Install An Automatic Overdrive

Wouldn’t it be nice if you could restore a classic Mustang in such a way that it would yield closer to 30-mpg highway, with less wear and tear on that vintage 289? Turn on and tune in to Ford’s automatic overdrive transmission for classic Mustangs. The automatic overdrive (AOD) transmission first appeared in new Fords and Mercs in 1980 and has steadily improved since. It’s a four-speed automatic transmission where Fourth gear is Overdrive—actually the reverse of what your engine experiences in gears 1 and 2 in conventional automatics.

In conventional automatics, such as the C4, C6, and FMX, First and Second gear are geared where the engine turns faster than the output shaft and driveshaft. This gives the engine mechanical advantage because it doesn’t have to work as hard under acceleration. Lower gear ratios allow us to make the most of the engine’s power output. Third gear, or final drive, puts the engine in straight drive at the same ratio as the rear axle gears. For example, if you have a rear axle ratio of 3.25:1, this means the driveshaft rotates 3¼ turns for every revolution of the drive axles and the engine is turning at the same speed as the driveshaft. This is how it is with a C4, C6, or FMX three-speed automatic.

The AOD has First, Second, and Third gears, as do the old three-speed automatics. However, it shifts a third time into Overdrive, which lowers engine rpm via a higher final-drive ratio inside the transmission. Most late-model automatics have a locking torque converter, which engages when the transmission shifts into Overdrive. This gives the engine a direct connection with the overdrive unit without the slippage of a fluid-coupling torque converter. The AOD doesn’t have a locking torque converter. Instead, it has a shaft within a shaft that goes straight from the torque converter shell—driven by the engine—to the overdrive unit. Lean on it in Overdrive and it feels like a stick shift in high gear. The engine labors mildly and you feel a solid connection with the drive axle. This saves fuel and reduces wear and tear because there isn’t the slippage we experience with a fluid coupling. Lean on the gas hard enough and the overdrive comes out of lockup, putting the engine back in touch with the torque converter, which gives us more power.

A torque converter does exactly what the name implies. It multiplies the engine’s torque via a fluid coupling that’s tied to the transmission’s front pump and input shaft. The input shaft turns clutches and planetary gear sets to get power to the driveshaft yoke. Whenever a torque converter goes into lockup in final drive or Overdrive, we’re tying the engine directly to the driveshaft, so to speak, for slip-free power transfer. In lockup, we no longer have torque multiplication—just a one-to-one direct link. AOD For A Classic

There are two basic answers for classic Mustang buffs looking for overdrive efficiency. The Gear Vendors overdrive unit for C4 and C6 transmissions enables you to spline overdrive into your antique slush box. All you have to do is shorten or replace the driveshaft and dovetail the Gear Vendors unit into your golden oldie.

Ford’s AOD offers a more economical answer for enthusiasts needing overdrive performance without the high price tag of a bolt-on, aftermarket retrofit. The Gear Vendors overdrive is an outstanding answer for enthusiasts looking to pump 500-1,000 hp through a driveline. Considering most of you who read Mustang Monthly won’t be building 500-1,000hp stump pullers, we’ll bet you’re game for the more affordable AOD solution. There are a lot of horror stories about AOD conversions and their complexities, but Windsor-Fox Performance Engineering makes it easy to bolt on an AOD behind your vintage Ford small-block. And we’re going to show you how.

Our test mule is a mildly modified ’67 Mustang convertible with a carbureted 289 engine. We’re going to retrofit a garden variety small-block Mustang with an ’80s vintage Ford AOD custom-built by Auto-Rite Transmissions in Van Nuys, California. Auto-Rite performed all the modifications necessary to bolster the performance of a Ford AOD: heavy-duty AODE components, more clutches, and the appropriate valvebody mods to firm up the shift. Now we’re ready for installation.