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is it heat that causes manifold to crack or vibrations?

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Just looked at my Esprit after thinking about things.

We all have issues with the manifold cracking, it struck me that the exhaust on most cars is fitted on with rubber hangers, the reason being so that the exhaust is not solid and does not transmit the vibrations to the manifold and crack it. The Esprit exhaust is very solid, even though it does have a small rubber fitting according to the parts manual, is this what may be causing the cracks? as the vibrations would appear to be trasnmitted all the way through to the manifold.

I know a few owners have fitted stainless steel manifolds, but have also had splits etc appear. So this could well be the same issue.

Anyone have any ideas or am I talking pooh?

Chris

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My theory is a combination of heat and having to support too much weight from the turbo and a big part of the rest of the exhaust system....

Edited by lotus4s

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I always thought they had put it down to the fact that the road wheel sprays water on the hot manifold when its raining causing it to crack...

But probably just a theory that has floating about for a while, the turbo option sounds ok but n/a esprits suffer the same problem aswell,

Not that the manifolds where casted 100% either, Regards danny

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Manifolds on every car are bolted directly to the cylinder head, the Esprit is no different in this respect.

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Most cars have the final parts of the exhaust hung from the body of the car, the Esprit hangs from the gearbox cradle, which is bolted securely to the engine, so the only movement is that which is allowed by those rubber mounts you refer to. With most cars the engine would move a lot relative to the car, and that's why their exhausts need more compliant mountings.

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Thanks Andy,

That would explian why it does not move, as you say other cars are mounted on rubbers, your explanation makes sense.

Ta

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Hi Chris

Not sure if he actually did it, but when Dave Freeman was installing his S/S manifold he was going to have a flexi joint added for this very reason.

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Chris,

It is worth checking the condition of those rubbers which hang the exhaust from the gearbox cradle as Sparky and I have found a couple in the past where the rubber was separating from the plates which hold the mounting screws. That would definitely cause excessive movement.

Edited by wookie

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FWIW - I'll pass on some hearsay, well beyond my understanding.

A while ago now I was speaking to a guy who makes SS exhausts and manifolds for competition use and he suggested that the problem was probably down to stresses given the different expansion characteristics of the head and manifold materials. We were talking about a straight six unit not a four, but he suggested that the head would grow in length by several mm when properly hot whereas a cast iron (or SS) manifold wouldn't. Since they are anchored together there are significant stresses around the locating points/bolts. In fact he reckoned that cast iron is better at dealing with these stresses than SS - though of course at a weight penalty. The grade of stainless that he had to use for competition manifolds meant they cost several £'000 and he reckoned even they would only survive so many heat cycles - not enough for the life of a normal road car.

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Ralph spoke to a colleague of his, and was told it's quite normal for manifolds to crack where they do, as Mike stated repeated expansion and contraction of material occurs. One thig to minimise this is t ensure the exhaust manifold studs are not tight in the holes of the manifold, thus the manifold can move relative to the head.

The coefficient of expansion of ali is grater than cast iron and stainless steel (AFAIK), however the head is kept cool whereas the manifold is only cooled passively by heat conduction to the head and be passing air (and occasionally water). So my guess is that the manifold expands a lot more than the surface of the head, so the holes (or slots) in the manifold need to accommodate that rather than the head expanding more.

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