Upgrading and modernizing vintage Corvettes is one of the hottest trends in the hobby today. More and more older Vettes are being built or modified for both shows and driving than ever. These owners are taking the necessary steps to retain the timeless looks of a classic Corvette, but are updating, where applicable, to make that classic drive similar to or better than a much later model. What's the purpose? To hit the open road and make lifelong memories with fellow enthusiasts, friends, and family. The NCM's Corvette Caravans and events that attract thousands of Corvettes, like Funfest and Corvettes at Carlisle, are wildly popular for reasons like this.
But driving a vintage Corvette with either the original Muncie, T10 four-speed manual gearbox, or an automatic (TH400 or TH350 or the dreaded Powerglide) at sustained high speeds on the interstates quickly brings several problems to light. The high rpm you experience with an older, non-overdriven transmission causes driver fatigue and poor fuel economy, and it contributes to reduced engine and accessory life. Swapping rearend gears or a complete differential to a long-legged (low-numerical) setup is a major cost and labor-intensive project in any older Corvette. And when a "tall" highway ratio is installed, acceleration is severely and quickly sacrificed.
Over the past several years, we have touted the benefits of modern transmissions with wider spaced gear ratios and overdrives, and we have published articles showing a do-it-yourselfer installation of a 700-R4 in a '72 small-block coupe and a Bowtie Overdrives' 700-R4 installation in a solid-axle.
All else being equal, a wide-ratio automatic overdrive should give both better performance and better fuel economy than a traditional close (or closer)-ratio, non-overdrive transmission, automatic or manual. So, why would someone want to change from the effortless operation of an automatic to the more involved operations of a manual transmission? Well, for many Corvette enthusiasts, a real sports car has to have a stick shift. But, there are some real, measurable advantages, too. More power reaches the rear wheels--automatics rob at least five percent, maybe more, horsepower and torque than manual gearboxes. And that lost power is turned into another form of energy, heat, which is transferred to the transmission fluid. The transmission fluid is pumped through the radiator's trans fluid cooler to be dissipated as radiant heat. Manual transmissions, due to their inherently higher efficiency, do not rob enough power or generate enough heat to even require an oil cooler in street applications. A modern manual transmission weighs substantially less than an automatic, with its large fluid capacity and related system components. Finally, there is less maintenance with the new manual transmissions; they are inherently simpler than automatics and require less time and money to maintain. Remember the old adage, "KISS?" Keep It Simple, Stupid!
We've been intrigued with the concept of putting a five- or six-speed manual gearbox in an older Corvette (without resorting to a full-on, "Newman" [Car Creations] reconstruction, including an entirely new powertrain)--just a modern manual box behind an original engine.
There are some great possibilities out there. On the low end of the scale are the old T5 five-speeds out of third-generation Z28s. Several notches up are the Tremec TKO five-speeds, the almost bulletproof (at least behind a small-block), wide-ratio overdrives that are a favorite with the Cobra kit-car crowd. Then there's the Tremec (ne Borg-Warner) T56 six-speed--a modern marvel with moderately wide ratios and two overdrives. Variants of the T56 are found bolted directly to the differential in all manual box C5s, in a traditional directly-aft-of-the-engine and "normal" tailshaft housing in non-automatic '93-02 Z28s and Trans Ams, and in all Dodge Vipers. We've been waiting for some enterprising company to do for manual tranny swaps what Bowtie Overdrives did for automatics.
Enter Keisler Automotive Engineering and their lineup of five- and six-speed transmission installation systems for classic Corvettes, as well as nearly every popular muscle car, classic truck, and street rod. One of their newest setups is for installing a T56 six-speed in the Shark-bodied, third-generation Vettes.
The Keisler kits are based on modern technology transmissions--brand new Tremec TKO five-speeds and T56 six-speeds. These transmissions have large diameter and width chrome-moly steel gears, quiet and efficient tapered roller bearings, all chrome-moly steel shafts, short throw internal shifters, and a trim weight of just 99-pounds for the five-speed and 138-pounds for the T56 six-speed (thanks to a pressure die-cast alloy ribbed case). Because these are OE boxes, there is full spare-parts support. Keisler's kits are custom-engineered to fit a TKO or T56 in each specific application, and the kits are shipped with all necessary installation parts, including a lightweight balanced chrome-moly driveshaft.
We've recounted the advantages of late-model transmissions in previous articles, but it's worth mentioning again. You get faster acceleration in first gear--with the TKO and T56 you get (in order) a 2.97:1 or 2.66:1 low gear for both great around-town drivability and spine-tingling acceleration at the stab of the throttle. You reap the highway benefits of low rpm, overdrive fifth, and (in the T56) sixth gears--gearing that will drop the rpm levels at an interstate-friendly 75-80 mph to below what was previously turned at a meager 55. There is less wear and tear; your engine will thank you for it and so will your bank account when you weigh the costs of rebuilding (or even replacing) a Corvette engine. You will see a notable improvement in fuel economy. No old Corvette will ever be a paragon of fuel efficiency, but in this day of premium unleaded nudging $2.50 per gallon, every penny--or mile per gallon--counts. Keisler told us that numerous customers are reporting a 30- to 50-percent fuel-economy increase when changing from a four-speed to a T56, and from 40- to 60-percent gains in gas mileage when changing over from a TH350 or 400 to a T56. With gas prices what they are (and probably destined to stay inflated), it wouldn't take too many thousands of miles for the gas savings alone to pay for the cost of a trans swap. And there are still the factors of improved performance and less wear and tear to be considered.
We think there are some very good reasons to consider converting from an early model stock trans, either four-speed or automatic, to one of Keisler's Tremec five- or six-speeds, So, let's see what it actually takes to make the switch. Our subject is a '79 Corvette L82, originally and until recently still equipped with a TH350, that is owned by Tom Aden of Tremec Corporation. Tom sent his car to Keisler's facility in Knoxville, Tennessee, for the conversion to a Viper-based T56. The conversion from old automatic to modern overdriven six-speed is basically a bolt-in operation with the changeover--including pedal and clutch actuation gear--requiring minimal fabrication and taking two days from start to finish.
Upon completion, Tom reported a marked and immediately noticeable improvement in off-the-line acceleration. Tom also stated that the L82-powered '79 achieved over 23 mpg over a 500-mile trip to last August's Corvettes at Carlisle--compared to 14 mpg on his previous trip with the Turbo350 automatic.
Improved performance and better economy--sounds like a winning combination to us.