Dennis,

We tested the pipes using the stock length provided by MACS. You should not need to change it......however, everyone knows tuning a glow engine depends on a lot of things like fuel used, altitude, heat, and humidity. I'm no expert like some of the pattern guys are but try the pipe with the stock length. These are not true "tuned" pipes but rather "sport" pipes that give the best all around performance at all rpms.