Quote Originally Posted by Jenson View Post
What your asking is do the High Velocity Merge Collectors help. Yes they help scavanging a good bit, in a comparison we did on an engine dyno we saw a 5Hp gain across the board and 7 peak using a burns HVMC (no spike) versus a standard stamped 4 into 1. This was on a 10.0:1/CR 2.3L. As Mike said, primary length and size are the biggest part, but I believe that the way the header comes off the head plays an important role too. Its just a theory of mine, one day I'll be able to test it on the flow bench. Like your second picture posted, that comes off the head very nicely. You dont want to just go 90 degrees out the head. The larger primaries are required once you start moving more air like going up in displacement.
Jenson prob has way more experience in this area than i do as well as Chunky on here too so you may want to get more in depth answers from them.

Jenson is right, the angle off the head is very important too. IMO its all about carrying the velocity out of the head through the primaries to the exhaust. The collector certainly makes a difference, but i think the "power" is well decided before then.