Pity the poor Powerglide. The two-speed automatic, when introduced in 1950, brought automatic transmission availability to lower-priced cars and became ubiquitous in Chevrolets of the 1950s and ’60s. Yet it made just about any car feel sluggish, especially in those situations where the car was traveling just a bit too slow for the torque curve in Drive, yet too fast to drop it in Low without winding out the motor.
“Sluggish” is not a good trait for a performance car, and yet Corvette buyers who didn’t want to shift for themselves had to settle for the Powerglide automatic for years—through the car’s first two generations, in fact. A three-speed Turbo-Hydramatic automatic wasn’t a factory option in the Corvette until the C3’s 1968 introduction.
Not surprisingly, the hot rodders at Hot Rod magazine figured out a solution to this problem as far back as 1956. HRM’s Ray Brock wrote about putting a “dual-range” four-speed Hydramatic out of a ’52 Lincoln in his wife’s ’56 Chevy 210 in the Jan. 1957 issue of the magazine. After the swap was performed at Transmission Specialists in Los Angeles, the results were “far above expectations,” Brock wrote. “Initial jump from a dead stop is fantastic compared to the old days with the Powerglide. Four forward gears instead of two give much better acceleration all through the range and the Third gear, which permits a speed as high as 75 mph before a shift into Fourth is necessary, really gives medium speed performance.”
With that kind of success, it wasn’t long before photographer Eric Rickman found a Corvette owner doing a similar swap into his ’56 Vette at the same transmission shop. “Corvette with a Hydro” in the magazine’s Nov. ’57 issue illustrated the procedures needed for the exchange that were different from Brock’s earlier story. (Rickman directed readers not familiar with that first installment to contact the magazine’s back-issues department and get a copy for 25 cents. The last time we were on eBay those 1950s issues were going for a bit more.)
Neither swap was simple. The transmission was modified with a new main drive gear and front planetary carrier; a pilot bushing was installed in the back of the small-block’s crankshaft; and the truck Hydramatic flywheel, torus cover, torus members, bellhousing, and starter replaced the standard Chevy car pieces.
For the Corvette, the transmission—a ’50 Hydramatic—was further modified with extra clutch discs and valvebody changes to convert it to a “Sur-Shift” model, “so that the transmission could be held in any of the four forward speeds as the driver wished,” wrote Rickman.
New rear transmission mounts were fabricated, and a hole was cut in the floor (later covered with an aluminum plate) to clear the shift and throttle-pressure levers. An additional bracket was fabricated and bolted to the crossmember to hold the reworked shift lever (not an issue with the Powerglide since its shifter bolted to the transmission itself).
Hooking the transmission to the driveline required an Oldsmobile U-joint and a shortened driveshaft. Rickman also noted that the truck starter interfered with the exhaust pipe on the right side of the car so it had to be re-routed. The weight difference between the Powerglide and the TH transmission “is small,” Rickman wrote. “Handling is unchanged.”
Rickman’s story doesn’t give a cost for the swap, but points out in a caption that the long parts list needed for the job “is one of the reasons why the conversion is fairly expensive.” Yet the Vette’s owner, Ted Northrop, was all smiles after his initial test drive with the new transmission. Rickman drove the car too and said its performance was “electrifying” compared to the Powerglide. “The owner had experienced trouble on three separate occasions with the Powerglide but since installing the Hydra-Matic, the Corvette has been trouble-free and it does charge.”