Header Runner internal diameters

[Mark Halligan]

Hi,

Does anyone have any idea which would be the best size for the internal diameter of the runners of a tubular exhaust manifold? I thought that the best size would be ~38mm (the same as the exhaust port on the head) but a few that I have seen on offer have a smaller diameter than this.

I would have thought that this would cause problems with flow due to the 'step' that the gasses would hit on leaving the head, or am I missing something?

Cheers,

Mark.

[Doug_RS2]

Does your MTM manifold not have a step in it? I am sure I saw that when it was off the car? I would have thought at least matching the ports would be the better way to go.

Are your exhaust ports noticeably large after your fiddling?

(Not much help!)

[twoqu]

Mark,
I started a thread a while back.

http://www.s2forum.com/forum/showthread.php?t=5140&highlight=manifold

I'm still confused over the firing order issue into the collector. Of all the manifolds I have looked at including the IMSA and ones from Lehmann, only the one on the 25V engine followed the firing order around the collector. Perhaps that isn't too critical on a turbo engine. But if you are going to do it, I guess the policy is too "reverse engineer" something that is known to work.
I think this is in the realms of another "black art". Anti-reversion, stepped secondaries, tuned lengths, collector merging, materials, coatings.....Agghhhh!

[S2central.net]

Agree with Robin's 'black art' comments... This is non trivial stuff. But my gut feel is that exhaust ports matching the manifold is the most logical way to achieve optimal flow. I'd need to hear a thoroughly convincing reason for doing it differently from that.


Paul

[Mark Halligan]

Are your exhaust ports noticeably large after your fiddling?

No I left the diameter the same, but the measurements I took are off of my spare cylinder head which is untouched.

[repo]

I had an Audi head inspected and flow tested by a Finnish wizard(Kansiviritys J. Kosunen) who has achieved VERY high torque values from his sprintrace-engine-projects.

We discussed about this issue a lot, what he claims is: Even when ported, the exhaust valves will not flow more than 34mm inside diameter tube will.

So, when You push the gas through the valves it turbulates a lot, this way requiring some space for the cylinder to push the gases out freely enough.

After this, the flow needs certain amount of flow capacity. By exeeding that You only get the velocity of the flow slower, but get no advance. This head - porting specialist's opinion was, that everything exeeding 35mm would be a waste of precious kinetic energy of the exhaust gas in these Audi multivalve - engines.

Step from exhaust port to runner is not what we want, but smooth line from 38mm to 34-34mm.

When we discussed about this, we "were building" +700hp 5-cyl race engine.

I have no first hand experience in differences of this magnitude, when it comes to very high power, but in 20v 1.8T my personal experience is that 34mm tube spools up quicker without compesating the top end. At least in 350hp level.

Exhaust ports are closely similar in 5'ers and in 1.8T's I think...

Any better ideas are welcome.

Edit: Called the man and checked the dimensions...

[twoqu]

Hi!,
The Sport manifold was supposedly developed to support 500bhp. It has 39mm ports at the flange!
Like Doug, not sure whether that is of any use!

[AudiS4]

Repo, do you have any more torque/hp data from this engine? Do you know anything about the importance of an equal length runner comared to one "as short as possible" type?

I had an Audi head inspected and flow tested by a Finnish wizard(Kansiviritys J. Kosunen) who has achieved VERY high torque values from his sprintrace-engine-projects.

We discussed about this issue a lot, what he claims is: Even when ported, the exhaust valves will not flow more than 34mm inside diameter tube will.

So, when You push the gas through the valves it turbulates a lot, this way requiring some space for the cylinder to push the gases out freely enough.

After this, the flow needs certain amount of flow capacity. By exeeding that You only get the velocity of the flow slower, but get no advance. This head - porting specialist's opinion was, that everything exeeding 35mm would be a waste of precious kinetic energy of the exhaust gas in these Audi multivalve - engines.

Step from exhaust port to runner is not what we want, but smooth line from 38mm to 34-34mm.

When we discussed about this, we "were building" +700hp 5-cyl race engine.

I have no first hand experience in differences of this magnitude, when it comes to very high power, but in 20v 1.8T my personal experience is that 34mm tube spools up quicker without compesating the top end. At least in 350hp level.

Exhaust ports are closely similar in 5'ers and in 1.8T's I think...

Any better ideas are welcome.

Edit: Called the man and checked the dimensions...

[repo]

Repo, do you have any more torque/hp data from this engine? Do you know anything about the importance of an equal length runner comared to one "as short as possible" type?

This was a virtual motor we "built" IOW we discussed about this matter with 700hp engine in mind.

The only +700hp 20v 5'er project I have participated in is mr. Lindström's blue Super Saloon S2, and with that we have tested some 5 different exhaust mannies. Unfortunately all of them were 42 x 2,5mm units.

The importance of the equal length of the runners is exaggerated a bit in my opinion. The funny thing is that You can actually hear from the exhaust sound if the runner length is equal: compare TransAm exhaust sound to IMSA one, and tell me which one has smoother exhaust sound.

Getting the pulses equally stated into a turbine sounds to be more critical than it actually is. When You use somewhat small or mid-range sized turbo, the difference in behaviour is not that big whether the runners are equally length or not. Especially peak power does not differ almost at all, this in case the headers are not restrictive in any manner.

What the length of the runners does do, is give better spoolup. But this difference comes apparent only with reasonably big turbos.

Most important thing that affects the charasteristics of the whole setup is the collector design. Linear collector with minimum amount of unnecessary inner area gives best results. Wich high-rate cams it's critical to get secondary pipe (at turbo end of the collector, if there is any) correctly matched, to avoid choking, but still keep it as narrow as possible to maintain the exhaust gas velocity.

Dunno if this helps at all... Just some messy thoughts written in a hurry....

Forgive me if my sentences are incorrectly built, English is not my mother tongue, and these things are a bit complicated to describe in simple manner.

[Mark Halligan]

Repo,

You have mail.

Cheers,

Mark.

[Jamo]

I had an Audi head inspected and flow tested by a Finnish wizard(Kansiviritys J. Kosunen) who has achieved VERY high torque values from his sprintrace-engine-projects.

We discussed about this issue a lot, what he claims is: Even when ported, the exhaust valves will not flow more than 34mm inside diameter tube will.

So, when You push the gas through the valves it turbulates a lot, this way requiring some space for the cylinder to push the gases out freely enough.

After this, the flow needs certain amount of flow capacity. By exeeding that You only get the velocity of the flow slower, but get no advance. This head - porting specialist's opinion was, that everything exeeding 35mm would be a waste of precious kinetic energy of the exhaust gas in these Audi multivalve - engines.

Step from exhaust port to runner is not what we want, but smooth line from 38mm to 34-34mm.

When we discussed about this, we "were building" +700hp 5-cyl race engine.

I have no first hand experience in differences of this magnitude, when it comes to very high power, but in 20v 1.8T my personal experience is that 34mm tube spools up quicker without compesating the top end. At least in 350hp level.

Exhaust ports are closely similar in 5'ers and in 1.8T's I think...

Any better ideas are welcome.

Edit: Called the man and checked the dimensions...

Very interesting post here from Repo