Always replace the torque converter. Why? Because stock torque converters won't hold up in performance applications. Aftermarket torque converters, such as those from B&M Racing & Performance and Mike's Transmissions, are designed for high-performance applications. They have furnace-welded fins and a tougher one-way clutch, which stay together under demanding conditions. The purchase of an aftermarket torque converter also allows you to dial in the stall speed, which gets the engine's power band more in synch with the transmission. The stall speed is the rpm range where the torque converter begins to transfer power to the transmission. This is the point where you step on the gas, and the engine begins to move the vehicle. For example, a 2,400-rpm stall speed torque converter begins to transfer torque at approximately 2,400 rpm. A stock torque converter's stall speed is typically around 1,500 rpm.
When you are shopping for a torque converter, shop stall speed while you are at it. If your engine's peak torque comes on strong around 5,500 rpm or higher, you want a torque converter that will allow the engine's revs to become higher before the torque converter begins to transfer power. This is why racers opt for torque converters with 4,000-rpm stall speeds. They get the engine revving at 5,000-plus rpm and mash the go pedal. The torque converter immediately transfers the engine's peak torque to the wheels.
If you are building an engine more for street use and cruising, a 4,000-rpm stall speed torque converter will not serve you well. You're going to need a stall speed between 1,800 rpm and 2,400 rpm for best results.
Whenever you are building a C4 transmission, always opt for components that are matched by the pros for best performance. When we consider this approach, we think of the B&M TransKit because it has matched components designed to work together in a performance application. B&M throws in additional clutches, as well as a Shift Improvement Kit, to get you back on the road quickly. As you can see from this C4 TransKit, it's a simple approach with all the parts necessary to do a complete rebuild.
A good rule of thumb for transmission building is extra care. All seals and gaskets need to be treated gingerly. A carelessly installed o-ring seal can be pinched or torn, which will cause internal leakage and, ultimately, clutch or band failure. Good sealing is critical with hydraulic systems because solid hydraulic pressure is what keeps clutches and bands firmly engaged. Soak the clutches and seals in transmission fluid before assembly. This ensures they're lubricated and ready for action.
Here are the two input shaft sizes, side by side. On the (left) is the '64-'69 input shaft. On the (right) is the larger '70-up shaft. Using the '70-up shaft in an earlier C4 means swapping the forward clutch pack as well. Ideally, you would use '70-up internals as a complete package whenever you opt for the larger input shaft.