This is one of the tests that need to be doen at the track on a back to back basis, hard to do in todays world unless your at a test and tune day where you can do run real close to each other We use to use a 18 inch, 12 inch and 6 ich colleteor extensions on both of the Hemi Stockers, 727 and 4 speeds, and the 18 inch was always faster After discusing our results with a SS/A 4 speed racer he had some big, 4 inch diameter, 18 inch long H pipe and extenions made up for his car, it went faster also