27. Normally, stock TH350s have a spring in the manual low control valve that automatically changes gears at a predetermined rpm. However, the B&M TransKit replaces the spring with a metal sleeve that blocks the valve, allowing the transmission to remain in First gear no matter how high the engine rpm goes. This is beneficial for racers who want to manually shift (at a specific rpm) their cars at the dragstrip for maximum power and lowest e.t.'s. But if you put the trans in Drive, the unit will shift normally for around-town driving.