Cast Manifold Problems
Westfield (and many other companies) offer some cars with a pair of cast manifolds. These present two main problems:
Westfield use 2 offside manifolds for the Seight instead of using a proper pair of nearside and offside manifolds. The reason for this is that the offside manifold is considerably smaller than the nearside version and fits within the engine bay more easily. The problem is that this manifold pairs cylinders 1&3 and 5&7. Because 7 follows 5 in the firing order this is not a good thing. This means that they are both on the exhaust stroke at about the same time (cylinder 5 starts it's exhaust stroke 90 degrees before cylinder 7) and hence both are trying to dump their gases down a pair of manifold pipes that join up into one (small) bore pipe after a few inches. The correct manifold for the nearside of the engine pairs cylinders 1&5 and 3&7.
The cast manifolds have very short primaries and are very small bore. This is bad enough on a standard head, but on a ported head it is difficult to open out the manifold to a large enough diameter as the wall is not thick enough once inside the manifold. Here is a cast manifold being opened out to match up to a ported head. An old gasket has been used as a template after it has been opened out to match the port on the head. Note the lack of available material on the manifold.
How much power will we free up if we replace the incorrectly paired manifolds with a decent 4 into 1 tubular system ? The following graph shows the power increase on a tuned 4.2 litre engine. Originally it had a pair of ported cast manifolds. These were replaced by custom built 4 into 1 manifolds with 1.75" primaries of 32" length and 2.5" bore systems.
Most Seights are not fitted with a balance pipe as the lack of space makes the fitment of one a difficult proposition. However the benefit of balance pipes to low end torque is well understood for a V8 engine. This picture shows a Seight (David Stephen's 4.8 litre car with Wildcat cylinder heads) which has had a balance pipe fitted. The 2" pipe passes through the bodywork, under the dry sump pan, and out through the bodywork on the other side of the car. Ground clearance is not reduced as the bellhousing remains the lowest part of the car. Here are power and torque graphs for this car before and after the balance pipe was fitted. The only other significant change to the engine was the addition of larger bore main pipes and silencers, up from the standard size to 2.5".
The standard Westfield side exhausts produce about 98db(A) when in good condition. This can rise to 100-102db(A) as they wear out. These readings are at 4000rpm and 0.5m from the exhaust. They should present no problems for competition but may be dodgy at places like Castle Combe for track days.
The tubular manifolds provided by Westfield have quite small openings. With standard heads this is not a problem but with ported heads they are often too small producing a step that can kill top-end performance. There is sufficient material available on the flanges to open them out if necessary. This is a particular problem if your head has curved roofs on the exhaust ports. Having modified the flanges make sure the gaskets you use are big enough. Standard SD1 gaskets are far too small for a ported head and even the latest Land Rover 4.6 gaskets are sometimes not tall enough for ports with curved roofs.
© Ian Crocker
Last updated on 24 Apr 2001