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Gearing Up for Fun and Fuel Economy

Installing a T56 Six-Speed in a Late Shark

Team Vette May 3, 2004

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.


This is the heart of the conversion--a Tremec T56 was modified by Keisler Automotive to provide correct shifter position. Completing the swap is a balanced chrome-moly driveshaft, custom crossmember bracket, rubber isolator, speedometer cable with driven gear, needle-roller pilot bearing, wiring, and mounting hardware. The transmission shown is the for the 650hp kit, based on a new Viper T56 six-speed. In addition to the revised shifter location, Keisler has prepared the transmission to provide direct mating with a factory GM bellhousing, the mechanical clutch linkage, the flywheel, and the clutch.

The Keisler kit works with GM factory 10.5- and 11.0-inch bellhousings. The bellhousing is surface milled (decked) to assure dimensional accuracy. Keisler plans to release heavy-duty reproductions of these units this summer. They also offer bellhousings for LS1LS6 and LT1/LT4 engines using the hydraulic clutch actuation. A pressed steel housing may also be used for drag race applications.

The original automatic's flexplate and torque converter will be replaced with a new flywheel, clutch pressure plate, and disk. Bolts, a new throw-out bearing, and a clutch alignment tool are included in the kit.

No scrounging around junkyards, swapmeets, or Ebay required! Keisler supplies a reconditioned OEM pedal assembly for the conversion from an automatic, along with new repop pedal pads and trim.

Keisler's hydraulic actuator kit for Corvettes completely eliminates the entire original mechanical actuator system, providing the easiest way to set-up clutch actuation while reducing pedal effort at the same time--just like in a C5! The system includes an application-specific master cylinder, slave cylinder with release bearing, reservoir, and all fittings and brackets for a clean installation. Keisler utilizes a hydraulic throw-out bearing that has been tested for a minimum life of 1.5-million cycles. It features a self-centering preloaded bearing, piston with wiper and silicon-molded dust boot, solid adjustable spacer mount, forged-aluminum hardened case, and a spring which keeps the bearing in contact with the clutch fingers at all times for quiet consistent operation. Additionally, this unit does not require an air bleeder line; bleeding the hydraulic clutch system is a one-man job.

It would take an expert's eye to tell that the transmission is 25 years newer than the car with one of Keisler's OE-style shifter handles. The handles are the correct height, use reproduction stock knobs, and offer a spring preloaded, reverse-lockout trigger to simulate the feel of an original shifter. Reverse lockout on T56s and TKOs is handled inside the transmission. A faux early-Shark shifter is seen on the left; the '77 and later is on the right. Shift pattern plates and correct repop shift boots are also offered.

Time to get started! The front seats, console, and stock shifter were removed.

Remove the driveshaft, cooler lines, torque converter bolts, transmission mounting bracket, speedometer cable, and wiring.

After disconnecting carburetor linkage and the tach cable, lower the engine.

Use a trans jack with a safety chain or strap to remove the transmission.

The flex plate was removed next. The engine mating flange surface and dowel pins were then thoroughly cleaned.

The transmission tunnel must be cut out at the rear where the shifter exits.

Some model years require an additional clearance cut to clear the top of the transmission. The hole was cut and patched with a sheetmetal panel, which was then sealed and painted.

After installing the flywheel and torquing it to specs, a heavy-duty needle-roller pilot bearing was carefully fitted into the recess in the end of the crankshaft using a socket and hammer. The pilot bearing plays a very important role in transmission life by supporting its input shaft within the crankshaft. Roller bearings offer a much longer life than the oil-impregnated bushings of yesteryear.

As part of the warranty validation, the bellhousing-to-crankshaft alignment must be checked and recorded. This keeps the crankshaft and transmission input shaft running in alignment with each other. If the alignment is out of spec, the straight dowel pins are removed and replaced with correcting dowel pins made by Lakewood or Moroso. Keisler supplies these and a low-cost dial indicator set, in case you don't have one. Mark the dowel pins alignment to the bellhousing with a paint marker when finished.

Remove the adapter plate from the transmission. Remove the bellhousing from the back of the engine engine and attach it to the adapter plate using the supplied bolts. Next, assemble the clutch fork and release bearing. These steps are not necessary if the hydraulic actuator kit is used, as there is no need for a clutch fork, release bearing, or an adapter plate.

The pressure plate and clutch disk were installed. It is important not to over-torque the bolts and to use the alignment tool to center the disk on the crankshaft so transmission installation will go smoothly. The Hays-manufactured diaphragm pressure plate shown offers positive engagement with reduced pedal effort compared to the Borg&Beck three-finger type that was used originally.

Due to limited space within cowl area, the instrument panel had to be removed to gain access to the pedal housing. With the I.P. removed, the automatic pedal assembly was unbolted and replaced directly by the manual pedal assembly. The clutch rod rubber boot was installed, the clutch rod was as attached to the clutch pedal arm, and then the instrument cluster was reinstalled.

With the new clutch and brake pedal in place, the '79 Corvette was beginning to look like a manual shift car. When the hydraulic actuator kit was used, the master cylinder mounted to the firewall in place of the clutch pushrod and had a spherical rod end (heim joint) to connect to the pedal--which provided a flexible, quiet, and low-friction operation. If you have an original pedal set with the pushrod-mounting pin, you will need to remove it by grinding off the back portion of the pin. The over-center assist spring was not used because the Keisler kit uses a diaphragm-type clutch with less pedal effort for performance street driving. When using the three-finger clutch, the assist spring should be used for lighter pedal effort. A firewall anti-flex kit is also included to eliminate firewall flex when using the hydraulic kit.

The automatic transmission crossmember must be modified prior to reinstallation. Keisler provides a template and the parts to perform the job, which requires cutting the crossmember then welding the new segment in place.

After bolting the bellhousing and adapter plate to the back of the engine, the T56 six-speed was hoisted in place with a transmission jack and bolted in place. With the trans jack still supporting the transmission, the modified crossmember was bolted to the framerails.

Next, a new GM rubber isolator was bolted onto the transmission.

Chores to finish up underneath included connecting the speedometer cable, reverse lights, neutral safety start, back-up lamp, and reverse lock-out wiring. The 475hp (GM-spec) six-speed uses the original GM speedometer cable.

The 650hp Viper-spec six-speed uses an electronic speedometer output and requires a signal transducer to convert the pulse signal to mechanical output drive. Both six-speeds have a safety solenoid to prevent going into reverse when in gear. All wiring connectors are provided.

The heavy-duty chrome-moly driveshaft was installed in two steps. First, the driveshaft with U-joints was installed to the differential. Next, the slip yoke was inserted to the transmission, and then the two were connected using a C-clamp and socket. The original strap-type yoke is not available on the 650hp T56, but the 475hp version can reuse the original slip yoke.

This shot of the completed underside shows a clean installation. The driveline angle remains within specs.

We're on the homestretch now. The shifter exits in exactly the same location as in an original four-speed car. Attach the lower boot and trim ring to the floor and seal with RTV. Next, attach the upper boot to the original automatic console top plate. Bolt the handle on with two bolts, remove the knob, slide console top plate over the handle, and reattach the knob.

Voila! The finished result looks so close to factory that most people would not know the difference. The shift indicator attaches directly to console top plate with no modification, covering up the original automatic selector.

This "Best of Both Worlds" table provided to us by Keisler Automotive shows the benefits of both the faster first gear launch and the "tall" overdrive sixth gear.


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