its a dicey subject, but having a single tube per exhaust port is beneficial no matter what design they take after this point. some standard manifolds (like the 20.8 v for instance) are just a big collector on the exhaust ports merging immediately into 2 downpipes. that kind of design leads to the exhaust equivelant of charge robbing. i.e the pulse from one exhaust port impeding flow from another. perfectly fine on a standard engine, but when you start upping the engine throughput its becomes a big issue.

 

so it sounds as if its the construction that is the effecting factor
cheers odders

 

The rough rules are just rough rules. A particular design can be developed on a dyno to be good at whatever it's supposed to do. As long as you enquire what the manifold is supposed to do then you will be alright. If it's a 4-1 meant for high revving race engines then chances are it'll be crap at low revs. If it's a road car 4-1 tubular then it'll hopefully add a bit everywhere.

The 2.0 8v cast 4:2 parts are supposed to be pretty good. It's the 2:1 downpipes that restrict. The charge robbing is beneficial. The flow of gas from one of the cylinders creates low pressure that helps pull the gas out of the other cylinder. Obviously that depends on a specific rev range so it probably is restrictive at high revs.

 

cheers Tom