I ran a custom 8" Coan behind my 355 on a 175 shot for several years. Went to a 9" Coan with my 427. Tried the 8" with the 427. It worked good on the motor only. Would not go below 6,500 on a 150 shot. Coan said they couldn't make the 8" any tighter so I had them build a 9" converter. Both converters worked great. They flashed nicely under full throttle but still only slipped 7% or less on the top end.
