Overdriven and Understressed

One of the hottest trends in our industry currently is the adaptation of modern overdrive manual transmissions into older vehicles. Road-going car crafters have been aware of the availability of overdriven gearboxes since they first emerged in the '80s, but these boxes also lacked strength, and it didn't seem worth it to merge them with a classic musclecar; why bother going though the trouble to swap in an overdrive if you can't bang gears without fear of catastrophic failure?

But as the '90s unfolded, steadily increasing power ratings from the OE manufacturers seemed to mandate more durable transmissions, and fortunately, the market demanded continued availability of do-it-yourself shifting. With the advent of somewhat bulletproof gearboxes that also featured overdriven high gears came the surge in swap interest from the musclecar crowd. The prospect of gaining a reliable overdrive meant the potential end to the classic street machiner's compromise: Put up with brutal continuous rpm levels on the highway or ditch your drag-race-oriented rear gears for comfortable cruising.

Keisler Automotive's five-speed kits are based on Tremec gearboxes. This design uses a modern internal rail arrangement, where all shift linkage, save for the shifter handle mechanism itself, is inside the trans, making for tight, precise shifting. Currently, Keisler is offering the new TKO 500 and TKO 600 models, rated at 550 lb-ft and 650 lb-ft respectively. The TKO 500 uses a 3.27:1 First gear while the 600 uses a 2.87:1 First with 0.82:1 or 0.64:1 overdrive Fifth gear.

Suddenly, a set of 3.73s seems like standard fare, and depending on the gearbox selected, even 4.10s can be acceptable for comfortable cross-country cruising.

So the motivation existed, but there remained the obstacle of actually mating that late-model gearbox with existing hardware and having it fit into the vintage host vehicle. The ambitious among us have taken steps to create these hybrid drivetrains, but it has often involved specialty parts that have to be fabricated, and then there's the issue of cutting and clearancing virgin floorpans, which simply doesn't fly with purists. Even the hardcore muscle freaks that are fortunate enough to have their cake and eat it too without permanent mods will sometimes have a change of heart if the finished product simply doesn't look right from the cockpit.

Keisler Automotive has recognized the win-win benefits of the overdrive gearbox, and has spearheaded an effort to streamline the swap process by developing complete kits, termed "transmission systems," to facilitate the installation process. The idea behind this approach has been to bridge the gap between custom installations and the average do-it-yourselfer by providing all of the necessary hardware to make a modern manual overdrive trans swap a bolt-in affair. The other objective is to enable these swaps without requiring major modifications to the host vehicle; in some cases, minor trimming of the floor is required, while in others, absolutely no alterations are necessary.

While the internal shift rails of the Tremec TKO and T56 transmissions make for excellent shift quality, this feature can also limit shifter positioning--a potential problem for swaps. However, Keisler manufactures custom application-specific internal-offset shifters to fit in alternate positions on the case, enabling greater adaptability.

To accomplish these goals, most Keisler kits include a custom-engineered gearbox with its application-specific shifter mechanism, a balanced custom high-performance driveshaft, a custom crossmember, and new speedometer cables and wiring harnesses to plug into the host vehicle, plus all required hardware. Some kits use the existing bellhousing while others require modified stock bellhousings. There are a few applications that require a new bellhousing, and Keisler is working on producing new high-quality replacements for vintage factory stuff. Keisler can also provide just about anything else required to make a manual overdrive trans swap, even if the car in question is currently running an automatic.

Nearly all the available kits are based on either a Tremec TKO 500/600 series five-speed or a Tremec T56 six-speed. For some applications, you can have either, and there are ratio and torque-rating options to choose from as well. All gearboxes are brand-new and many have been fitted with Keisler's own tailhousings and low-profile shifter mechanism, which reduce the overall diameter of the gearbox and assist in relocating the shifter. We've outlined the offerings and their benefits and compiled an application chart, but new apps are being added regularly, with a full range of Ford kits, including modular V-8s, FE, and 429/460, plus a kit for the new Chrysler 5.7 HEMI is on the way. Take a look at the swap details and check the Keisler Web site for updates.