Turbo Exhaust Size?

[crazymonk]

Hi everyone, I'm using the M52 engine, (2.5L 24v non-vanos), for a turbo conversion, it's got loads done to it, and is using a Garrett GT2871R Turbo, but does anyone how to work out what size exhaust tubing to use for the downpipe and then the pipe running to the rear silencer. I'm using a custom exhaust manifold shaped so that all of the primaries going to the turbo are of equal length. I'm trying to do the best I can with the conversion, but I haven't got endless money to make the downpipe, and the rest of the exhaust system in loads of different sized pipes to see what works best, which is the rubbish advice I've been given so far. I'm aiming for abour 330 to 350hp if that helps at all. Any advice would be great.

[Turbo-Brown]

Fit the largest down pipe and system you can would be my advice.

You're looking to generate the largest pressure differential accross the turbine housing you can and to that end, the best exhaust would be no exhaust at all. Obviously that's not practical so the next best thing is to make it a good 3" or so (that will fit BTW )

[blingsta]

mines a 2.5" system all the way, ive been told and also read that a 2.5" system is good for upto 500bhp and anything more than that you will need a 3" system...

[keggy]

whats the spec on your engine?

[Gunni]

mines a 2.5" system all the way, ive been told and also read that a 2.5" system is good for upto 500bhp and anything more than that you will need a 3" system...

well who ever told you that was just lying or doesn´t know any better when in regards to turbo exhausts.

It has nothing to do with what anything can flow but the pressure release from behind the turbine wheel, the better, the effeciency of the turbine section gets better and results in better scavenging and will result in quicker spool and "better" HP.

Turbo Brown nailed in the first post.

[e30_Turbo]

I've got three inches

It's about the only place on earth I'd big up three inches tho

[Colonel Sanders]

I've got three inches

It's about the only place on earth I'd big up three inches tho

me too.
but its cost me alot of cash though!!!

[Turbo-Brown]

Have you guys squashed the pipe as you go under the rear subframe out of interest?

[Colonel Sanders]

i went 3" downpipe then into 2.5" under the car, as long as the downpipe is big for the really hot gas..
3" is a real pain in the arse though, if your not lookin for over 300whp i would stick with 2.5" downpipe.

[crazymonk]

As a reply to Keggy, the spec on my engine is this,
M52 2.5L 24v engine non-vanos
Some internal tweaks (arp bolts, headwork etc)
E36 M3 Throttle Bodies Match Ported
Custom Inlet and Airbox
Forge Rad, and custom Forge Intercooler front mounted at the front below and infront of the radiator but runs almost from one side of the car to the other
Custom Six into Two into One exhaust manifold with all of the primaries equal lengths
Garrett GT2871R with the A/R .86 version
Turbo Spec Piper Cams
Emerald ECU
444cc injectors
I've read a few things that say anything over 300hp needs a 3" downpipe and system. Some places talk about running a twin pipe system after the downpipe, is this any good or would a 3" system be better?
Also has anyone ever run an intercooler and a charge cooler together?

[fuzzy]

although my engine is obviously different ive got a straight through 2.5" system with a single rear silencer good for about 350bhp ish im told. i was also told that if you go to big you lose the backpressure you need to keep the seals in the turbo tight. boll0cks or not i dont know.
has anyone kept to the original twin 2" pipe system used on the 325 rather than a larger single pipe? 2 x 2" pipe should be better than a single 3" pipe?

[Turbo-Brown]

The standard 325i system is more like 1 3/4" so it's not much cop really.

I made a twin 2" system for mine, but that works out a reasonable amount smaller in terms of available flow area than a single 3" system.

I've heard some of the 205 turbo people talk about needing back pressure to stop seals blowing. IMO, you shouldn't be relying on gas pressure in the exhaust to stop that!

[fuzzy]

I've heard some of the 205 turbo people talk about needing back pressure to stop seals blowing. IMO, you shouldn't be relying on gas pressure in the exhaust to stop that!

this is something that a tuner told me but considering the bullsh1t that some of them talk i wouldnt believe it to much but it was said that standard exhausts are designed to allow a certain amount of back pressure for this reason
i dont know as i only break them. i get others better qualified than me to fix them

[e30_Turbo]

Have you guys squashed the pipe as you go under the rear subframe out of interest?

No, that's held up with a bracket and hangs about 1/2 inch from the subframe, as yet it's not been smashed off, one day a speed bump will get me though....

[Turbo-Brown]

Awesome, I was having visions of needing to fabricate a fan shaped thing to go under the frame which kept the area the same for a reduced height