A C-6 is bigger, heavier, and beefier then a C-4. Both have 3 forward gears. A well built C-4 will handle 300 horse ok, the C-6 would only take some minor mods. Both trannies work good. I actually prefer the C-4 if it's going in a lighter car. C-6 will handle bigger horse and heavier cars and pickups, though.