Just wondering what are the best 4:1 merge collectors for our cars. Looking for 1.75" primary and 3" at the collector. I will be having a set of custom headers built based on the SLP style.

From what I have seen the slip on style seem to be best but the internal design matters a lot for horsepower. Need one with the least amount of turbulance yet not overly long to fit a 3rd gen with dual cats and a single pipe.

I notice that Hedman, Hooker and Edelbrock make some that look right on the outside from the pictures but not sure what the design is on the inside.