I think the only way we could ever get anywhere near the truth on these things would be to set up the following test. FIRST flow test the top 6 or so mufflers of the same pipe size and get some numbers. THEN test them on a dyno mule engine and see if flow corresponds to HP. THEN test different brands of the SAME flow to see if their claims of "anti-reversion", "pulse technology" or whatever actually help HP,, EVEN WHEN THE FLOW IS THE SAME. Until something like this happens by an independent tester who does NOT sell exhaust products,,, We'll never know!! Ya pays yur money and ya takes yur chance. I still maintain that flow is flow and NONE of that internal magic does ANYTHING except change the sound! But hey,,, that's just ME! Bruce Hevner -- No virus found in this outgoing message. Checked by AVG Anti-Virus. Version: 7.0.300 / Virus Database: 266.11.11 - Release Date: 5/16/2005