Depends on the HFC and its’ cell count. I run a 3.5” 100cpi cat on my GTR with a custom 4” dump prior to the CAT that I can open and close. Did a back to back with dump open and closed on the dyno, which showed ~10hp drop when valve was closed and running through the CAT. That was at 700hp@wheels. If you have a 200cpi or less HFC I suspect you’ll see little to no gain with test pipe.





Red is 4" Valve Open
Blue is 4" Valve Closed