The most reliable automatic transmission Ford has ever produced has been the C4 Cruise-O-Matic. The C4 is a simple three-speed automatic transmission you can rebuild in your home garage with common hand tools. Now you can improve its performance dramatically with new parts from B&M Racing & Performance and Mike's Transmission.
Before you even get started, it's important to understand the basics that make C4s different from one another. In the very beginning (1964), the C4 was a dual-range automatic transmission. It functioned in two ranges, depending on where you positioned the shifter. The shifter had a large dot and a small dot. If you put the shifter on the large dot, the Dual-Range C4 would do a normal 1-2-3 upshift. Placing the shifter on the small dot kept the transmission in second gear, for slow, slippery road driving.
Ford quickly figured out the Dual-Range was twice the headache for motorists and transmission shops alike. People would place the shifter on the small dot and never get out of second gear, taxing both engine and transmission. In 1967, Ford phased out the Dual-Range C4 Cruise-O-Matic, going to a more conventional valvebody and shifter with a P-R-N-D-2-1 shifter pattern that was easy to use. Placing the shifter in "D" delivered a normal 1-2-3 upshift. Moving it to 1 or 2 got you a lower gear range for slower driving or towing.
In 1970, Ford made significant improvements to the C4, including a larger input shaft, better shift programming, and a host of other changes that refined it for better driveability. In the '80s, Ford gave the C4 a locking torque converter for greater efficiency and renamed it the C5. It was found mostly behind the 3.8L Essex V-6 and some 5.0L small-block V-8 applications. The C5 is not recommended for performance use.
We're going to address how you can improve your C4's performance by showing you what the pros recommend for performance or just plain cruising.
Over the C4's two-decade production life, there were a number of significant changes you need to be aware of, especially if you're shopping for a core to build, and parts to build it with. The '64-'66 C4 transmission has the dual-range valvebody, which is not what you want for your performance build. C4 transmission cases can be identified by their casting numbers, located on the left-hand side of the case. If the casting number begins with C4, C5, or C6, that is the C4 transmission to avoid. However, having an early C4 case doesn't make these transmissions a lost cause. You can use a '67-up valvebody, which gives you the P-R-N-D-2-1 shift pattern and performance. These valvebodies are plentiful.
Beginning in 1970, Ford went to a larger C4 input shaft, which changed everything from the torque converter to the forward clutch package. It becomes beneficial to go with the larger input shaft and forward clutch assembly whenever you're building more power into your in-line six or small-block V-8. But, it's more than that. Rebuild kits and performance parts for early C4 transmissions are becoming harder to come by. For example, B&M makes TransKits, Shift Improvement Kits, and torque converters for '70-up C4 transmissions only. This means your early C4 Dual-Range transmission will not accommodate the B&M kits and torque converters.