at our track most are running lightspeed brushes. we are using lightspeed 3910, they produce more rpm then the 767, but are not as hard as the putnam greens. we have found good power out of f brushes on the neg and 3910 on the positive and very low wear

as far as springs, I have used anywhere from green\green to purple\purple. Usually purple is too much tension, but I have one motor that produces quite a bit more wiht purple\purple. like I can get 22k in rpm and torque is still quite good. but generally most of the time green\green to green\red.