Exhaust Header Design. 3

[scrimshaw]

Hello again
Once more I need some technical help from the experts this time on a header system that I am looking at building.

I have a fairly old straight six engine with an aluminum cylinder head that comes with two cast iron manifolds. I want to replace this with a stainless header system and a one piece flange - all stainless probably about 3/8 or 1/2 inch thick. Anyone think of any reason not to go with a one piece design?

Also I have looked into some after-market stainless systems that use a mild steel flange. Apparently this is to do with "heat absorbtion or something" according to the salesperson. Can someone give me a better technical explanation for this? These are high end systems so I doubt it's a cost issue but that is always a possibility I suppose.

I am not worried about tuning primaries, collector box sizes etc., optimum power output is not our primary concern. Thanks for all help.

 

[MikeHalloran]

A thick one-piece stainless, or steel, flange may, potentially, warp the head, or cause the gasket to slip. ... which may or may not induce failure of the gasket.

I'd be inclined to make the flange axially compliant, e.g. by laser or waterjet cutting it with slots between the ports. The idea is to allow the port flanges to act semi- independently of each other.



Mike Halloran
Pembroke Pines, FL, USA

 

[Bribyk]

One aftermarket header design for similar cast-iron head engines that I have seen used a two-piece flange split between ports 3 and 4, presumably for the reasons Mike stated. It also used slotted or half-moon mounting bolts to allow for expansion. Since you have dissimilar materials I would be inclined to use a two- or three-piece flange to allow enough differential expansion.

Brian Bobyk - Hoerbiger Canada

 

[Tmoose]

The first year or two after Audi came out with their inline 5 cylinder our metal disintegrator division burnt out MANY broken exhaust studs for the local dealer. Always end studs, as I recall. No idea what the manifold configuration was.

 

[scrimshaw]

I must be missing something.

Why would a (relatively thin) stainless flange cause these problems when the cast iron block does not, if the block has a lower rate of expansion and has a larger temp difference to the Al. head?

Also why would a manufacturer use a mild steel flange on a stainless header system to overcome these problems.

As always all input appreciated.

 

[MikeHalloran]

Because of the intimate contact and the coolant, the temperature difference between block and head is not that great, and because of the relative masses, the gradients are smaller.

Stainless costs a lot more than mild steel and is harder to punch or cut. It's also weaker, at engine block temperatures. If you're going to put a ceramic coating on the header anyway, the difference in corrosion resistance of the steel makes no difference.



Mike Halloran
Pembroke Pines, FL, USA

 

[izzmus]

The Audi exhaust manifold is just a log. The one that I played with located the manifold via the center port's stud holes, and the other eight holes were significantly larger. Cone washers, locknuts, and expensive-looking multipiece gaskets were also in use.

In the aftermarket, they just use five individual port flanges.

I'm curious as to which engine the OP is designing for.

 

[Tmoose]

"Why would a (relatively thin) stainless flange cause these problems when the cast iron block does not, if the block has a lower rate of expansion and has a larger temp difference to the Al. head?"

Aluminum heads on iron blocks forced gasket engineering to a whole new level. Well into the 80s Toyota was having difficulties keeping their 4 cylinder engines head gaskets in place beyond 100 kmiles. The regions responsible for containing the coolant were wearing out, or something.

Underestimating thermal expansion can be costly, or at least create a lot of extra work. Thoughtful Flexibility can take take most of the sting out of it.