Changing Your Ford Transmission - Overdrive Transmission Swap Guide
Thinking about an overdrive-transmission swap for your vintage Mustang? This look at the parts and procedures is a good place to start.
For more than a decade, installing an overdrive-automatic or manual transmission into a vintage Mustang has been a popular upgrade. It's more common these days, but some people still aren't convinced of the numerous benefits of such a conversion.
If you could only do one upgrade to your vintage Mustang that's a fairly nice driver and is reasonably complete, installing a transmission with an overdriven top gear is the single best thing you can do to improve the car's overall usefulness.
Even though we first owned a '66 fastback with an AOD more than 10 years ago, the novelty never wears off when cruising in an early Mustang on the freeway. Shift an AOD into Fourth gear or a T5 into Fifth, and engine speed is reduced by more than 30 percent while vehicle speed remains the same.
That said, there are numerous other benefits to an overdrive transmission, and reduced engine speed at swift highway velocities is only the beginning. Other major advantages include better fuel economy, less engine wear, quieter cruising on the highway, and in the instance of the typical T5, even better off-the-line acceleration due to a lower (numerically higher) First-gear ratio compared to four-speeds like the vintage Top Loader.
We're covering swaps for both automatic and manual transmissions, as well as another popular swap, converting a car with an automatic to a manual. There are separate sections for each of these three possible avenues.
If an automatic to a five-speed manual is a swap you want to do, then it's easiest to go from an automatic straight to a T5 five-speed. Avoid doing the process twice by swapping to a four-speed, then realizing later that a five-speed is clearly the better option.
In any case, an overdrive transmission, either an automatic or a manual, is an upgrade you'll love more and more every time you drive your vintage Mustang. Having owned and driven numerous early Mustangs and other vintage Ford cars with either a T5 or an AOD, we can tell you the appeal of the conversion never wears off.
Since a majority of the vintage Mustangs produced came with either a C4, FMX, or C6 automatic, we'll begin with installing an overdrive automatic in their place. These days, it's nearly a bolt-in since much of the research has already been done.
To answer the most common question of "what overdrive trans should I use?" by far the best and most popular choice is the four-speed automatic-overdrive unit known as an AOD. The Ford AOD has at least as much aftermarket support as a C4, and there are numerous options for obtaining one.
New AODs with all sorts of modern internal updates are available from several sources, such as Lentech Automatics, Performance Automatic, and B&M Automotive, among others. New AODs also come with the correct torque converter for your application based on discussions with the companies that build the transmissions.
Another route is a used AOD from a donor car. The best choice is one from an automatic-equipped '86-'93 5.0 Mustang, all of which were equipped with AODs. For a carbureted vintage Mustang, the AOD-Es and 4R70-Ws found in later 5.0s and 4.6s aren't good choices as they are computer-controlled and can't be set up as easily in a vintage car. Stick with a Fox-body AOD from a '93 or older 5.0, and you'll be in good shape. The torque converter in a Fox 5.0 will also work in a typical vintage Mustang.
The next main component you'll need is an aftermarket crossmember because the stock one won't work. Several sources offer automatic- and manual-transmission crossmembers for '65-'73 Mustangs, including Ron Morris Performance, California Pony Cars, DB Performance Engineering, National Parts Depot, Mustangs Plus, and CJ Pony Parts. One stock piece that will work is your car's existing C4 transmission mount. It's just the right thickness to fit between an AOD and a crossmember designed for AOD swaps into vintage Mustangs.
Moving on to the smaller but equally important pieces to complete a conversion, you'll also need a throttle-valve (TV) cable that does the equivalent for an AOD that a vacuum modulator and kick-down linkage does with a C4, FMX, or C6: It makes the transmission shift properly. With a carbureted engine, both Lokar (PN KD-2AODHT) and Ron Morris Performance offer TV cable setups that will work with an AOD.
Wrapping up the laundry list of required parts for the typical vintage-Mustang AOD swap, you'll also need an AOD dipstick and tube (the ones from a Fox 5.0 donor car will work). A shift linkage that connects the AOD trans to a car's stock shifter (which will also work) and a flexplate are both available from Ron Morris Performance. An AOD yoke can be ordered from a Ford dealer. You'll also likely need to relocate the transmission lines where they connected to the C4 from the radiator.
In addition to an AOD yoke to be installed on the driveshaft, the car's driveshaft needs to be shortened, around 1 inch in most cases. Each car should be individually measured to confirm.
Parts Needed for an AOD Conversion
Any vintage Mustang equipped with a three-speed manual, T-10 four-speed, or Top Loader four-speed is the ideal candidate for a T5 five-speed swap. Several other five-speeds are available these days, including the newer Tremec TKO-500 and TKO-600 five-speeds, as well as the burly T56 six-speed, all of which are available from Keisler Engineering or D&D Performance. While these three transmissions are pretty much bulletproof and installation is similar to a T5, we're going to focus mainly on the T5 conversion as it's the ideal five-speed overdrive transmission for any stock vintage Mustang with a 289, 302, or 351 engine.
There are similarities with T5 and AOD swaps; the most apparent is that you'll need an aftermarket crossmember. As with AODs, numerous sources offer T5 crossmembers, including Ron Morris Performance, California Pony Cars, DB Performance Engineering, National Parts Depot, Mustangs Plus, and CJ Pony Parts. There are two ways to do a T5 swap, depending on what you want and/or whether your car is a manual or automatic to start with. The next section is more applicable to cars with automatics, so here we'll assume an existing vintage Mustang is factory-equipped with a three- or four-speed manual trans.
There's lots of good news with this. You can retain a car's factory clutch linkage and bellhousing because California Pony Cars offers an adapter plate that allows you to join a T5 with a stock Top Loader four-speed bellhousing. The stock bellhousing is modified with two holes so it will accept the adapter plate.
From this point, the T5 installation is virtually a bolt-in. A car's existing clutch can be used along with a stock Fox Mustang or late-model aftermarket shifter. Sources such as Mustangs Plus or National Parts Depot also offer vintage-look shift handles that bolt to a T5 shifter. You could also use the stock late-model T5 shift handle and knob if you want.
To wrap up a T5 swap, the tasks at this point are about the same as an AOD conversion. You'll need to install a T5 yoke on the driveshaft and shorten it a predetermined length, again around 1 inch.
Parts Needed for a Four-Speed to T5 Conversion
Automatic to Five-Speed Manual
Since the vast majority of vintage Mustangs were equipped with automatic transmissions, it stands to reason that many enthusiasts will want to convert to a manual trans, which usually meant a Top Loader or T-10 four-speed back in the day. Frankly, we think going from, say, a C4 to a Top Loader is a complete waste of time these days when it's the same amount of work and only a little more cost to go to a T5, especially when you factor in that you can get a freshly rebuilt T5 from D&D Performance for $999.
Swapping from a C4 to a T5 is an excellent upgrade, and the task is similar to going from a four-speed to a T5. However, since a car with an automatic doesn't have a factory clutch linkage, there's an alternative that works perfectly for this swap. You can also go this way with an existing manual-trans car if desired.
Of course, the most obvious thing needed for this conversion is a manual-transmission pedal set with a clutch pedal. You'll also need a complete clutch setup with the correct flywheel, as well as a late-model T5 bellhousing and clutch fork. Where this swap differs compared to cars already equipped with a stock clutch linkage is that the best way to go is a cable system, similar to what's used in a late-model Fox 5.0. Both Ron Morris Performance and DB Performance Engineering offer a clutch-cable conversion designed for vintage Mustangs. The DBPE system takes care of two issues at once by offering a complete pedal setup along with the cable, while the RMP system is an excellent option as it works without having to drill holes in the firewall. RMP also offers a corresponding clutch pedal to work with its cable system, though you'll still need to procure a pedal setup.
Other than the late-model T5 bellhousing, pedals, and cable kit, the rest of the swap is the same as the conversion from four-speed to T5. That is, you'll need the same crossmember, shifter, C4 transmission mount, and T5 driveshaft yoke. You'll also need to shorten the driveshaft about 1 inch.
Parts Needed for an Automatic-to-T5 Conversion
Manual to Four-Speed Automatic
Though not likely done very often, it's possible to convert a car from a manual transmission to an AOD. The needed parts are basically the same as going from a C4 to an AOD, although you'd likely want to get an automatic brake pedal. A radiator designed to work with an automatic and an auto-trans shifter are required. Everything else listed for an AOD swap remains the same.
Four vs. Five
One of the biggest benefits of a T5 over any standard four-speed manual is that it has an overdrive (less than 1:1) top-gear ratio. However, a standard Fox 5.0 or Ford Racing T5 "Z" trans (PN M-7003-Z) also has a lower (numerically higher) First gear ratio as well, which, of course, means better acceleration in First gear. In a car with a 3.25 rear-axle ratio, the final drive with a four-speed and its 1:1 top gear is 3.25. Add a five-speed to the mix with a typical 0.68 overdrive ratio and the final drive is a dream on the freeway with a ratio of 2.21 (3.25 x 0.68 = 2.21). The chart below further illustrates what we're talking about. The numbers are also similar for AOD conversions.
|Close Ratio||Wide Ratio||T5||FRPP T5Z|