Jim Smart
April 1, 2004

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.

Changes-C4 Yourself
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.

Torque Converter
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.

TransKit It
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.

The intermediate band servo size is critical to performance. The 289 high performance V-8 C4 was fitted with the popular "C" servo, which was the largest size available. Shown here are two C4 intermediate servos. On the left is the small "A" servo, common with most C4s. On the right is the "H" servo, which has a large diameter bore and piston to improve band-holding pressure. It just isn't as large as the "C" servo, but it is plentiful and cheap. The C4 has two bands. The intermediate band holds the reverse/high drum, while the low/reverse band holds the low/reverse drum. Believe it or not, the best intermediate band available for the C4 is the original Ford type because it is so durable. Use the best friction materials available for your C4 performance build. Mike's Transmissions suggests dressing the drums before assembly, which improves band hook-up during upshifts.

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This is the early C4 transmission case, identifiable by its vent tube. The C8DP part number tells us it is a '68 casting.

When the C4 was first introduced for the '64 model year, it had a smaller, five-bolt bellhousing for the 260/289ci small-blocks of the era. Beginning in 1965, C4s and small-blocks got a larger six-bolt bellhousing pattern to reduce noise, vibration, and harshness. The five-bolt bellhousing is not shown here. What is shown is yet another difference, known as flywheel size. This one will stump you more times than not. Two flexplate (flywheel) sizes were used-157 and 164 tooth. On the right is the smaller 157-tooth flywheel bellhousing. On the left is the larger 164-tooth bellhousing. Most Mustangs had the 157-tooth bellhousing and flywheel.

In the '70s, the C4 transmission case changed, deleting the vent tube and moving the vent to the tailshaft housing.

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With the changes made to the main transmission case also came changes to the tailshaft housing. Early C4s, with the vent tube, didn't have a vent on the tailshaft housing (right). When Ford deleted the vent tube, it went to a tailshaft housing vent that looks like a mushroom (left) on top. This vent allows transmission heat to escape.

Here are the two flexplate (flywheel) types. Above is the smaller 157-tooth flexplate. The larger 164-tooth flexplate is pictured below.

Mike's Transmissions in Lancaster, California, has a dust shield designed to fit both the 157-tooth and 164-tooth bellhousings. All you have to do is cut the dust shield for your 164-tooth application. Take a close look where the starter fits, and you will see the perforation for your cut.

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Another important difference with C4 transmissions is the vacuum modulator, which controls shifts points based on intake manifold vacuum (load and throttle position). Early C4 transmissions have a screw-in vacuum modulator. Later models have a press-in vacuum modulator held in place with a bracket as shown.

Greater Friction
When we improve C4 transmission performance, we do it by getting power to the rear axle reliably. In its most basic form, the C4 has room for additional clutches. The more clutches and plates we can add to a C4, the more bite (less slippage) it's going to have. The forward clutch has room for more clutches and steels. So does the reverse/high clutch. This will greatly improve the transmission of power. Another performance trick is to sand the clutch steels, which offers the same result as refacing a flywheel or turning brake rotors. It roughs up the surface for greater friction and hold.

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Install A Shift Improvement Kit
A B&M Shift Improvement Kit isn't just about racing or blasting from traffic light to traffic light. It is about longer transmission life. In stock form, automatic transmissions have a certain amount of clutch and band "slip" programmed into their valvebodies. This gives a smoother upshift. However, every time there is slippage during the upshift, you are wasting clutch and band friction material. Friction material works its way into the transmission fluid. Think of the fluid as a friction material freeway, carrying this material throughout the transmission. Friction material tends to cut the seals, which allows hydraulic pressure to escape, causing clutches and bands to slip even more. Eventually, the slippage cycle becomes so bad that clutch pistons and control valves stick, or fail to function at all, causing transmission failure. Installing a Shift Improvement Kit isn't just about getting there faster, it's about getting there in the first place.

Cooling It
Through the years, we have consulted with dozens of automatic transmission shops. When we asked them what the greatest reason for transmission failure was, they have all said the same thing-heat. Heat kills automatic transmissions. Ideally, your C4 transmission should operate around 250 degrees F sump temperature. In racing and in towing, automatic transmission sump temperatures can run upwards of 300 degrees F. An external transmission cooler can help get fluid temperature down where it belongs. B&M Racing & Performance has a variety of transmission coolers sized to work in your application. Hint-the larger the better. Install one of these and help your transmission live.

Free From Leaks
Not only do we want our C4 to perform well, we want it to be free from leakage. Too many transmission builders fail to take care of all of the seals during rebuilds. Everything from the front pump seal to shift control seals get overlooked during builds. Whenever your C4 is torn down for a complete rebuild and nose powdering, make sure it gets all new rubber parts, including the shifter shaft seals on the left-hand side. These simple o-ring seals are notorious for leakage when they're overlooked.