Looks like my engine is coming out so it seems to make sense to change the manifolds if I'm going to.
The question seems to be is there really any advantage to a "performance" tubular stainless manifold on a brick and if so which one. Janspeed do one as do rimmers etc. Is it the same part for a 4.2 as a 3.9?
Any suggestions?
A basic understanding of engine tuning and exhaust dynamics will help loads in this regard.
Afraid I don't know what spec engine you have, or what you are planning.
The main aim in exhaust tuning is:
-more flow
-scavenging
Flow rate is important, but only to an extent. Forced induction setups will generally want the most flow rate possible.
Scavenging however is very important on n/a engines and can release a lot of performance potential. Essentially what this aims to do, is the exhaust pressure wave from one cylinder, helps 'suck' or pull in air to the cylinder with the inlet valve open. This means you get more air into the combustion chamber, and essentially a bigger bang and more power.
A good exhaust will be part of entire setup, so the rest of the exhaust, the induction and the engine spec can all play a part. But good exhaust manifolds can and will yield good gains.
I don't know what is available. But you want something that has long primary pipes, this will promote scavenging at lower rpm. Short primary pipes look cool and will out flow factory log style manifolds. But yield little to no gains in power unless you are running forced induction (turbo or super charger). Short primary pipes do scavenge, but only at high rpm, maybe 8000rpm or higher. Hence their limited appeal on a OHV V8.
Long primaries come with the downside of costing and packaging. Which is why they are normally rarer.
Engine displacement doesn't really have a bearing on exhaust manifolds. Not at this level anyhow.
A good set of manifold will result in the car needing to be tuned however, as the fuelling will be out. And don't believe anyone that tells you that you need back pressure on a petrol engine.