there are different opinions bout the 7 vs 8 tooth, I have the extended case and C3 belt drive on a Mod Doo , running 156X3 .awesome track!. Anyway I ran her with 8 tooth and with 7 tooth, When i ran the 8,s the approach angle was almost too flat with very little ski lift, Too planted and nose would plow and the 8,s hung down so far they are exposed to every hit, the drivers were smacking everything instead of being tucked up and protected, The extended case and 8,s was not nearly a fun to ride, I put the 7,s back on. there was no performance change between the two with same track speeds. The extended case with 7,s works flawless especially with the belt drive.