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C3 Corvette Transmission - Maximum Overdrive

Installing An American Powertrain ProFit3 Five-Speed In Project C3 Triple-Ex

Dave Young Oct 21, 2010
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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.

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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.

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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.

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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.)

Project C3 Triple-Ex began life as a manual-transmission '71, and since it was equipped with a T-10 four-speed, we were able to retain the bellhousing and relatively new clutch that came in the car. If your application requires these parts, they will be included in the American Powertrain five-speed conversion kit for your car.

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Since we had all the parts we needed for our conversion, we decided to take our Stingray to Inline Performance Specialists in nearby Webster, Florida, to make use of one of the shop's lifts. The job is no more difficult than installing a factory transmission in a Corvette, but because the fixed crossmember does require the rear of the engine to be lowered and the transmission to be at an extreme angle during installation, performing it by yourself with a transmission jack can be a challenge. Using an extra pair of hands, we found it easier for two people to lift the trans in place manually.

After bolting the transmission in place using the new mount and crossmember bracket, it took only a half-hour or so to bolt the driveshaft in place, hook up the speedometer and reverse lights, adjust the clutch, and reinstall the boots and console pieces. The professionals at Inline Performance Specialists had the factory trans removed and the new Tremec TKO bolted in place before lunch, leaving just the ancillary equipment to be hooked up in the afternoon. For the average mechanic in his own garage, the American Powertrain ProFit3 conversion could easily be accomplished in a weekend, if not a single day.

With the transmission installed in our Corvette, it was time for the fun part: testdriving the car. Having become accustomed to the long throws of our C3's original T-10 and its external shifter linkage, we were immediately impressed with the precision of the American Powertrain shifter. Gear selection resulted in precise clicks, with short throws between the gears. We hadn't even made it out of the parking lot, and we already knew this conversion was a great choice.

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On the road, we were even more impressed with the performance of our Stingray. The position of the shifter is just like it was from the factory, but accelerating through the gears tells you immediately that this is not a 1971-vintage trans. First gear of the Tremec TKO is noticeably lower, providing easy rapid launches and great throttle response. The remainder of the gears are nicely spaced in terms of ratio, all the way up to the 0.64 overdrive of Fifth. Prior to the American Powertrain conversion, our 350 would be spinning above 3,000 rpm to maintain a speed of 70 mph on the interstate. Now, we simply click the shifter into Fifth, and rpm at 70 mph drops to 1,900, making the car quieter, more fun to drive, and more fuel efficient as well.

As our project car has progressed, we've made major improvements to C3 Triple-Ex's aesthetics and mechanical hardware, resulting in a car that handles, stops, and accelerates better than it did when new. We must say, however, that the American Powertrain five-speed conversion is one of the most noticeable upgrades we've performed, improving the car in numerous ways and making it better suited to the more-powerful engine we have planned (see sidebar). Follow along now, and we'll show you the steps it took to convert our Stingray to five gears of fun.

What Do You Think?
We've spent a lot of time building C3 Triple-Ex so it can handle a powerful engine, but deciding which engine to build for the car has proved a difficult task. Of course, we could always rebuild the 350 small-block that's in the car, and we also considered a big-block (who wouldn't want a big-block Stingray?), as well as a conversion to some kind of LS engine.

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Then one day at lunch, the subject of LS conversions came up, and someone suggested the LS7 as a viable candidate. Editor Heath then countered with a challenge: "Can you build a traditional small-block of the same displacement (427 ci) that will out-power the LS7 on pump gas?"

Which option do you think we should exercise? We must admit that an LS conversion would be cool, but the challenge of topping the LS7 with a first-generation SBC might be even cooler. Log on to or write us a letter and let us know what you think!



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