For the longest time, if you ran a Ford small-block or Windsor engine and wanted to run an automatic, your transmission of choice would likely be a C4. For heavier duty applications you'd likely opt for a C6. As with everything, time and technology move on and today many overdrive automatics are available, mostly computer-controlled. With internals loosely based on the FMX, the four-speed overdrive AOD which was offered from 1981-93 is the only non-computer controlled overdrive Ford transmission that will work with a V-8.
Ford first introduced an automatic transmission in the Fifties (discounting the Hydramatic offered in 1949-54 Lincoln Continentals), with the three-speed Ford-O-Matic, in service from 1951 to 1968. With a separate bellhousing, this cast iron cased trans evolved into the FMX, which lasted until 1981. However, unless you're restoring your Ford, you'll likely want to upgrade from the Ford-O-Matic, which was also known as the Merc-O-Matic, or Cruise-O-Matic, though we've also heard the latter used to describe an FMX trans.
Used in Ford, Lincoln and Mercury passenger cars and Ford light trucks until 1979, the FMX is easily recognizable by its cast-iron main case, which differentiates it from other Ford three-speed automatics, with separate aluminum bellhousing and extension housing. While the FMX can handle plenty of torque-indeed it was used in trucks into the Eighties-it's not as well catered to by the performance aftermarket as the C4 and C6 are, and it's heavy, thanks to that iron casing. Rated as a medium duty transmission, the final drive ratio is 1:1.
Ford's most adaptable auto transmission for years has been the C4, the light-duty three-speed introduced in 1964 and offered behind four-, six- and small eight-cylinder engines, namely the 260, 289, 302 and 351W motors. You'll find them in Galaxie, Fairlane, and some Falcon models. The C4's removable bellhousing means it's long been a favorite automatic to bolt behind a Flathead too, thanks to companies like Flat-O-Matic offering bellhousing adaptors. With an aluminum case, the C4 is light in weight and efficient, comparable to the GM Powerglide as one of the least power-robbing automatic transmissions. There are two variants, however. Those in passenger cars have the dipstick in the case, while van, truck, and some fullsize car versions have the dipstick in the trans pan. Larger bellhousings accommodate a 12-inch converter; an 11-inch converter is found in the versions with the smaller bellhousing. They are identical internally. A lockup converter version called the C5. which uses different valve body programming, ran from 1982-86, when C4 production ceased. It too has the larger bellhousing and converter, and can be found in LTD II, Ranger, Fairmont, Cougar, T-Bird, and Mustang models.
Moving up to Ford's longtime heavy-duty transmission, the C6, which can handle large amounts of torque, and came from the factory bolted to the rear of 351W, 351C, 428, 429 motors and even the 7.3 liter diesel, Ford moved to a one-piece casing with integral bellhousing. Produced from 1966-89, again with a final drive ratio of 1:1, the C6 is to the C4 what the GM TH400 is to the TH350. The other gear ratios are: 2.46 First gear and 1.46 Second gear. Available in one of three bellhousing bolt patterns, depending on which series engine it was coupled to, the C6 was the transmission of choice for Ford drag racers for a long time, until strong race-prepared C4s became available.
Ford's first overdrive automatic transmission, and indeed the first for any American automaker, was the AOD (Automatic Overdrive) 4-speed, introduced in 1980 and available until the early Nineties when the AODE began to replace it, starting with the new modular V-8. With mechanical lockup, it can be found behind many Ford motors from the 3.8 liter V-6 to 351ci V-8s. With an overdrive ratio of 0.67:1, gas consumption and engine cruising speed were greatly reduced over its predecessors, though obviously rearend ratio also affects this. The other ratios are First: 2.40:1, Second: 1.47:1, Third:1:1. According to Phoenix Transmission Products' Greg Ducato, "The AOD is a good swap choice from a C4, C6, or FMX transmission and shares many common dimensions with them. The AOD also uses the small-block bellhousing pattern which fits 250 and 300 six-cylinder engines, as well as 289, 302, 351 Windsor and 351 Cleveland."
The AOD transmission and torque converter are a matched pair, as the AOD uses a hollow two-piece input shaft for mechanical lockup, and as such requires a specific torque converter.
By 1994 the AOD had been superseded by the AODE (the E denoting "electronic"), sharing most internal parts except the input shaft, and electronically controlled by a computer, doing away with the TV cable. Advantages are that the shift points and firmness, as well as the lock-up point can be programmed. Incidentally, a wide ratio version of the AODE, using a 2.84:1 First gear is also available, known as the 4R70W.
For extreme tow or race duties, there's also the E40D, which is huge, essentially a C6 with an overdrive added on, but it can handle serious power, and was used in Lightning pickups.
We're not sure how many of you might contemplate dropping a late-model Mustang (2002 onwards) drivetrain in your project, but the 5R55S transmission they use is a five-speed, with four forward gears and an overdrive. First gear is 3.25:1, low enough to provide rapid acceleration! Don't confuse them with the 5R55N or 5R55W though, which are also five-speeds but came behind four- and six-cylinder engines despite the similar numeric designation!
Before you rush to buy and bolt an overdrive behind your engine though, take some time to calculate whether your engine/rearend/tire combo will support one. With a normally aspirated motor and maybe a mild cam, you should be looking at a 70 mph cruising speed of around 2,000-2,200 rpm. If you are running 28-inch diameter tires and a 2.56 rear gear, you're not going to be able to use Overdrive. Of course you may want to switch to a shorter gear (numerically higher number) to take advantage of the Overdrive and quicker acceleration in the other gears, but now your bill is mounting up! What we're saying is you need to know your objective when selecting a transmission, and that your engine needs to be within its operating range for fuel efficiency. For more on this refer to our "Hitting the Sweet Spot" article in our February 2009 issue or in the tech section at www.rodandcustommagazine.com.
Okay, now you're armed with some history and model designations, which Ford automatic transmission is right for your project? How much power do you need it to handle? Do you need an overdrive? Will the trans of your choice fit in your car? To help with that decision we'll look at some strengths and weaknesses of each model, physical sizes and lengths, and power ratings, with the help of some aftermarket transmission suppliers. First though, let's compare weights and lengths. The following are all without fluids or torque converter:
The C6 has always been regarded as a heavy transmission, but as can be seen, the AOD outweighs it by another 10lbs. However, the advantages of the overdrive gear make up for this.
If you're planning on swapping out your existing transmission, or are planning a project, some dimensions may help too. We'll assume you're not going to use a Ford-O-Matic or FMX here:
All the Ford automatics have good and bad points. The C4 is light, efficient, and small enough to fit under the floorpan of most vehicles, and is easily adapted to other engines thanks to its removable bellhousing, unlike other Ford automatics. It is a light duty transmission though, and in stock form the planetary system can fail, but C4s can be built to accept tremendous amounts of horsepower, up to 1,200 bhp depending on how they're modified, and by whom. This obviously depends on how much you want to spend and how you use your car. They also have the advantage, depending on your point of view, of being simple, with just a kick-down cable and no computer controls, electronics, or even TV cable to worry about. There are two tailshaft housing lengths available on C4s, the common version being 131/8 inches long, with some pickups and vans using a much shorter 65/8-inch version.
The C6 can withstand tremendous power in stock form, and has no inherent weaknesses, though they will rob power and are really suited to use with high horsepower big-block engines, because of their heavy rotating mass. The one-piece casing with its integral bellhousing means each C6 is limited to the series of engines its bolt pattern will accept, and its sheer physical size and length means it is awkward to fit it in many cars. Again, depending on the application, three different length tailshaft housings are available; 7 inches for trucks, 14 inches for passenger cars, and 17.4 inches in Lincolns.
The AOD can handle about 300hp and 250ft lbs of torque in standard form, but can be modified to withstand 700-800hp. However, in stock form, one of the most common problems is that the overdrive band has a tendency to burn up, and the lockup input shaft is notoriously weak. These are nothing that cannot be fixed however. There are actually two input shafts in an AOD, a hollow outer shaft that is driven by the converter and operates First, Second and Reverse gears, while the inner shaft, which runs inside the outer one, provides direct drive in Third gear, and is driven mechanically from the front cover of the converter. This is known as a split torque path. In Overdrive, all the torque is transmitted mechanically, as in a manual transmission. Upgraded and hardened input shafts, usually manufactured from chrome moly or billet are available. Upgraded valve bodies, often using an electric solenoid to control Overdrive, also improve the AOD.
If you're sourcing an AOD from a car, rather than buying an aftermarket version, you'd do well to grab the associated hardware, such as bolts, the four pin electrical connector for the backup lamps and Neutral sensor, dipstick tube, block plate, linkages, hydraulic fittings and even the driveshaft yoke to make your conversion easier. Note also that the AOD is physically larger than a C4, more akin to a C6 "around the waist," and may not fit your transmission tunnel without some modification. Also, remember the AOD doesn't use engine vacuum and a modulator valve like its predecessors to sense load, but has a throttle valve (TV) and hence requires a TV cable for the kick-down function. An improperly adjusted TV cable will trash the transmission so make sure it's fitted and set up correctly. The AOD also comes with two lengths of tailshaft housing, the shorter and more common one of just over 10 inches providing a transmission length similar to the C4. As can be seen in the preceding chart, while the C4 and AOD are only 1/2-inch apart in overall length, there's a 2-inch discrepancy in bellhousing to trans mount measurements, useful to know should you be swapping from one to the other. Oh, and the AOD uses a 164 tooth flexplate, while the C4 can use 148, 157 or 164 tooth ring gears.
While some people will prefer the AOD because it's not computer controlled, others will opt for the AODE, because it is! According to Phoenix Transmission Products' Greg Ducato again, "with the level of refinement in our daily drivers these days, we have new expectations of our hobby cars. The AODE makes such a difference in drivability. The curse of the AOD is its mechanically locked converter. In Third and Fourth gear it's locked up and loses all torque multiplication. Most AODs we sell are non-lockup. We use a stand-alone Compushift with our AODE transmissions, as you can manually raise or lower the shift points and adjust the lockup, without needing a laptop computer. It's all done via the Compushift display panel." So if you are planning on the AODE, you will need an AODE computer and transmission wiring harness, either OEM or aftermarket, from a company such as Compushift or Baumann Engineering.
Ford -O-Matic 228lbs FMX 160lbs C4 110lbs C6 140lbs AOD 150lbs Trans Bellhousing