Thanks for the replies.
yes, I guess that's what I'm after. The motor is going into a triumph spitfire and I purchased a s/h set of extractors but the angle sloping towards the rear was too shallow and it fouled the fire wall. I will sell the extractors as soon as I've solved this issue. I've been told that these manifolds are 'fairly' good in terms of flow. I know that the GX manifolds are probably better, but I'm better off with the single pipe setup as room is pretty tight between the chassis and motor.