If you've ever spent any amount of time working with a Doug Nash 4+3-equipped C4, you've probably found yourself scratching your head, uttering expletives that would make a politician blush, pounding your fist against the wall-or all of the above. The concept behind the 4+3 was hatched at a time when automotive engineers were grasping at any and all available straws as they fought to produce cars that delivered decent performance, while meeting the then-current emissions and fuel economy standards.
Back in the day, I remember thinking the 4+3 would be a kickass gearbox for a Corvette. The main case and internal components are derived from the Borg-Warner Super T10 four-speed-a trans that established a reputation for strength, durability, and power shiftability during the '70s and early '80s. Unfortunately, the overdrive portion of the trans proved to be quirky at best; at worst (the more or less normal situation), its performance ranged between outright annoying and downright aggravating. Now, some 20-odd years after the fact, durability is a bigger question than ever, and the challenge of finding parts at a reasonable cost often makes replacement a more viable option than repair.
As an aside, the overdrive control used in some model years is apparently connected to the same wiring as the torque converter lock-up control. I once did a chip for an engine builder who had built and installed a 383 in an I-don't-remember-the-year C4. He was pressed for time, as engine builders usually are, and the only chip he had on hand to be reflashed was for an automatic. He said it was a real thrill when he took the car out for a testdrive and, without warning or provocation, the 4+3 shifted into and out of overdrive.
Obviously, installing a late-model, five-speed transmission isn't a drop-in proposition. it has a different external configuration than a 4+3 and also employs an internal rail shifter, rather than a side-mounted external type. Fortunately, companies like Classic Motorsports Group offer complete kits that greatly simplify the task.
These kits are typically built around Tremec TKO 500 or TKO 600 five-speed transmissions and include most, if not all, of the necessary conversion hardware. We used Classic's Ultimate Fit TKO-600 kit, which includes a specially modified transmission with a custom Hurst Sidewinder shifter mechanism, a Spicer 1330 slip yoke, a custom torque arm bracket, a shifter handle with five-speed knob, a template and close-out panel for modifying the transmission tunnel, a speedometer interface kit, hardware, and a comprehensive installation instruction manual.
We selected a TKO-600 model because of its higher torque capacity and more attractive gear ratios. With 2.87, 1.89, 1.28, and 1:1 first through fourth ratios, the 600 has a nice spread between gears. Its first gear ratio is also better suited to a 3.42:1 rear axle ratio than the TKO 500's 3.27:1 first gear; its .64:1 overdrive is also a better match than the 500's .68:1 fifth gear ratio.
Bolting the transmission into Project C4orce required a few alterations to the standard installation, necessitated by the LSx engine. The Tuned Port engine that had been residing in Project C4orce's engine bay was fitted with a 153-tooth flywheel, so the original bellhousing couldn't be used. In its place, we installed a traditional, old school, small-block bellhousing designed for a 168-tooth flywheel and mechanical throw-out bearing. These bellhousings are relatively easy to find and are reasonably priced.
However, since these bellhousings have been designed to work with mechanical clutch linkage, they have no provision for mounting a slave cylinder, which is required with a hydraulic throw-out bearing. That problem is easily resolved by installing McLeod part number 1363 and routing the hydraulic lines through the opening in the bellhousing that was formerly occupied by a clutch fork.
Pilot bearing location is another issue that must be addressed during installation of a transmission designed for a traditional small-block. Standard bearing location in an LSx crankshaft is farther forward than in a Gen I small-block, so if a traditional-style bellhousing is bolted to the rear of an LSx engine, the input shaft on a Gen I-compatible transmission isn't long enough to engage the pilot bearing. The remedy for this is to install GM pilot bearing number 12557583, which is designed to fit the larger diameter bore in the very end of an LSx crankshaft.
A third area of modification is the C-bridge section that contains the bolt holes through which the transmission tailhousing is connected. These have to be elongated approximately 1/4-inch to accommodate the LSx engine's bellhousing face position, which in our installation is slightly forward compared to that of the original engine.
All in all, installation of Classic Motorsports Group 5-speed's TKO kit is a straightforward task that is a no-brainer for anyone with a reasonable amount of wrench-turning experience. The kit is supplied with excellent instructions, and the modifications required to install the transmission behind an LSx engine are minimal. Any question as to whether the additional effort required to install a TKO transmission (as opposed to a rebuilt 4+3) is justified, is emphatically answered during the first few minutes in the driver's seat. The TKO's smooth, positive shifting, combined with no surprises as to which gear has been selected, makes driving much more enjoyable. And the fact that you have to be stupid, completely uncoordinated, or both in order to damage the transmission allows you to repeatedly enjoy vigorous acceleration without fear of suffering the effects of automotive edentulism (if you're wondering, edentulism is the condition of being toothless). Purists may cringe at the thought of replacing an original 4+3 with a TKO 5-speed, but if you want to drive a C4 and enjoy the experience, it's clearly the right thing to do.