M50/M52 exhaust port diameter?

[e30gentleman]

I am going to build my own turbo tubular exhaust manifold, I am debating which to go with sch 10 1-1/2 or 1-1/4.
I would appreciate if anyone had the measurements laid out horizontally and vertically. I don't have the head with me yet :(

[e30gentleman]

I see some manifolds run with 1-1/2 and 1-1/4. I searched around and saw one of the flanges multiplex were selling with 1-1/4 counterbore.
I think that is best way to go with some step on top and bottom of the flange for anti-reversion and keeping the velocity high.
Anyone of you running 1-1/4 had the restriction perhaps past 600 whp?

[e30gentleman]

Mods can you please move this to forced induction forum? I was just asking for the diameter but obviously its related to FI now because of manifold design. Thanks :)

[e30gentleman]

Come on its not like everybody doesn't have spare head laying around lol. Positive rep if you posted up the specs!

[nando]

1 1/4 is way too small for basically any BMW engine. also, turbos are highly sensitive to exhaust restrictions. I'd go with 1 5/8 or similar.

[e30gentleman]

Interesting, 1-5/8 weld els will be hard to find though. 1-1/2 should be good enough? Bigger than common 1-1/4 most of manifolds have.
Can you recommend any website with 1-5/8 sch 10 butt welds? I'm having hard time finding those. I like acestainless.com for their 1-1/2 fittings.

I will be fabbing up 3.5 downpipe to race cat and resonator then transition to 3" oval straight to the back. 3" oval would be equivalent to 4 inch round pipe with plenty of clearance so it wouldn't be an exhaust restriction after hotside!

[e30gentleman]

I don't think 1-5/8 is good idea to lose velocity, 1-1/2 should be better option to keep speeds high, turbo response, and low end torque. I still need the measurements so I will confirm which diameter I will go with.

[nando]

on a turbo application, the turbo itself is a huge restriction. there's basically no velocity anyway, because it hits the turbo soon after it exits the port. you can do some pulse tuning with a twin scroll turbo, but otherwise it doesn't relaly work the same way as N/A with respect to velocity.

I've never seen a 1 1/4" turbo manifold for any BMW engine?

[e30gentleman]

I see, that's interesting. Lot of turbo guys says its evident that there are improved spool up with smaller diameter piping. 200 to 400 rpm earlier spool up wouldn't bother me though.
Here is the flange for 1-1/4 diameter by multiplex

http://forums.bimmerforums.com/forum/showthread.php?t=923295

After seeing that flange, 1-1/2 should be good enough according to port diameter otherwise he didn't match it to actual port size.