Originally Posted by
Speedracer1600
Hey all, what size restrictor is recommended for the 21-4c in a MP9 with a 9901 and a 41021 Pipe/header combo? Tried a search but came up empty. Thanks!
I used 7mm a couple races last season in my TKI2. It doesn't have the same top end compare to my OS speed so I switched it back. But I found the 21-4C work better in my STRR EVo with OS 2060 pipe.