5.0 Mustang & Super FordsHow To Drivetrain
Upgrading Ford's Automatic Overdrive Transmission - Blowing Into Overdrive
An Old Hand Shows Us How To Make An AOD Live Behind A Supercharged 5.0
The AOD was not a clean-sheet-of-paper design from Ford-it was based off the venerable three-speed C4 automatic.
Now that Ford's automatic overdrive transmission has been around for so long, we sometimes forget it was really the first widely used overdrive automatic gearbox in the United States. But it was, making its debut in 1980 on passenger cars, then showing up in Mustangs from 1984 to 1993. Even then, the AOD was replaced by itself, dolled up with electronic shifting controls as the AODE in 1994.
While not thought so at first, the AOD has proven a fairly stout gearbox-stout, that is, if given the proper upgrades. And considering the numerous methods of arriving at major 5.0 horsepower, it's a good thing for auto-shift fans that the AOD can be readily modified to near-bulletproof status.
This is especially true for Ricky Best. Well known in the Mustang world as racing coordinator at Vortech Engineering, Ricky recently acquired a '93 coupe as a street/strip project vehicle for himself. And, while cumshawing a V-1 T-Trim blower onto it was reasonably easy (do you think?), he knew his little coupe's stock AOD would never make it without upgrading.
Recalling that one of the original AOD pioneers was reknowned automatic-transmission specialist Art Carr, Ricky headed off to California Performance Transmission, where Art does business these days. That's where we tagged along to review what an AOD should get when being prepped for modern performance use.
At California Performance Trans-mission, we reviewed what makes an AOD fail to tick when belted with more torque than it was designed to handle. A four-speed automatic with an overdrive top gear, the AOD has several weak points in high-power applica-tions. For starters, the Third gear (sometimes called "direct") clutch pack has only five "clutches and steels." This is a minimal number, and the arrangement has difficulty transmitting torque without slipping.
Next, the Overdrive (Fourth gear) is achieved by applying a band around the direct drum (Third gear). This band is narrow at 1.5 inches wide, and it too often fails by slipping.
Furthermore, the valvebody is set up to supply 30 percent less line pressure to Third and Fourth (OD) gears. This was Ford's misguided attempt to increase fuel mileage via reduced power losses to drive the transmission's pump-miniscule mileage was gained at the expense of numerous Third and OD failures due to low clamping pressure.
If you fix those conditions, you'll find the AOD's two-piece input shaft is not quite the stuff of legend. Given just a little extra power and half sticky tires, the stock AOD input shaft will instantly snap at the first full-throttle shift. In addition, a stock AOD will not shift into Overdrive at full throttle (and if you try, you'll smoke the direct clutch pack), normal shifts are a bit squishy, and the shift pattern is undesirable.
Late in the game as we are these days-Ricky's coupe is 10 years old and has 125,000 miles on it-a few maintenance items are also important. As with all automatics, heat is the enemy, and the constantly moving fluid and slipping, power-on shifts lead to plenty of heat. Seals dry up and need replacement, the clutches are typically worn, and the stock oil cooler is woefully inadequate. The Throttle Valve (TV) cable is also prone to stretching around the 50,000-mile mark, so this is now a standard replacement item.
From a performance standpoint, the torque converter is also in need of upgrading. The stock 12-inch unit is most often replaced by a 10-inch converter with a touch more stall speed, so converter construction and integrity come into play.
Addressing these issues is relatively straightforward, with the typical trade-offs and costs associated with hot-rod parts.
The direct drive (Third gear) clutches and steels can be increased by recutting the drum's snap-ring groove or using a different drum, which is what California Performance Transmission does. This allows seven clutches and steels, which are also of considerably higher-grade material.
The narrow OD band is replaced by a higher-quality, wider, 2-inch unit, and the line pressure is increased via drilling several holes in the valvebody plate. This firms the shifts, and other valvebody work allows safe full-throttle upshifts into OD.
Curing the weak, two-piece input shaft is accomplished by using a vastly stronger one-piece shaft. This eliminates breakage concerns, but the trade-off is that a lock-up torque converter can no longer be used. CPT makes its own non-lock-up converters, which is what Ricky received. This results in a gain of approximately 400 rpm at freeway cruising speeds, so fuel mileage may fall a tick, although historically this hasn't shown up much. The better acceleration and other efficiencies from the upgraded transmission probably offset the higher freeway cruise rpm-or rear-axle gear-ratio changes, power adders, and so on made at the same time as the transmission upgrade mask this relatively small effect.
CPT's 10-inch converter brings the foot-brake stall speed to 2,600-2,700 rpm, although CPT says you may see 3,000 rpm without creep on a sticky dragstrip and good tires.
Finally, a proper, high-quality 19-row oil cooler is fitted for the sake of dura-bility and consistency.
All told, CPT figures the average performance gain is 0.750 second and 3 mph in the quarter-mile, provided you have the traction. The increase in applied low-end torque is considerable, so the tires and suspension need to be up to the task.
As with all major improvements, getting this amount of work done costs more than just lunch money. The CPT Extreme Duty transmission retails for $1,999, the CPT Mega-Torque 10-inch converter is $549, and the oil cooler is $189 with fittings and brackets. Toss in tax, installation, and possibly shipping, and getting an AOD into shape can be a $3,000 proposition. The reward is a CPT transmission that will shift worlds better and happily bang out the thousands of shifts behind big-power engines.
Predictably, after driving it home, Ricky was excited about his new gearbox. The big deal is the fast, positive shifts in a streetable package. "It's not a neck snapper," he says. "It's solid, not laggy-just a good shift in all gears ... It has some stall to it-not too much ... It bangs the shift at 5,100 rpm every time at full throttle ... It chirps the tires at 4,600 rpm on the one-two shift. The car definitely feels faster and is obviously more responsive."
We didn't have a chance to baseline Ricky's car at the strip before the transmission went in, but previously we tested another, nonsuperchared AOD 5.0 prepped by Art and it gained 0.2 second in the quarter. We did chassis dyno Ricky's car before and after the transmission work, but we didn't see any power gain. The track improvements that gearboxes such as these exhibit come from getting what power there is to the ground better and faster, not so much in increasing available power.
Also, increased power to the ground from a performance transmission is really secondary. The major advantage from a prepped transmission such as the CPT Extreme Duty is it will live through hard use and increased power levels the stock AOD could never withstand.
And now, with the supercharger on the engine and the transmission taken care of, it seems the only parts Ricky has to worry about blowing away are the rear tires.
Ricky Best's project coupe is a typical, modern 5.0 project car. A nearly stock coupe when he recently bought it, Ricky's ride sports a 2-1/2-inch Bassani X-pipe with cats, along with a Bassani after-cat on the exhaust side and an Anderson Ford Motorsport Power Pipe on the intake-well, that and a Vortech V1 T-trim set at 8.5 pounds of boost.
Although the car is quite clean and there's life left under the hood (the odometer shows 125,000 miles), Ricky really doesn't need to push his luck until he can spiff up the basic engine. One change he will be making shortly is from the stock 2.73 rear-axle gears to probably 3.73s. That job will likely be handled by Extreme Automotive in Canoga Park, California. They got Ricky through a pan-gasket change in record time and would probably enjoy a straightforward gear change after the gasket hassle.
California Performance Transmission can tailor almost any part of the transmission's function as desired by the customer. Clearly the major changes by the torque converter are a common adjustment, but so is shift rpm. Often CPT sets an AOD to shift at 5,500 rpm, but given the high mileage and possibly weak valve-springs on Ricky's coupe, CPT set it up for a 5,100-rpm shift to avoid any overspeed issues.