Ford's various three-speed automatic transmissions came in a number of applications and a variety of types. The lightest duty of the three-speed automatic transmissions, the C3, and the oddball "Cruise-O-Matic" series probably won't be of any interest to many car crafters. From a V-8 performance standpoint, there are really only two transmissions to consider  -  the C4 and the C6.

The C4

     Introduced in 1964, the C4 was Ford's first light-duty three-speed automatic transmission. It was used behind four-cylinders, six-cylinders and small V-8s by all Ford divisions (Ford, Lincoln and Mercury). An interesting feature of the C4 is its bolt-on bellhousing, which allows it to be easily adapted to a variety of engine applications. There are two major C4 case variations with respect to dipstick location: Those with the dipstick in the case were primarily used in passenger cars, while those with the dipstick entering directly into the trans pan were used mainly in trucks, vans and some fullsize cars. On transmissions with the dipstick in the pan, the bolt-on bellhousing was also make larger to accommodate a 12-inch torque converter (the smaller-cased units housed an 11-inch torque converter). Internally, both versions of the C4 are the same. A lockup version called the C5 was introduced in 1982 and continued in use until production ended in 1986. The C5 used a lockup converter and different valvebody programming, but it was otherwise similar to other C4 transmissions. C4s were original equipment behind 260, 289, 302, and 351W V-8s.

     Harvey Baker of Performance Automatic Transmissions, makers of some of the toughest C4s available, says the company prefers the C4 over other Ford offerings because they are light and efficient, ranking along with the GM Powerglide and the Chrysler 904 as the least-power-robbing automatics. If there is a weak point, Baker says, the stock planetary is the component most apt to fail on a C4. To address this, Performance Automatic builds a rollerized planetary system (see Fig. 4), which is used in it's "Super Comp" series of race-ready C4s rated for engines producing up to 1,200 hp and 800 lb-ft of torque. Super Comp transmissions feature standard gear spacing (see chart below), a hardened 26-spline input shaft, the rollerized planetary system, six-gear pinion, six-clutch drum, and a lifetime warranty. Performance Automatic claims to have yet to experience breakage with one of its Super Comp C4s, and the company is so confident in its strength that it recommends this unit over even a beefed-up C6.
 

Fig. 01 - Fords automatic transmission offerings

Fig. 02 - 3-spd ID

Fig. 03 - The Ford transmissions to avoid

Fig. 04 - Rollerized C4 planetary system from Performance Automatic Transmissions.

The C6

     The C6 is to the C4 what the GM Turbo 400 is to the Turbo 350. Introduced in 1966 and produced until 1989, the heavy-duty C6 was used behind engines ranging from the 351W to the 7.3 Diesel. Before the advent of race-prepped C4s, the C6 was the auto trans of choice in performance/race applications. One-piece-case C6s came in three different bellhousing patterns to accommodate the three different series engines offered by Ford:

• small-block (260, 289, 302, 351W, and 351C)
• FE-series big-block (332 to 428)
• 335-series (429 and 460, plus the 351M and 400, which shared the same bellhousing pattern

     Three different extension housings (tailshafts) were offered: the Lincoln version at 17.4 inches long; the truck housing at 7 inches long; and, most commonly, the 14-inch length used in passenger-car applications.

     According to Pro-Formance's Sean Wiley, the C6 really has no basic weaknesses, and it can withstand obscene amounts of power in stock form. A good intermediate lever and servo, and a later-model four-clutch drum modified to accept five clutches are pretty much all that's needed to fortify the already beefy C6. Nevertheless, Wiley only recommends their use behind very powerful big-blocks where the power required to drive the transmission is less significant. Mike Stewart of Mike's Transmissions considers the C6 to be ideal for tow vehicles where the owner isn't concerned with squeezing out precious tenths of a second, as the internals are so heavy that they eat inordinate amounts of power.

     ATI's Chris Esterly concurs with Stewart, informing us that ATI rarely builds C6s for performance applications but does so occasionally for tow vehicles or restorations of musclecars originally equipped with the C6. With today's crop of small, high-horsepower motors, the heavy rotating mass of of the C6 simply kills power. Packaging is also difficult, as the massive C6 doesn't fit easily into many cars.
 

The Cruise-O-Matic

     Introduced in 1951, the Cruise-O-Matic series of three-speed automatics (which includes the FMX) was used in Ford, Lincoln and Mercury passenger cars until 1979, and in light trucks from 1968 through 1979. They came in three configurations:

• the small case (from 1951 to 1966)

• the medium case (from 1955 to 1968), and

• the FMX (from 1967 to 1979)

     The Cruise-O-Matic transmissions are easily identified by their design, which incorporates a cast-iron main case (unlike all other Ford three-speed automatic offerings) with separate aluminum bellhousings and extension housings bolted to it. The Cruise-O-Matics should be avoided in performance applications as very little is available in the way of aftermarket performance parts other than shift kits. The Cruise-O-Matics were also offered with all three bolt-common V8-pattern bellhousings.

     Ford's automatic overdrive (AOD) for rear-wheel-drive applications was the first of it's kind offered by an American automaker back in 1980. Featuring mechanical lockup, it was available behind everything from the 3.8L V-6 through Ford's Windsor-style V-8s (the 5.0L and 5.8L). The later automatic overdrive electric (AODE) incorporated computer control.

     According to Mike Stewart of Mike's Transmissions, one of the most common problems with the AOD is its tendency to burn up the overdrive band. Mike's Transmissions offers a variety of parts to bolster the capabilities of the AOD, including an "A+" servo with grater capacity than stock and a hardened 4340 chromoly input shaft to replace the notoriously weak lockup input shaft. Mike's Transmissions also retrofits the larger and stronger drum/band assembly from the newer AODE to non-electric  AODs. Stewart considers a well-prepped AOD to be capable of withstanding 700 to 800 hp comfortably, making it safe for use in street applications less aggressive than all-out race cars.

     Canada's Lentech Automatics has a reputation for building the world's toughest AODs. When asked about the weakest link  in the AOD, Lentech's Chris Nugteren quickly points to an inherent problem with the power flow through the transmission. The AOD incorporates a somewhat rudimentary overdrive design featuring two input shafts. The inner shaft, which is driven directly from the front cover of the torque converter bolted to the flywheel, provides direct drive in Third gear and bypasses the hydraulically driven components of the torque converter entirely; the hollow outer shaft is driven hydraulically by the converter and drives First, Second, and Reverse gears. This is commonly referred to as a split torque path, and it allows 40 percent of the engine's torque to be transmitted  via fluid coupling (through the torque converter) and 60 percent to be transmitted mechanically while the transmission is in Third gear. In Fourth gear, 100 percent of the torque is transmitted mechanically  -  similar to the direct-drive of a manual trans.

     When building an AOD transmission for a mild street/strip applications, Lentech first substitutes a nonlockup-style torque converter, which allows hydraulic damping to reduce the shock transmitted to the inner shaft compared to a lockup converter. Another benefit of the nonlockup converter is that Third gear now receives torque multiplications from the converter rather than being driven directly by the crankshaft as with a manual transmission. The result is more potential speed and smoother operation without the lockup feature. Lentech offers hardened inner and outer input shafts in the standard spline count.

     Another major component of the AOD that needs to be addressed in a performance applications is the valvebody, which, in Nugteren's words, "has a shift pattern that sucks." The AOD was introduced at a time when Ford's automatic-equipped cars rolled off the assembly line with only three-position column detents. Rather than design a new column, Ford (during it's "Better Ideas" days) opted to provide only three shift positions for the new transmission: First, Drive, and Overdrive, bypassing Second gear entirely in a manual-shift situation. Mustang owners quickly learned that if they upshifted from First to Third (Drive), then quickly downshifted back to First before the trans made the shift into Third, the trans would hold Second gear until the driver manually upshifted to Third.

     While this technique will work for a while, the overdrive band and direct clutch will prematurely wear out. Lentech changes the shift pattern by making a specific gate for Second gear and combining Overdrive with Drive. Overdrive is controlled with an electric solenoid in the valvebody (like most overdrive transmissions).

     Lentech covers mild, but what about wild? The company starts by replacing the two-piece input shaft with a one-piece nonlockup 4340 chromoly unit (see photo) offered in two states. Stage One shafts feature a standard spline count, while State Two one-piece input shafts are offered in larger diameters and feature higher spline counts. They also offer a matching direct drum and torque converter.

Fig. 04 - Ford's AOD comes equipped with a two-piece input shaft (left) but Lentech Automatics offers a one-piece 4340 chromoly replacement to handle brutal horsepower.

     Next, the valvebody is replaced with a Lentech unit that applies two clutch packs to Third gear. Normally only the direct-drive clutch pack would be used, but the reverse clutch pack is added to the function, spreading the load over two sets of splines and friction elements (drums), and allowing it to hold huge horsepower. It's this single development that has made Lentech's ultimate AOD transmissions virtually bulletproof. They further modify the transmission by removing the Fourth (Overdrive)  gear and replacing many of the components with lightweight pieces. In applications where a "loose" or high-stall converter is used, Lentech maintains the lockup function of the converter, enabling it to eliminate the slippage that plagues high-stall converters. On of its race units is in a Mustang running 8.20s at 165 mph with no breakage in a full season's use. We would say that it works!

     The AODE is essentially an electronically controlled AOD transmission, sharing most of its internal parts except for the input shaft, which has its lockup function controlled electronically by a computer. As mentioned above, the drum/band assembly is slightly larger and made of stamped steel (unlike the cast-iron drum used in the standard AOD), and it is interchangeable with its counterpart. One advantage enjoyed by the AODE is that the shift functions and characteristics of the transmission can be programmed for greater versatility, but this creates its own unique set of problems.

     Ford programmed the AODE to provide seamless transitions between gears, accomplishing this with a pressure-control solenoid (PCS) that manipulates the line pressure in the transmission to soften shifts. Ford also attained seamless shifting by slipping the converter clutch, which cycles the converter in and out of lockup. Unfortunately, this prematurely wears out the clutch materials, which can send debris through the system. Lentech insists that the first thing to do with an AODE is to add a performance computer chip that corrects the pulse width of the PCS and eliminates the multiple personality of the lockup feature.