "american big"

See what other opinions come up, go on see what the americans think Mark, I bet they say 4!

[e30_Turbo]

It does all come down to power I suppose, at the mo I'd say it's pushing 250-280bhp @ 8.5psi and I'd like to see 21psi by the end of tuning it.

I know it sounds a lot but as I have lowered the C/R, bolted the head down tight and fitted injectors for up to 500hp I may aswell see what it can do.

Now the exhaust place are happy to do whatever I want, I just need to make up my mind before it goes in.

John, as soon as I hit boost the wastegate opens and stays open till I drop clutch or back off, I'm thinking that due to my large compresser really.

I've read about power gains to be had with 3inch but no-one has an external pipe aswell to know if 2.5inch would be enough.

If I aint sure I may go 3inch anyway, bigger bragging rights in the pub

[Jhonno]

go for 3" imo

you getting it done @pipecraft at all?

[Jon_Bmw]

It does all come down to power I suppose, at the mo I'd say it's pushing 250-280bhp @ 8.5psi and I'd like to see 21psi by the end of tuning it.

I know it sounds a lot but as I have lowered the C/R, bolted the head down tight and fitted injectors for up to 500hp I may aswell see what it can do.

Now the exhaust place are happy to do whatever I want, I just need to make up my mind before it goes in.

John, as soon as I hit boost the wastegate opens and stays open till I drop clutch or back off, I'm thinking that due to my large compresser really.

I've read about power gains to be had with 3inch but no-one has an external pipe aswell to know if 2.5inch would be enough.

If I aint sure I may go 3inch anyway, bigger bragging rights in the pub

Yeah but as you wind up the boost to 18-21 psi later on, this i think is when the wastegate will be slammed shut and you will need a good flowing exhaust, so as jhonno says 3 inch for the win

[gcorky]

im getting a stainless 3 inch system(single box) m8,go for it.

[WillG]

yeah go for 3" if your aim is 21psi i dont think 2.5 is worth messing about with, even if its more money its better then risking it. How did you lower your C/R?

[fuzzy]

i wouldnt think bragging about having 3 inches in the pub will get you far . i was told 2.5" would do for 300bhp but 3" is needed for over 350bhp.over and above that i dont know because thats as far as im going

[e30_Turbo]

i wouldnt think bragging about having 3 inches in the pub will get you far . i was told 2.5" would do for 300bhp but 3" is needed for over 350bhp.over and above that i dont know because thats as far as im going

See thats part of my question, I would have in total 4 inches if the screamer pipe is included but I'm just not sure enough to decide yet.

[RowdyBurns]

Mark,

I would go with three inch.

Who are you getting to make your exhaust and what sort of money have they quoted you?

Did you say they were going to make you a chromed exhaust???

Best bet is to go for a decent 3" stainless system.

Things to ask them are:

What grade stainless are they using? 304 is pants, 316 is better. Most places use 304 to save cost.

How thick is the pipe. Must places use thin stuff that can hole with one touch of a speed bump!

How are they going to weld it up? Stainless MIG looks crap and the welds always seem to rust due to the crap quality stainless. TIG welding is the only proper way to weld stainless. Much neater and will last longer.

If you want to have a go at making it your self. I can get 1.5mm 3" 316 grade stainless for £25 a meter and the same in constant radius bends at about £16 each (90 or 45). If i still have acess to a TIG i can weld it up for you or i know a man who will do it for about £40.

If you want to pop over at the weekend i can show you one that I've made for a friends 400bhp zetec turbo 3dr sierra.

Got those sparks sorted yet?

Doug.

[buster]

isnt 304 stainless better as it doesn't rust ?
316 lasts a long time but does rust and discolour.

[e30_Turbo]

Mark,

I would go with three inch.

Who are you getting to make your exhaust and what sort of money have they quoted you? .

Knight Automotive designs on Bishops Hill, £400 including some odd bits of floor welding and a new downpipe section.

Did you say they were going to make you a chromed exhaust??? .

They are a custom chrome dealer so it's all polished chrome stuff, never thought to ask if it was stainless or not but I'm not that worried about it, my std steel system last a few years, I may ask and find out tho.

Best bet is to go for a decent 3" stainless system.

Things to ask them are:

What grade stainless are they using? 304 is pants, 316 is better. Most places use 304 to save cost.

How thick is the pipe. Must places use thin stuff that can hole with one touch of a speed bump!

How are they going to weld it up? Stainless MIG looks crap and the welds always seem to rust due to the crap quality stainless. TIG welding is the only proper way to weld stainless. Much neater and will last longer.

I will ask and find out.

If you want to have a go at making it your self. I can get 1.5mm 3" 316 grade stainless for £25 a meter and the same in constant radius bends at about £16 each (90 or 45). If i still have acess to a TIG i can weld it up for you or i know a man who will do it for about £40.

If you want to pop over at the weekend i can show you one that I've made for a friends 400bhp zetec turbo 3dr sierra.

As soon as I can I'll be up for popping over but I need the exhaust for the MOT so kinda stuck till it's done.

Got those sparks sorted yet?

Yes mate, fully standalone now mate, not had much time to play but it's getting better everytime and I've managed to get a base map from a guy in Sweden with a similar setup, low comp etc, so will bang that in later and see how that goes!!

[RowdyBurns]

Yes mate. Give me a bell when she has a ticket and we can go out and have a bit of a mapping session. See if we can get that ignition map optimised.

[WillG]

308 is good aswell, i think 316 is more for manifolds and 308 for the system. Im having mild steel manifold and 308 system

[Simon13]

am i missing something! u will need a silencer of somekind no?!

bigger bore will help but won't a high flow centre box of some kind be needed also? if u have standard type silencers with bigger bore pipe, wont it defeat the object of it all?