I would like to see more then two runs to get an idea of the HP gains. Four HP seems to me to be within a range of error even if nothing was touched. Yes, there might/should be a gain but I think it is negligible. On my car I can hear the difference but I do not feel/see much of an increased flow of exhaust coming from the side with the discs out. It would be nice but 10 or even 4 would be a lot, I could be wrong but I just don't see it.

What Gary is talking about would make a much bigger difference I assume.

 

I wonder if the quad pipes coming right out of the crossover in the H-pipe takes advantage of the reason the crossover is there in the first place and enhances the effect. Look at the graph - the curves separate at two points and if you do a little quick math you'll note that the second area starts & ends at twice the the RPM of the first area and comes on right at the point where the curve starts hitting peak power. Perhaps the Ford engineers know something about pipe resonances? It could just be a coincidence, but I thought it was worth pointing out.

 

I think the Ford engineers knew exactly what they were doing here.