Chevrolet’s two-speed Powerglide automatic had a long and somewhat curious relationship with the Corvette—both the first and last Corvettes to use PG came with triple carburation. The first Corvette (1953) was only available with the PG but even after the new 3-speed (1955) and 4-speed (1957) manuals became available, PG was still chosen by over 10% of buyers. Powerglide wasn’t available with the new big block 396 V8 that arrived in mid-year 1965 and the 427 in ’66. But in 1967, the last year for the C2, the seemingly odd combination of 427 and PG was available, the ultimate version being the 400 hp version newly 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? Oops; I already gave that away.
Before we take a stab at answering that, let’s also pay homage to the 1953 Corvette. It was the first production sports car to have an automatic transmission, something that was seen as both radical and heretical. It was a choice made out of necessity (lack of a suitable manual). Automatic transmissions was seen as futuristic at the time, seemingly suitable for a Motorama car that was put into production.
With its 150 hp tri-carb 235 cubic inch six and light fiberglass body it was no slouch, with a 11 second 0-60 time, the same as a ’55 V8 automatic Thunderbird.
Not only was the Corvette the first production sports car with an automatic, it was also the first car with a floor shifter for an automatic. 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.
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. It was 10.1% in 1967, overwhelmingly with the 300 hp 327.
And in the last few years of the C2, the shifter ended up like this, as would also be used with the ’67 427 PG.
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 high-revving mechanical-lifter 435 hp engine was still off-limits, as was the 350 hp 327.
The question you’re undoubtedly asking is why didn’t they use 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 fiberglass transmission-hump floor panel. The Powerglide was a pretty compact box. In 1968, the new C3 and its giant transmission tunnel and console finally played host to the THM.
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. How many of these unicorns were built in 1967? Exactly 207. Another 392 were built with the 427/390/PG combo. A true gentleman’s express.
Here’s a look under its hood. The L68 had three Holley two-barrel carbs, set up to run on the middle one only until the vacuum-operated linkage opened the two secondary carbs.
Here’s how that looks without the distinctive triangular air cleaner. We have some vintage reviews of these in our archives, links at bottom.
So let’s get back to how I answered the question of how fast one of these 427 Corvettes could go in Low gear. It took the tallest available rear axle ratio of 3.08:1, with the 427 spinning a bit over 6000 rpm. The 427/400 made its maximum hp at 5400 rpm, but as the tach here shows, red line for both the 390 and 400 hp 427 wasn’t until 6000 rpm. Peak power invariably is below safe maximum engine rpm. 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 a 427 PG 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, prior to 1953, like the original Buick Dynaflow (these two transmissions were fundamentally quite similar). Now that’s something to mull over, at high speed.
Related CC reading:
Automotive History: From Powerglide to 4-speed – The History of Transmissions on Early Corvettes by VinceC
Powerglide: A GM’s Greatest Hit Or Deadly Sin? by PN
Vintage R&T Road Test & Tech Report: 1954 Chevrolet Corvette – “Is It Really A Sports Car?” by PN
Vintage Review: 1968 Corvette 427 – “All Of The Virtues And Vices Of American Technology” by PN
Vintage R&T Road Test: 1969 Corvette 427 435-HP – “More Of A Race Car Than A Road Car” by PN
One lives, and, sometimes begrudgingly, one learns.
You see, when I was young and bulging with the fullness of my own opinions, I’d hear stories from the bogan* class of how they’d put a Holley and extractors and such on their 20 y.o. ’68 GM Brougham* and that “mate, that thing could do 100 in first gear.” Yeah, I thought, right, sure it did.
Turns out they weren’t wrong. (Note the refusal, even now, to actual admission that they were right). Their cars had a 2.7 LSD as standard, and one presumes you could extract a fair few more revs from a standard Chev 307 with a bit of porting and whatever. So if 3.08 in the PG Corvette could get 6K revs, then no reason the little engine could too, and actually, less than 6K revs with the taller ratio. Just slower to do so. Bugger!
Do we have any performance figures to 60, 100 mph for this Corvette combo? I wonder if it’s the quickest of the lot to 60, with no gearchange.
*equiv of redneck, only fonder in concept. One of my brothers was always one, and i love him to death (though didn’t back then!)
*an absurd-looking looong booted standard Holden made for “prestige” in ’68-’71 (it got an honorable mention in CC’s excess overhang list just recently) that was popular with the hotrod crowd in the ’80’s for its V8. Then valueless, now, even more absurdly, expensive.
Yes, it wouldn’t be that hard to get 100 mph in Low. I seem to remember that Aussie 307 being quite rev-happy.
I’ve never run across a road test of this 427/PG combo in a Corvette (or any Chevy). I suspect it was quite quick to 60, especially if it had lower (higher numerical) rear gears.
Commented, instant rejection, weary. Sure, I’m wanted in all 11 continents for crimes against humanity, but damned if i know why this must affect my monitored internet, or why there can’t be SOME bloody solution to it!
Your comments will always be rescued from comment hell.
This car is a head scratcher for me, however, the Paul Revere & the Raiders air cleaner is iconic.
I have only minimal time driving a PG – is there a forced upshift (which would presumably be before 90 mph) or will it hold in low until you manually upshift or something breaks, whichever happens first?
I had never thought about the design constraints that prevented use of the more modern automatic, but certainly see it now.
Up here in Canada, lots of 1965 & 1966 Canadian Pontiacs (like Parisiennes, Custom Sports, etc.) in the 60’s were using Chevy drivetrains such as Powerglides with their 283’s, and 327’s and they would stay in Low if you put it there until you decided to manually upshift it.
I believe the more modern THM350 did not come out until the late 1960’s.
I recall the larger, hd THM400 was introduced around 1965 or so, and was used on a lot of big block engines in GM’s full size cars. The main benefit of the Powerglide was its light weight, compact size and esp. its low parasitic power loss. The numbers vary slightly between internet sources but a Powerglides power loss was abt.18 hp, whereby a THM400 was abt. 44 hp. The THM350 was abt. 36 hp. So, the Powerglide was ideal for the Corvette at that time with its small block engine and chassis size.
Nope. Unless I could part it out or flip it I would not take it to free
Why is the tripower only 400 ft lbs while the 390 hp 4 bbl is 460?
It’s a typo. it makes no sense, obviously, and in other brochures it’s listed at 460 lb.ft., including the ’68 Corvette brochure.
Typos in brochures were not all that uncommon.
Further to my last submission, bit of internet searching , found that even some big block 396 Impala’s in 1967 could be had with a Powerglide. Correction to my last entry, re-read the article and the Corvette has the 427 engine (a big block).
1968 was the last year that PG was available with the 396. But never the 427, other than the Corvette.
No forced uplshift with PG. TH400 was pre-programmed to upshift, but one could hold the pg ’till kingdom com. My ’67 Parisienne 2+2 convertible would hit 75mph in low. Gotta’ love the free reving 283. Mine eventually got 3x2s and TH350…. Ahhhh… the joys of teenagehood…..
https://photos.app.goo.gl/MD3h13jPeVJ7RoUNA
Google cloud link to above 2+2
I’ve only ridden in (and once driven, a C5) in manual transmission Corvettes, including one C2. I think the idea of the Powerglide is appealing. In the case of the C5 that I drove, the six gears seemed 3 or 4 too many.
After my years of searching for a used C4 (only ended because three sports cars in a row showed me that on a nice day I go for the motorcycle anyway) and finding nothing but automatics for sale, it’s so heartening to find out that there was a time when ‘Corvette’ and ‘manual transmission’ were virtually synonymous. Unfortunately, by that time, any C2 was out of my price range.
After all my searching, I came to the conclusion that manual Corvettes were owned by true enthusiasts who kept their cars forever, while automatic versions were owned by someone who wanted a new toy for a couple of years.
This is an interesting combo, I’ll bet you got a real kick in the butt when you floored it to pass a slower car! The downshift combined with the 2 end carbs slamming open would be exciting to say the least.
For me, Corvettes of this vintage have to have that big chrome tree trunk of a 4 speed shifter though. Even though this particular engine certainly didnt need it. With A/C and an automatic this would be a very nice car to cover some ground (rapidly) in, though I suspect it would have some cooling problems on a hot day as big block ‘vettes were known for.
Very generally speaking, the Corvette has been better-suited for road racing than for drag racing. But if you chose to drag race a Corvette, a Powerglide would be the way to go, I would think.
As for tri-power vs. QuadraJet… of course the tri-power was more powerful on Day One, but I have to believe that (even with meticulous tuning) that by about 40,000 miles you would start to get slop in the linkages controlling the outer carbs and the advantages of tri-power would start to slip away.
Actually, the 2 end carbs were controlled by vacuum diaphragms. Just like the secondary’s on a Holley 4 barrel with vacuum secondaries. You could adjust how soon they opened by changing the spring in the diaphragm but they tended to come open very quickly.
It looked cool, but I suspect a properly sized and set up 4 barrel made as much power with less hassle.
Back in the day there were kits to convert to mechanical linkage. I knew a guy who did that to his tri-power GTO. It was a fiddly setup. The vacuum setup was like fuel injection, set it up right and the don’t mess with it.
I forget, did Chevrolet offer a progressive mechanical linkage for those engines? Pontiac did, first as a dealer-installed option, later on manual-shift Tri-Power cars. I assume there must have been something like that in the vast reaches of the Chevrolet order form.
If they did I’ve never seen or heard of it, and I did a fair bit of work with these systems once upon a time. The Holley 2300 carbs used on the end of these were not intended for that. The only linkage they had was a one way return to insure the carbs closed fully and quickly once the throttle was released, and they had no accelerator pumps. I’m sure somebody tried a home brew setup but all I can see that doing is making an already tricky setup even worse. If you took the time to install an “indexing kit” from Holley to insure both diaphragms saw the same vacuum, and install appropriate springs in the housing these worked reasonably well as long as it got excercised enough to keep fresh fuel in the float bowls.
It was also important to make sure there were no vacuum leaks anywhere else on the car ie: headlight mechanisms, wiper trapdoor actuator, power brakes etc.
I honestly dont think these offered any noticeable advantage over a Holley 4150 on Chevy’s High rise manifold, but damn they did look cool!
I think the PG was a tough transmission. A lot of dragsters used it back in the day.
I had a 3.08 in my ’66 427/425 C2 with the CR Muncie 4spd and that would go to about 60 in the long1st gear, so I can well believe that a solid lifter BBC with its increased rev range would go even faster than 90 with a PG if it had been offered. Trouble with the 2 speed and torque converter is that you would miss out on the engine braking and hence controllability in corners that made the C2 so entertaining and also saved the brakes. I,m not sure I would want a kickdown in the middle of a corner either.
I always heard that street racers preferred the single 4bbl 780Cfm to the Tripower setups as it was more consistent and predictable and the 3x2s were a pain to keep synchronized. Performance figures seem to be fairly similar, so I really wonder if the Tricarb set up was worth it. I hardly ever touched the carb in 27 years of ownership. Notably the L88s had single 4bbls…
I’d love to know what the original what the owner wanted from this car – easier driving, couldn’t drive a stick, or dragstrip performance?
These “unicorns” are fascinating in any case.
Imagine what they would do with the Buick switch pitch .