Exhausted!

[Heuer]

I have been advised by Andrew at CMC that my beloved huge bore mandrel built exhaust system is actually reducing the performance of my engine. It sounds superb, obviously, and is great for tearing around the local roads and announcing its presence (106db at 1 metre!) but it seems time for a change.

Suggestion is to dump the tubular manifolds (headers for you guys in the US) and fit the standard Jaguar cast manifolds feeding a big bore stainless system. My current exhaust is steel and unprotected but refuses to rust. To quote Andrew from his report " I would recommend changing to a large bore stainless sports system using Jaguar original cast iron manifolds This would reduce heat radiation, cockpit noise and allow the engine to run more efficiently at sub 2000 rpm?s. Power loss above 4500 rpm would be minimal. But low down torque/drivability would improve greatly."

Seems these much touted open exhausts do little for the XK engine which rely on the narrow pipes to increase the speed of exhaust gas and improve low down torque. They add power above 4,500 rpm but it is actually rare to extend the 4.2 beyond this. It seems no-one has been able to improve on the Jaguar items. Car will be dyno'd before/after any change to see the difference.

Only problem I foresee is the less than attractive sound produced by stainless exhausts - they are not musical! The sound from a plain steel system is so much better.

So big bore steel and standard manifolds - any views or suggestions?

David

[Moeregaard]

David, this is not an unusual problem. Everything I've read says that tubular manifolds work well on XK engines if the secondary pipes are kept to the stock diameter. Using two-inch secondarys is what degrades low- and mid-range driveability.

If you read the Jaguar factory tuning manual for the E-Type, it becomes obvious that the real restriction in the exhaust system lies in the steel-packs under the floor, and Jaguar recommended replacing these with straight-through pipes, leaving only the resonators at the rear. I once tried this and, while performance improved throughout, the noise above 3,000 RPM was incredible. I found myself looking for routes with long tunnels! It may have been fun for fifteen minutes at a stretch, but neither the dog or girlfriend shared my enthusiasm.

In my eyes, the real benefit to tubular manifolds (aside from the look) is that they do away with the troublesome packing pieces between the cast manifolds and the downpipes.

[andyp]

David

If you were happy with them before the conversation with Andrew I'd leave them alone until they got rusty. Even then I'd pass on the Jag cast manifolds. Having seen pictures of your car I'm sure rusty, flakey cast manifolds would drive you mad.

Done neaty, exhaust wrap can look good (I favour the black). This would help keep the heat in the pipes which the theory suggest would speed the exhaust gases so perhaps make up what you are losing by having bigger bores. Obviously it would also cut down the under bonnet temp.

If I was looking to replumb my exhaust I'd have a look at E Type Fabs nice polished items.

Andy

[MarkE]

My understanding is that you want as smooth a flow rate (of the exhaust gasses) as possible, and that is affected by two main issues.

Firstly, bends will affect the flow rate. The fabricated manifolds are designed to smooth out the bends as far as is practical to minimise turbulence, and each of the ports has the same length / flow pipe to make them equal. This is where the cast iron manifold looses out, as the gasses have to suddenly make a 90 degree turn as soon as they are out of the head. Any turbulence caused by a sudden change of direction leads to a reduction in flow.

Secondly, temperature will affect the flow rate. As the gasses cool, they become more dense and slow down. As you mentioned in a recent thread, the fabricated manifolds get a lot hotter than the cast manifolds due to more effective conductivity of the thin-walled metal, hence they would seem to be less efficient. But if they are wrapped, they will lose less heat than the cast iron one which, combined with the lack of turbulence, will produce a better flow.

Once the gasses are away from the manifold, they must go into a similar sized pipe to complete their exit. Any sudden expansion (into a very big bore system) will again cause turbulence and rapid heat loss due to expansion, so will slow down the flow.

So everything has to be matched up. If you have a big bore exhaust system, you really need a big bore manifold, and the exhaust ports need to be opened up to match the manifold. If you use the cast manifold, then stick to the standard system.

I really do think that the bottom line is to go with what you prefer. The differences on a road car will be virtually imperceptible. Except for one thing?the feel good factor of the noise coming from the big bore system.

Mark

[Heuer]

Andy

Several people have mentioned E-Type Fabs for manifolds but all they seem to do is frames. Are people getting confused with this company?

http://www.classicfabs.co.uk/full_manifolds.html

David

[andyp]

I think I was David

[Moeregaard]

From what I have heard, the gentleman making the manifolds for E-Type Fabs passed away and they have not found a replacement. If you've ever seen the subframes produced by E-Type Fabs, you'll understand because their quality standards are extremely high.

[e-bygum]

Uryk (E Type Fabs) passed his exhaust business over to Classic Fabs some while ago because he could not replace his exhaust man and retain the quality. I believe Uryk did a tremendous amount of R&D with his pipes. Presumably his designs went to Classic Fabs so the exhausts that they make should be equally good technically if their quality control is equal to Uryk's. I have not heard of any comparisons being made between the two manufacturers.
Pete

[Moeregaard]

This is good to know. My post was based on information that was circulating over here a few months back, and many of us feared that stainless tubular manifolds had become a thing of the past. XKs Unlimited just released a set that look really nice and are priced very reasonable as well.

Apologies to the group if my statements were incorrect.