Dr Jamie Meyer
August 28, 2007
One of the most dramatic modifications you can make to your Mustang is the installation of a new, high-tech, overdrive transmission. For this swap, we followed along as Tom and Roy Honsaker from Paul's Automotive Engineering in Cincinnati showed us the right way to install a five-speed gearbox.

With the introduction of the '05 Mustang, Ford Motor Company has tipped its hat to the retro styling that drives our segment of the hot rod world. It seems like overnight the restification of classic Mustangs has become the it thing to do. Aftermarket companies are joining in the fun of modifying classic Mustangs utilizing the timeless, iconic good looks of the '60s Mustangs to showcase all sorts of new technologies.

After the efficiency of fuel injection and those funky overhead-cam modular V-8 engines, the next system everyone wants to upgrade is the transmission. The advances in transmission technology are just as impressive as many of the engine combinations available today. Lighter yet stronger materials lead the billing, but the option to have one and in some cases two overdrive gears (a gear ratio numerically lower than 1:1) has pushed these transmissions to the top of the aftermarket industry.

It's impossible to overemphasize the advantages of an overdrive gear. Allowing the engine to operate at a lower-rpm range while pushing the car at highway speeds does several things. First, by dropping the engine rpm at, say, 65 mph, you dramatically increase the fuel efficiency of the car. Less rpm equals less fuel consumption. Second, you drop a lot of highway noise. Yes, it was cool at one time to own the loudest car on the block, but isn't it cooler to sneak up on your prey? In addition, a more comfortable ride allows you to enjoy the car more, and your passengers will appreciate the drop in noise as well. Third, with the extra gearing, transmission manufacturers have built modern gearboxes with a wider range of gears. This usually translates into a low First and Second gear that can dramatically increase the acceleration of the car if the necessary traction is available. Go faster and get better gas mileage? It's true! Last, but probably most important, by decreasing the rpm that the engine has to turn to reach the same final drive speed, you in essence decrease the wear on all parts in the engine. The fewer times you turn that engine over, the less wear and tear on the rotating assembly, valvetrain, and accessories. When you add all of those benefits together, you can see why modern overdrive transmissions have become so popular.

OK, so you now know that you want a five-speed or six-speed manual transmission in your classic Mustang to take advantage of everything we just described. How do you go about choosing the right transmission, and how do you get it into your Mustang?

The new transmissions are much lighter than the older ones. Our Top Loader four-speed tipped the scales at 97 pounds, while the high-tech G-Force five-speed weighed only 73 pounds, making for a 24-pound reduction in overall weight, a lot of which is rotating mass.

To investigate the latest in modern transmission design and the applications in first-generation Mustangs, we visited Paul Faessler's Paul's Automotive Engineering (PAE), a supercenter for Mustang modification and reengineering, and the shop that brought you The Blender, a '66 coupe with all the modern conveniences, including an overdrive transmission (Sept. '05 ssue).

Paul concurs with our assessment of the advantages of these new transmissions: "As an owner of several early Mustangs, the biggest advantage I see is how much quieter the cars are when you are rolling down the highway. The difference in noise level from 3,200 rpm in Fourth gear to 2,000 rpm in Fifth gear is dramatic. New Mustangs (which we drive daily) are more rigid and better insulated than our old Mustangs, and the drop in rpm at highway speeds really makes these older cars more enjoyable to drive. The second thing I like is the more precise shifting, especially with an aftermarket short-throw shifter. Third, the higher-ratio First gear really helps get the car rolling from a stop. You are looking at a 2.94-3.25:1 ratio compared to a 2.78:1 wide-ratio Top Loader or a 2.32:1 close-ratio Top Loader. This reduces clutch slippage and increases clutch life. You also get a 25-30-pound weight savings and increased fuel mileage.

"At Paul's Automotive Engineering, our typical application for these transmissions are cars that are driven. Subtle performance improvements (which do not detract from originality) are very popular, and owners want their classic Mustangs to perform as well as the new cars."

Photo Gallery

View Photo Gallery
PAE uses an alignment tool to verify that the clutch, pressure plate, and flywheel all align perfectly. This is cheap insurance that your expensive components are installed properly.

Other Parts
In addition to the overdrive transmission itself, there are several other parts you should upgrade at the time of installation. A modern diaphragm clutch like those offered by Centerforce is at the top of the list. They offer a tremendous advantage in clamping force and efficiency. Trust us; they will make you forget all about those three-finger units of old. The clutch fork will also need to be upgraded to match that high-performance clutch, and you might run into some pedal-linkage adjustments on your path to adding another forward gear.

Actually fitting the trans to your engine will require the right bellhousing adapter, something you can source from many shops. We know JBA makes several for the classic Mustangs.

You will need to pick up a good shifter for your new transmission. Luckily, the 5.0 Mustang market has forced dozens of companies into this market, so take your pick. PAE uses a model from Hurst because the company also manufactures chrome shifter handles and vintage shift knobs that match the interiors of the earlier Mustangs.

Also, a new driveshaft will be needed on most cars. At the minimum, you will need a new driveshaft slip-yoke that will allow your current driveshaft to fit the newer transmission's output-shaft spline count.

One other item to change is the crossmember that holds the transmission into the body of the car. Here Roy is placing an aftermarket crossmember into position. Several companies offer these swap units, and it's also a good time to upgrade to a urethane tranny mount.

Alignment
Remember to keep an eye on driveshaft alignment. This can get out of whack if the transmission doesn't quite line up correctly due to some interference with the body or other component under the car. Another common problem is the wrong crossmember and transmission mount for your application.

Tremec-Specific Installations
As a general rule, the Tremec line is beefier and stronger than the Borg-Warner stuff it was designed to replace. As such, you will run into more clearance issues with a Tremec. As a general rule, PAE classifies the swaps like this: A Tremec T5 is pretty much a bolt-in as far as fit. A Tremec 3550 or Tremec TKO takes minimal modification, which includes moving the shifter bezel in the floor approximately one inch toward the passenger side of the trans tunnel. This means you will have to massage the floor of the car. A Tremec T56 is a serious modification which will require moving the trans-tunnel crossmember up to retain the correct driveshaft angle. You'll have to clearance the floor as you would with other Tremec units, but you will be rewarded with a sixth forward gear.

As for the year of Mustang involved in the swap, the '65-'66 models are the tightest with the '67-'68 cars offering a bit more working room. On some '68s, PAE can get away with retaining the stock driveshaft. Again, this depends on the car and application, but you will need a different yoke to go from the old transmission to the new.

During the installation of this G-Force unit, Tom makes sure everything lines up correctly. Don't forget that the driveshaft will likely have to be a different size. That's a good reason to upgrade to an aluminum or carbon-fiber unit.

Conclusion
Paul sums up the installation process: "In the world of modified Mustangs, I don't consider them big hurdles, just items that need to be addressed. Some of these are driveshaft length, exhaust clearance to the trans crossmember, different methods of clutch engagement, and shifter appearance. Every aftermarket parts manufacturer designs its part to fit a bone-stock car. Once you stack up a combination of aftermarket parts not related to the tranny install (such as exhaust, headers, subframe connectors, and so on), you sometimes find interferences. What makes for a great result is finding a shop that is knowledgeable enough and experienced enough to pay attention to all the little details that result in a rattle-free installation.

Stepping up to a modern, overdrive transmission is one of the best upgrades you can do to your classic Mustang. While we've outlined a few problem scenarios here, remember to consult all of the manufacturers before starting your project. Question other Mustang owners on similar transmission swaps. In the end, the work and planning will be well worth the reward: a quicker, more efficient Mustang that is fun to drive

Photo Gallery

View Photo Gallery