(first posted 9/23/2014) Chevrolet’s two-speed Powerglide automatic had a long career, and a somewhat strange one at times, especially so in its relationship with the Corvette. The first Corvette (1953) was only available with the Powerglide, a choice made perhaps out of necessity (lack of a suitably-strong manual) or because the automatic transmission was seen as futuristic at the time. But an even odder Corvette was the final (and ultimate) one to sport the Powerglide, the 1967 with the optional 427 (7 liter) big block, and crowned with triple carburation. Chevy’s newest engine and oldest transmission were mated in a final blow-out tribute to the Powerglide. Care to guess how fast one could go in Low gear?
Before we take a stab at answering that, let’s also pay homage to the fact that the 1953 Corvette was the first production car to have a floor-shifted automatic transmission. It emerged directly out of the side of the transmission, undoubtedly an expedient way to add a lever to the Powerglide’s existing shift mechanism that exited there.
The gear markings were discretely labeled on the knob, which reflected the older PG shift pattern, and in this case, was also the reverse of the typical floor shift to come, with P at the bottom. The Corvette’s PG gear pattern went through several convoluted evolutions, but maybe that’s a story for another time (or not).
Eventually it morphed into the familiar PRNDL, as seen here in this 1963. The Powerglide’s take rate dropped steadily through the years, and was available only with the mildest engines, the 250 hp and 300 hp 327, the latter being standard since 1966.
Although the big-block engines became available in 1965.5 (396) and 1966 (427), they were only offered with the four-speed manual in those first two years, even the hydraulic cam 390 hp version of the 427.
But in 1967, Chevrolet graced slush-box lovers with a one-year only offering: the Powerglide behind either the 390 hp four-barrel 427, or the 400 hp tri-power 427. The mechanical-lifter 435 hp engine was still off-limits, as was the 350 hp 327.
The question you’re undoubtedly asking is “why not the excellent THM-400, which had been in production for several years and was available on big-block Chevrolet sedans?” Well, I didn’t have to Google for an answer: it obviously wouldn’t have fit under the C2’s rather petite transmission-hump floor panel. In 1968, the new C3 and its giant transmission tunnel and console finally played host to the THM.
The pictures of this restored red ’67 Corvette convertible I’m showing you is one of these rather unusual and rare cars, a 1967 427 with the 400 hp tri-power, Powerglide, and even air conditioning (power steering wasn’t available). How many of these unicorns were built in 1967? Exactly 207. Another 392 were built with the 427/390/PG combo. A gentleman’s express.
So let’s get back to that question, of how fast one of these 427 Corvettes could go in Low gear. Would you believe 90 mph? That’s my calculation based on the tallest available rear axle ratio of 3.08:1, and assuming the 427 could be coaxed to spin a bit over 6000 rpm. The 427/400 made its maximum hp at 5400 rpm, but as the tach here shows, red line wasn’t until 6000 rpm. Just how fast one of these would rev is subject to debate and how good of a tune-up it had. With an even taller axle ratio, it could have topped 100 mph in Low. Top speed (in High gear) was in excess of 140 mph.
We can safely assume that this Corvette was the world’s fastest two-speed production gasoline-powered car. For that matter, the 427 Corvette had the kind of abundant torque that would have worked pretty well even if the Powerglide had still been in its original one-speed mode. Now that’s something to mull over, at high speed.