Exhaust manifold runner diameter
 

So I'm about to have a new turbo exhaust manifold made and I was presented with the options of either 1.25" or the standard 1.5" diameter runners. I tried to do some reading on this but am only coming across NA applications for runner diameter. I've seen some headway in a manifold thread, but it sort of ended.

Does anyone have any input on this?

When I was talking to a manifold builder he suggested I go 1.25

This was because there is no real top end gains with similar manifolds until you exceed 500hp on a 4 cylinder.

I know it's a very general statement but that's all the input I have to offer.

Matt

Is that due to these 1.8's in the lower hp bracket not being able to capitalize on the 1.5" diameter of the current manifolds?

1.25" runners can be ovalized at the ends to match the exhaust parts, AFAIK.

You don't want the exhaust gas to expand exiting the ports - the exhaust "puffs" lose energy doing that, which reduce the energy available for the turbine, and this affects spoolup.

This is correct measure the area of the port exiting the head at the flange. If you are going to neck it down to force it through a turbo at some point anyway then there is no pont in going any bigger pipes gatting to the turbo than this. More energy is lost by decreasing the velocity to flow in a bigger tube then accelerating it again to fit through the turbo than you will loose in skin friction forcing it through a slightly smaller tube at a more constant velocity all the way to the turbo.

I know my long equal length runner 1.25" schedule 40 weld ell manifold works well and spools a GT3071R quite well and makes plenty of power.

Bob

I tend to agree with the above points. I used a 1.625 runner on a previous manifold build. After switching to a 1.375 runner, the turbo spools 200-300 rpm sooner.

The spool seems more consistent as well. It used to spool slowly from 1-5 psi then skyrocket. The runner change seemed to alleviate this problem. It now spools up at a uniform speed from 0-20 psi.

I should point out that 1-1/4 shedule 40 pipe actually has a 1.375" ID which can cause some confusion. I think it is a real good match for the outlet of the head.

My manifold has eqal length runners 16-1/2" long from the head to the turbo flange made of 1-1/4" shedule 40 pipe. nice linear spool to a 3071R turbo and it is making over 3000ft-lbs of torque by 4000 rpm at 18psi with the 2.0L. The spool is still good with a stock enternal 1.8L.

Bob

Incredible!

The few manifolds i've made using a weirtech head flange and 1.5" schedule 40 pipe the tubes were slightly smaller than the ID of the exhaust primaries on the flange. I actually had to port them a little bit with a dremel. More so on the 1.6 than 1.8. However, schedule 10 pipe does leave room for the exhaust to expand once it leaves the head.

damn. like a freight train engine swap.

oops got an exta 0 in there.

I used a JGS flange.
http://www.jgsturbo.com/index2.html

It comes with the ports machined to adapt a gasket match on the head side and almost perfectly fit the 1.375 Id round pipe on the other. Some minor smoothing done.

Bob

It appears my ETD was 1-1/4 "STD" pipe which happens to be the same wall as sch40. it's 1.38 ID.

http://gallery.y8s.com/d/394-3/waterlines03.jpg

So far all of mine have been 1.5" sch. 40 pipe (well I did do one sch 10). It matched the profile of the port better when ovalized with a bearing press. But I can see 1.25" pipe being better to a certain extent.

Damn and I thought you were even studlier than I previously thought.

Those JGS flanges sure are slick but I just cannot justify the $80 price tag when weirtech flanges are 22 dollars.

Well, I've decided to go with the 1.25". I hope it does great things.

I had a thought today and I was wondering if anyone had tried 1" with a flared end toward the head. In my head it seemed it would help with gas velocity and overall manifold dimensions.

Any experiences?

have not done it, but purely based on flow you would be capped at around 250whp just because of the flow restriction based on the cross sectional area and experience on pipe size vs power.

you wont gain much in terms of response. maybe 200rpm.