The Corvette started its life in 1953 with options and features well ahead of its domestic counterparts. Since then, it's maintained that lead using cutting-edge technology all the way through the current model. Anyone who has driven a recently produced Corvette, however, notices immediately what the classic models were lacking: a durable, precise transmission with an overdrive. Yes, the transmission is a central part of the driving experience for the Corvette owner, but being limited to three or four forward cogs (with a manual) requires a sacrifice in either low-end acceleration or highway-speed driveability, due to the 1:1 ratio of the high gear.
Our project Stingray was suffering this affliction, being geared in the 3.55 range and equipped with a Borg-Warner T-10 four-speed. This setup actually resulted in two compromises. First, the 3.55 gears combined with the relatively tall First-gear ratio of the T-10 caused the car to be temperamental off the line. Leaving at too low an rpm would bog the engine, and the car was sluggish until the small-block built a little steam. Additionally, highway driveability and top speed were reduced, as our rear gearing caused the engine to rev excessively at anything over normal highway velocities. We decided to find a five-speed conversion kit that could handle the power we had planned, while offering a lower First-gear ratio for rapid acceleration and an overdrive top gear for smooth highway cruising.
Our research led us to American Powertrain, located in Cookeville, Tennessee. American Powertrain has been providing transmission conversion kits for Corvettes and other classic cars for years, including conversions from automatic to five-speed manual, as well as manual to five-speed manual. The ProFit3 system offered is the latest iteration of the company's ProFit series for the C3, and is based around the legendary Tremec TKO five-speed. While it's no easy task to get the TKO into an early Corvette, due to the fixed crossmember, narrow tunnel, and shifter location, American Powertrain has engineered its installation components, and specifically the ProFit3 offset shifter mechanism, to make the job much simpler.
Starting with a clean slate, American Powertrain designed the shifter featuring a direct, offset link to the main shift rail, providing a 1:1 relationship with the rail. The result is more-precise gear changes, with none of the slop associated with the offset mechanism, and no buzz or vibration transmitted to the cockpit. By design, this places the shifter in the exact location of the original four-speed shifter, so no cutting is required. The shift tower is also designed to be easily removable, which makes it a lot easier to get the transmission properly aligned in the tight space the Stingray provides. We simply removed the shift tower, and then easily reinstalled it once the transmission was bolted in place. The provided factory-style shifter was then bolted on, followed by the boot and factory console cover for a clean, stock-appearing look.
We were equally impressed with the remainder of the kit. The new, MIG-welded and powdercoated rear crossmember bracket requires no drilling or modifications, and it mates the transmission to the chassis via a new polyurethane mount. A reverse-light switch is installed and wired just like the factory switch, and even comes with a sealed wiring harness to protect it from moisture and road grime. The driveshaft included with the kit is professionally built, race balanced, and customized for each application using solid universal joints to match the rear yoke of the vehicle.
Instead of including a full-length speedometer cable with the kit-which would require fumbling around behind the dash to change the cable-American Powertrain provides a cable adapter for the speedo. As anyone who's tried it will attest, disconnecting the factory cable either requires removing the entire dash assembly, which is time consuming, or reaching behind the dash among all the wires and instruments, which risks breaking critical items. By using the adapter, the original cable doesn't need to be removed, and the entire process is easily completed under the car. (Full-length cables are available, however, if that's your preferred approach.)