Yeah I was researching that point for the longest time on why stillen and motordyne made so much compared to the rest. And almost all the other exhausts used the Hpipe or x pipe design. Granted in theory they would make the most HP and tq design, they don't. Now this is actually where getting into power under the curve matters... An X pipe/ypipe design will give you the most power down low, because the exhaust gases and closer to mixing or mixing directly which in turn creates back pressure without the use of a muffler or resonator. So the exhaust gases are not slowed down by the packing, which helps with exhaust scavenging. The h pipe helps in high rpm power cuz it equalizes exhaust pulses and they will flow better in upper RPM.
The point I cannot directly explain is how motordyne, and stillen still make the most power in the high rpm utilizing the ypipe design. Granted motordyne uses the indirect resonator on the tail pipes(I forgot the actual name for that) and it creates even more exhaust scavenging. These points are why I chose the ypipe design with a non resonated midpipe and two straight through mufflers as my exhaust of choice. Least path if resistance for me.
