Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

[Wayne M.]

I just read an article in the Winter 2025 edition of Restorer entitled "Identifying Automatic and 3-Speed Corvette Transmissions". In the text and a table included in the article it indicates that the Powerglide (PG) transmission used from 1963-1967 was a 3-speed. I have a '67 427/400 PG car and it has 2 forward speeds. I think all C2 PG's have 2 forward speeds? I assume reverse isn't included as a 'speed'. Can someone with greater knowledge than me clarify this? Thanks.

Wayne

[Duke W.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

I haven't received this issue of the Restorer yet, but maybe you misinterpreted. Remember that the base transmission for all Corvettes beginning in 1954 (1953 was Powergllide only) was a three-speed manual transmission until the four-speed first became standard in 1970.

The early three-speed manuals have a non-synchronized first gear, but the base three-speed manual got a synchronized first gear, IIRC begining in 1967.

All "Powerglides" are two-speed torque converter automatic transmission with a 1.82:1 first gear and direct drive second. The first available three-speed torque converter automatic for Corvette was Turbohydramatic 400 in 1968, 2.48, 1.48, and 1.00:1 ratios.

Both of these automatics have maximum torque multiplication ratios of about 2:1 and 1800-2000 stall speeds except the TH 400 available with '69 L-88 and later L-82s that have higher converter stall speeds.

Duke

[Harry S.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

Here is the text from the PDF, It says 3 speed.

I will first address the evolution of the automatic in1953-82 Corvettes: two-speed cast-iron Powerglide
(1953-61), three-speed aluminum Powerglide (1962-67),
Turbo Hydra-Matic 400 (1968-77), Turbo Hydra-Matic
350 (1977-81), and finally four-speed automatic 700R4
(1982).

From the chart in the article

Year................Speeds
1962 ALUMINUM 3 RPO313 POWERGLIDE (250 hp), (300 hp)
1963 ALUMINUM 3 M35 POWERGLIDE BASE (250 hp), L75 (300 hp)
1964 ALUMINUM 3 M35 POWERGLIDE BASE (250 hp), L75 (300 hp)
1965 ALUMINUM 3 M35 POWERGLIDE BASE (250 hp), L75 (300 hp)
1966 ALUMINUM 3 M35 POWERGLIDE BASE (250 hp), L36 (390 hp)
1967 ALUMINUM 3 M35 POWERGLIDE BASE (250 hp), L36 (390 hp), L68 (400 hp)

[Terry D.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

I'm confused, I had a 62 powerglide and it was only two speeds forward.

[Harry S.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

Terry & Wayne. I believe we will find the article is in error.

[Wayne M.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

Thanks Harry. I believe you are correct. Other than the 2-speed/3-speed (typo?) issue, the article was good.

[Wayne M.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

Duke, the online version of Restorer and Driveline is now available.

[John D.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

I recall in Dave McClellan's book he mentions the torque converter stall speed on a PG acts like an additional (low) gear.

[Duke W.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

That's what all torque converter automatics do and did back in the day including TH350, 700R4 2004R, Ford-O-Matic and Chysler Torq-Flite. Buick's Dynaflow was just a single speed torque converter transmission like diesel hydraulic locomotives, but it did offer a "low" than you had to manually shift to. Then there was the short lived Chevy Turboglide... ah, never mind...

The original four-speed Hydramatic that I recall was first used by Oldsmobile in 1948 just had a fluid coupling, so no torque multiplication. First was really short followed by modest drop to second, fairly big drop to third, and modest drop to fourth.

Modern dual clutch automatics like the eight-speed in the C8 don't have torque converters, but first is super short and then there are seven more gears.

Duke

[Roger W.]

Duke, How do the new transmissions work without a torque converter or fluid coupler?

[Mike M.]

Re: Current Restorer Article: 3 Speed Powerglide in C2 Corvettes

That's what all torque converter automatics do and did back in the day including TH350, 700R4 2004R, Ford-O-Matic and Chysler Torq-Flite. Buick's Dynaflow was just a single speed torque converter transmission like diesel hydraulic locomotives, but it did offer a "low" than you had to manually shift to. Then there was the short lived Chevy Turboglide... ah, never mind...

The original four-speed Hydramatic that I recall was first used by Oldsmobile in 1948 just had a fluid coupling, so no torque multiplication. First was really short followed by modest drop to second, fairly big drop to third, and modest drop to fourth.

Modern dual clutch automatics like the eight-speed in the C8 don't have torque converters, but first is super short and then there are seven more gears.

Duke

Duke: without a doubt, you are the most knowledgable TDB contributor I've ever experienced , both here and on the corvette forum. it pains me to correct your posts on this power glide/ hydromatic issue, but all 54 corvettes and all but a handful of 55 vets had the crummy cast iron poweglide trannys. and the first Oldsmobile to offer a hydromatic tranny occurred in 1939, not 1948 as you recalled. I'll forgive you and am very thankful for your participation on this TDM. happy new year old buddy. mike mccagh

[Duke W.]

Thanks for the correction, Mike. I confused the date of first Hydramatic with the first post war OHV V-8 (a Joe Biden moment?). I'm familiar with the operating characteristics of that first 4-speed, fluid coupling Hydramatic as the Williams famiy's '49 Cadillac Fleetwood 60 Special, '54 Pontiac Catalina Coupe, '59 Caddy Series '62 Sedan, and '59 Pontiac Bonneville station wagon were so equipped.

Other than riding or driving the above vehicies, I have actually never owned a car with an automatic transmission and never intend to.

As far as Corvette Powerglide is concerned, checking the NCRS Spec. Guild I see that though Powerglide is listed as a 1954 option (along with several other options) EVERY option was included on ALL 3640 jobs built. PG is listed as a mandaroty option for '55. A footnote indicates a three-speed manual became available late in the model year, but no production numbers are listed for either transmission.

So that leads to the question of how many jobs had the three-speed manual and how were they divided between the I-6 and V-8?

Duke

[Kent S.]

I believe the 6 cylinder was only built in the very early model year and the 3-speed manual was only in the very late model year, so I don't think you will find that combination.

[Duke W.]

Duke, How do the new transmissions work without a torque converter or fluid coupler?

Being as how I have never overhauled an automatic transmission I only have a high level understanding of how they work. I'm not sure if I fully understand current "dual clutch" automatics like like the 8-speed in the C8, but I'll give it a try.

First of all, all automatics have clutches, but back in the day they were referred to as "bands" that engaged /disengaged planetary gear sets. Current dual clutch automatics' architecture is more like a manual transmission, but they have two countershafts. The even number gears are on one countershaft and the odd on the other, and each has a transfer gear to a single output shaft to a final drive gear set built into the transaxle that transfers power (in the case of C8) 90 degrees via axle shafts to the rear wheels. So when upshifting or downshifting one clutch engages on one counter shaft and the other simultaneously disengages, which is what allows gear changes to happen in about 100 milliseconds.

The clutches are wet, like a motorcycle.

The base C8 first gear is 2.91:1 and the final drive ratio (product of the transfer and axle ratios) is 4.89:1 (5.17:1 with Z-51), which is 14.2:1 overall (15.0:1 with Z-51). Compare that to a CR four speed (2.20:1 first) with a 4.56 axle is only 10.0 overall and only 6.77 with a 3.08. That's why I say my 340 HP SWC with a 3.08 axle is like a five speed with no first gear. I believe the Z06 final drive and even some, if not all, transmission ratios are shorter due to its higher rev range.


I found this chart on the CF C8 Forum:

speed_in_each_gear_6d33d8d63d82938588b33c85282b0d4b48ee9eb0.jpg
If I normalize this to a 1:1 8th gear the ratios are:

8.82, 5.33, 3.69, 2.66, 1.96, 1.54, 1.21, 1.00:1

So the overall gear spread is a whopping 8.82:1 compared to 2.20:1 for a vintage CR 4-speed and on the order of 2.5 for the WR.

BTW the "intergear ratios" (the ratio of the ratio between gears) for the above set is:

1.65, 1.44, 1.38, 1.35. 1.28, 1.28, 1.21:1

If the 1-2 shift is at 3000 the revs pick up in second at 3000/1.65 = 1818. Note that the spacing becomes closer as we move up through the gears. This is what I call a "progressive ratio" gearbox and the closer the spacing the less power loss in the next higher gear, and Ideally, especially in the upper gears, shift revs should be beyond the power peak with the intergear ratio dropping revs to the same power level in the next higher gear. This maximizes average power for maximum acceleration rate through the gears.

Note that the base C8 will achieve top speed in 6th, so 7th and 8th are "fuel economy gears" and 70 MPH in 8th is just under 1500 (about 1400 without Z-51), but the excellent torque bandwidth of the base engine should allow it to easily pull the car smoothly on level pavement and even mild grades at freeway cruising speeds while achieving around 30 MPG.

A few years ago I developed a "gear chart" on MS Excel that might be here on the TDB, but I know it's on the CF C2/C1 section. (search for threads started by SWCDuke). It allows analysis of various transmission and final drive gearing and tire revs per mile. If you're good at Excel you can print out a speed in gears graph. and even overlay power curves to find optimum gearing for both acceleration and top speed. It only accommodates five transmission gears, but can easily be expanded to as many as you want.

So, to answer your basic question, a super short first gear and many more gears more closely spaced than traditional automatics is why current dual clutch automatics don't need a torque converter or fluid coupling.

If anyone sees any errors in my explanation or can do a better job of explaining modern dual clutch automatics, feel free to chime in.

Duke

[Duke W.]

I did a Web search and Wikipedia has a pretty good explanation of dual clutch automatics that includes an easy to understand schematic.

https://en.wikipedia.org/wiki/Dual-clutch_transmission

How does the old expression go?... "picture worth a thousand words."

Duke