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.