Recently Paul posted a vintage Road and Track article on the early Corvettes which asked if the Corvette was really a sports car. Much discussion ensued in the commentary about the exclusive use of the Powerglide on the first Corvettes. Selecting an automatic as the only transmission for a sports car was a controversial choice, especially in 1953.
As Paul has mentioned in the past, although the Powerglide was a 2-speed automatic, it was fairly comparable in performance to a contemporary 3-speed manual, which in 1953 was the only manual transmission available in American made cars. Undoubtedly the Powerglide was seen by Chevrolet as a more advanced transmission compared to its old-fashioned three-speed. However, with Zora Arkus-Duntov becoming heavily involved with Corvette shortly after its release, he helped introduce a close ratio three-speed manual transmission which was followed by a 4-speed manual transmission in 1957. So, let’s take a more in-depth look at the early Corvette and the history behind the transmissions it used.
To begin, it will be necessary rehash some history involving the development of the Corvette. In early 1952 Harley Earl decided that his next Motorama show car was to be a sports car. Inspired by European sport cars, he organized the project in secrecy, code named Project Opel. Earl used a studio of the 3rd floor of Plant 8 at the old Fisher Body plant, to keep the project hidden from the other stylists. In May of 1952 Earl showed Ed Cole Project Opel, who expressed immediate interest in the idea. Cole was already in the midst of attempting to revolutionize Chevrolet to have a more youthful image, but most of that wouldn’t occur until the 1955 model year. Cole figured that if Chevrolet introduced an affordable sports car, this would only further help his mission of establishing a more youthful and performance oriented image for Chevrolet. Cole believed that he would be able to engineer a worthwhile sports car relatively quickly using the massive Chevrolet parts bin.
Both Earl and Cole were able to convince Chevrolet General Manager, Tom Keating, on moving forward with the sports car, but GM President Harlow Curtice was a little more difficult to win over. Curtice decreed the project could only become a production model once they had positive feedback from the public at the Motorama Show. That wasn’t until January of 1953, which was about six months in the future. Nevertheless, Cole didn’t wait the six months to start working on a production worthy model. Cole was certain that the sports car would receive a lot of public interest. He knew that he could easily work on the project and hide the expenses within Chevrolet’s massive budget without Curtice being the wiser.
Consequentially, Project Opel moved ahead in secrecy. Since they were limited to the Chevrolet parts bin, the engine was to be the conservative 235 Stovebolt six. The 1953 Chevrolet passenger cars used two different 235 six engines, one rated at 108 hp for the manual shift cars, and another version rated at 115 hp for the Powerglide equipped cars. On paper, it appeared that the two versions of the 235 six were very similar, with the only significant difference being the higher compression ratio on the Powerglide engine. However, this actually was not the case. Chevrolet had performed some major engineering improvements to the venerable six for 1953, but only the Powerglide engine received the big updates.
For the first time, the Chevrolet six used a fully pressurized oil system, rather than the previously used partially pressurized and partially splash system. The new 235 also used modern style insert bearings verses the older Babbitt bearings. As mentioned, these improvements were only made to the Powerglide Blue Flame 235 six, which also used hydraulic tappets. The 235 Thrift-King six, continued to use the old oiling system, Babbitt bearings and mechanical lifters.
When it came time to select an engine for the Corvette, the obvious choice was to start with the more advanced Powerglide Blueflame 235 six. For use in the Corvette, the engine underwent substantial modifications which resulted in a significant increase in power. These included three side draft Carter YH carburetors, a twin exit exhaust manifolds, a more aggressive mechanical camshaft, and higher compression. The result was 150 hp (gross). So why stick with the Powerglide? According to automotive historian Karl Ludvigsen, because they used the Powerglide version of the 235 six as the basis for the Corvette engine, it only made sense to use the Powerglide as the transmission. With the short timelines that Cole was working under, it certainly made sense to make as few changes as possible. Furthermore, the Chevrolet 3-speed manual didn’t have ideal ratios for a sports car, with a 2.94:1 first and a 1.68:1 second gear ratios.
The Powerglide Transmission
That said, I tend to also agree with the belief of many, that the Powerglide was seen as the more advanced transmission, the path to the future. This certainly fits the criteria for a Motorama or halo car. I think this attitude is best captured in a quote by Maurice Olley, the renowned chassis engineer who designed Corvette’s chassis and suspension, when he stated:
“The use of an automatic transmission has been criticized by those who believe that a sports car enthusiast wants nothing but a four speed crash shift. The answer is that the typical sports car enthusiast, like the ‘average man,’ or the square root of minus one, is an imaginary quantity. Also, as the sports cars appeal to a wider and wider section of the public, the center of gravity of the theoretical individual is shifting from the austerity of the pioneer towards the luxury of modern ideas. There is no need to apologize for the performance of this car with its automatic transmission.”
This use an automatic transmission in Corvette incidentally caused another first – the first use of an floor shift for an automatic transmission. While I am not entirely sure if the Project Opel team initially attempted to use a column shift, it was quickly learned that it was not possible. The Corvette had a considerably lower steering column than the Chevrolet passenger car reducing the space for a shift linkage. In addition, the triple carbs rearmost carburetor also didn’t allow for adequate room for the shift linkage. The simple solution was to have a small shifter protrude through the floor.
Note how little room there is for a column shift
Along with the major revisions to the 235 six, Chevrolet also made major engineering changes to the Powerglide for 1953. The original concept of the Powerglide was for to derive all of its torque multiplication through the torque converter. The car was to start in direct drive and only the torque converter would provide the torque multiplication, which gradually decreased without any shifts until the torque converter was fully coupled. While one could start in low gear and then manually shift to drive, low gear was not robust enough for frequent use and was meant as an “emergency low.” The Powerglide worked and made for a smooth shiftless transmission, but also made a very lethargic car.
For 1953, the hydraulic control system was redesigned to allow the transmission to start in first gear and then shift to second automatically. This change required the use of a stronger low band and clutch. The revised Powerglide resulted in a major improvement in performance and it was deemed to be sufficient for use in the new Corvette.
A three element Powerglide Torque Converter
The Powerglide had a stall ratio of 2.10:1 and once this is applied to low gear ratio of 1.82:1 ratio, the overall ratio becomes 3.82:1. This is actually considerably lower than the 2.94:1 gear of Chevrolet’s three-speed. However, since the toque multiplication is done through torque converter rather than gears, there are some frictional losses, meaning it isn’t as efficient as traditional gears. Nevertheless, it did make for a seamless and shiftless torque multiplication as compared to using an additional gear ratio.
For use in the Corvette, the Powerglide saw additional enhancements, including higher hydraulic pressure to help it handle the additional torque, the number of torque converter drive bolts were doubled and the tailshaft had to be modified to use the open driveshaft of the Corvette, as Chevrolet sedans used torque tubes. After all that said, how did the Corvette actually perform with the Powerglide? It did reasonably well, especially for something built in 1953. It ran a 0-60 time of 11.0 seconds and a quarter mile time of 18.0 seconds.
Nevertheless, during this time, manual transmission swaps were fairly common in early Corvettes. Despite the fact that the Powerglide transmission was pretty comparable a three-speed manual, performance car enthusiasts wanted a manual transmission in their Corvette “sports car.” I dug up an old article from Hot Rod magazine, which is available online here, that discusses an early swap of a three-speed into a Corvette. In this particular case, a Ford three-speed manual is used from a 1951 Ford truck which used a 2.78:1 first gear and a 1.61:1 second gear.
Installing the Ford truck three-speed on the 235 six.
The article tells the story of how the owner of the Corvette raced a Ford Thunderbird and was beaten by a small margin. He wanted to beat the Thunderbird and thought that a manual transmission would improve the performance. So the swap was performed and the new transmission did result in a slight improvement in performance. A follow-up race allowed the Corvette to beat the Thunderbird by about the same distance that the T-Bird originally beat the Vette when it had the Powerglide.
This article demonstrates when all else is equal, that the Powerglide gave up very little to a three-speed transmission. And although we don’t have the test data to back up the performance difference between the two transmissions, this story shows that the performance difference was modest. Most of the difference would likely be attributed to the Powerglide having less efficiency resulting in more parasitic losses compared to a manual transmission.
Zora was an experience race car driver. This is him at LeMans in 1952 with an Allard J2X.
With Zora Arkus-Duntov heavily involved with the Corvette, he wanted to make things more serious. Through his actual experience with sports cars and racing, he knew that a manual transmission was a must. He deemed the existing Chevrolet 3-speed transmission was unsuitable. Maurice Olley asked Zora Arkus-Duntov to evaluate several other transmission options. This included a 4-speed Hydramatic, an Oldsmobile 3-speed and a Jaguar 4-speed gearbox. The Olds three-speed had closer ratios than the Chevrolet 3-speed, with a 2.39:1 first gear and a 1.53:1 second gear. After extensive testing by Zora Arkus-Duntov, he concluded the Hydramatic first gear ratio was too low to be useful. He also concluded that the Olds 3-speed’s steeper first gear was still too low. He did, however, like the second gear on the Jaguar gear box, which was 1.21:1.
A Saginaw Close Ratio Three-Speed Transmission. This Corvette 3-speed transmission was used from 1955-60 without revision to its gear ratios. In 1961, revised wider ratios were used for the 3-speed.
Zora thought that a first gear ratio of 1.84:1 would be ideal for the three-speed box. After testing it was found that this was too steep for a first gear and it caused significant performance loss. As a result 2.21:1 first and 1.31:1 second were found to be the best compromises, allowing for minimal difference between the three speeds, while first gear didn’t hamper the low end performance. This Saginaw close-ratio transmission was released near the end of 1955. There were very few 1955 Corvettes produced with the three-speed manual, only about 75 cars. None of the 7 six-cylinder Corvettes produced for 1955 left the factory with the three-speed.
For 1956, the close ratio three-speed became the Corvette’s standard transmission. Road and Track tested two versions of the 1956 Corvette, both with the 225 hp dual-quad 265 V8. One had a Powerglide and the other the close ratio 3-speed. From the test data, the 3-speed car was considerably faster than the Powerglide, but it’s not as simple as the numbers suggest. The Powerglide Corvette had a green engine that only had only logged 600 miles, while the 3-speed car was well broken in with over 3000 miles. The effect of the green engine hampering performance is partially supported by the fact that the 1956 Powerglide Corvette actually had slower acceleration at lower speeds than the less powerful 1955 Corvette Road and Track tested. Furthermore, the ’56 Powerglide Corvette was the heaviest car of the bunch.
Road and Track commented that both ’56 Corvettes had a pronounced flatness on the acceleration take-off due to the carburetion. It seems Chevrolet using two 4-barrel carburetors still needed some tuning or at least these test cars needed some work. Zora’s close ratio transmission had a significantly steeper first gear than the Powerglide car. With a 2.21:1 first gear and a 3.55:1 rear axle ratio, the overall ratio was only 7.81:1. This compares to the Powerglide Corvette’s 13.56:1 overall launch ratio. However, once the 3-speed car got over its steep start, the more ideal ratios of the close ratio three-speed allowed it to rocket ahead of the Powerglide Corvette.
Corvettes at Sebring in 1956 used a ZF 4-speed transmission
Despite the close ratio 3-speed being developed for the Corvette, Zora was still not entirely satisfied. As early as August 1954, he had written a letter to his friends that worked at Porsche to get suggestions on a suitable 4-speed transmission that could be used in the Corvette. He received a response that suggested the four-speed gear boxes from the Jaguar Mark VII and the Mercedes Benz 300SL. Chevrolet also used a ZF four-speed in the Sebring raced Corvettes for 1956. None of these options were going to fly with GM, so that led the in-house development of a 4-speed.
An early T-10 4-speed, note the Reverse gear is outside the main case.
Consequentially, by late 1957 the Borg-Warner T-10 was added to the Corvette option list. Although built outside of GM, it was actually designed by Chevrolet engineers. The primary engineer, James W. Fodrea, used a Borg-Warner T-85 three-speed transmission as the T-10’s design basis. The T-10 shared the same case design, gear centers, and 3-4 synchronizer with the T-85, and the T10 was full synchronized unlike the Saginaw 3-speed. The transmission case didn’t have room for a fourth speed, so the reverse gear was moved to the tailshaft housing to allow for the four speeds to be contained within the main case. The ratios were very similar to the close ratio three-speed, simply with the addition of a new gear between the first and second speeds. The end result was the T-10 had a 2.20:1 first, 1.66:1 second and 1.31:1 third and 1:1 fourth ratios, along with a 2.25:1 reverse.
Sold under RPO 685, it was an expensive $188 option, due to the higher cost of GM buying the transmission from Borg-Warner rather than building it in-house. It was officially released for sale on April 9, 1957, making it a short lived option. Only 664 Corvettes got the 4-speed in 1957, making it even more rare than a than a 283-hp Ram Jet Fuel Injected 283 V8 engine. By 1958 the T-10 four-speed became far more popular, supplanting the close ratio-three speed as the most popular transmission, which it would remain for many years to come.
The Borg-Warner T-10 was introduced in 1957 in the Corvette but was used in many other applications.
General Motors had an exclusivity contract with Borg-Warner for the T-10 transmission, which expired in 1960. Once this contract ended, Borg-Warner T-10 was also used by other American manufactures, including Ford, Chrysler and American Motors in an assortment of cars with a variety of different ratios. In the Corvette, the close ratio T-10 remained the sole 4-speed option until 1961. For the 1962 model year a wide ratio T-10 was added to the Corvette option list which used a 2.54:1 first, 1.92:1, second and 1:51:1 third, but the close ratio T-10 was still used for the higher powered engines. During the 1963 model year the two T-10s variants were replaced by GM made wide and close ratio Muncie 4-speed transmissions.
The Muncie transmissions were a design evolution of the T-10, based on James W. Fodrea’s original design. He originally applied for a patent in 1957 which he further revised in 1960. His patent, number 3088336, was granted in 1963. All Muncie transmissions had this number stamped on their cases. The close ratio Muncie used almost identical ratios as those developed for the original Borg-Warner T-10 in the 1950s. Eventually when the Corvette was redesigned in 1968, it had a transmission tunnel with enough room to allow the 3-speed TH-400 to fit, an automatic transmission which gave up little to a 4-speed. By 1974, an improved Borg-Warner Super T-10 replaced the GM made Muncie transmissions on the Corvette, albeit it used revised ratios. The Super T-10 was used by Corvette into the early 1980s. However, by this time the automatic transmission had become the primary choice for the Corvette customers. Today the latest Corvette is sold exclusively with an automatic, just like in 1953. So maybe Maurice Olley was right defending the automatic back in 1953, he was just almost 70 years ahead of his time.
Well Vince, I feel like a kid who just got handed a giant piece of cake with ice cream; dessert to follow the meal of the ’54 Corvette review. You’ve answered all the questions that it raised, and most thoroughly, as is your style.
But there’s one question left: if the GM engineers designed the T-10, why did they start with a Borg Warner box, and not their own Saginaw? Because of physical constraints? I know the T-85 is a heavier duty affair than the Saginaw, so maybe they just acknowledged that it had better bones? Or?
I am a bit fascinated by gear boxes and gear ratios. I’m also a bit surprised that Duntov didn’t find that Olds three speed acceptable. Although the ratios weren’t absolutely optimal, they were certainly much better than the existing Chevy three-speed.
Duntov doesn’t seem like the type of guy to go – “eh, good enough.” Which is probably why he rejected it.
Thank you kindly Paul. I guess you dug up your own answer below on why the engineers started with the Borg-Warner T-85. Thanks for sharing the link.
Based on that story I read about with Duntov, it’s my belief he was trying to pick the gear ratios that would be ideal for sport car type racing. My best estimation is that he wanted a very high first gear so as to allow first to be more usable when one had to downshift for a really sharp corner. With a 2.50:1 or so first, he probably thought it was too low to be useful for that type of driving. His choice of a 1.84:1 first gear was probably good for this type of driving, but it hurt the off the line acceleration too much. Of course the 4-speed was the real answer, which is why he pushed for and eventually got one .
I also recall Dave McLellan commenting that Duntov always preferred close ratio transmissions, even with big block engines. McLellan seemed to be of the opinion that high torque big block engines were better suited to wide ratio transmissions. The wider ratios used in the Super T-10 also coincided with McLellan’s take over as the Chief Engineer.
Would it be possible to do another article on how the 4 speed manual became such an iconic part of the Corvette’s identity that the 80s brought us the “Doug Nash 4+3” transmission in an attempt to keep 4 speeds AND meet CAFE at the same time?
(Wouldn’t it have just been easier to use a 5-speed OR pay the gas guzzler penalty?)
The Nash was a convenient response to the general trend to more gears and modest power. GM already was stretched for money and people to develop powertrains for its mainstream products.
At the time, GM was using the Borg-Warner/Tremec T-5 in the Z-28, but felt it wouldn’t handle the torque of the 350, so only the Z-28 only offered the manual with the 5.0.
The Doug Nash transmission offered a higher torque rating, so they put it in the ‘Vette, then replaced it with a ZF 6-speed in ’89.
Because of this, the Corvette never offered a true 5-speed manual as a factory option.
The C4 Vettes initially used the Super T-10 with the Doug Nash OD unit simply because the T-5 was too weak. It seemed to barely hold together behind the 5.0L engines. Even the 5.7L F-bodies were automatic only because the T-5 couldn’t handle the torque. There was no other manual transmission options for the Vette, so the Super T-10/Doug Nash combo was the best compromise. It was really nothing special, they just took the existing transmission and added an OD unit. No different than adding a Gear Vendors to an old 4-speed today (which many have done).
It wasn’t until 1989 that they started to use the ZF six speed, but GM switched to the T56 for the C5 Vette. The F-bodies went from the T-5 to the T56 in 1993.
One thing leaves me curious. This post discusses how the passenger-car 3-speed really wasn’t suitable for the Corvette engine’s power curve. I don’t doubt that.
But there were other manual transmissions available to Chevrolet’s engineers. I’m fairly certain the 3+OD was available in passenger cars in ’53. If that was combined with a steep rear axle gear, it would make for a very low 1st gear, and still would have cruised reasonably well in OD.
Then there’s the entire line of truck transmissions, in both 3- and 4-speed, and the 235 was certainly available in trucks. Were they too physically big?
Just curious.
The truck 3-speed used the same ratios as the Chevrolet cars. The four-speed truck transmission was more of a three speed with a bull-low. It had far less ideal ratios than the three-speed. The OD was an add-on to the end of the transmission. I don’t think a low first gear was the concern of Duntov, he seemed to be selecting ratios that were more ideal for sporting events, rather than a hard launch.
Thanks, Vince!
Excellent article, Thx!
One question, are you sure about the ‘Vette returning to the T-10 in ’74? I worked for Chevrolet dealership back then and I recall them still using the Muncie close and wide ratio units. The ratios listed for ’74 are the same as in use for ’73.
From what I know, the change took place near the end of the 1974 model year. So many (probably most) 1974 Vettes left the factory with Muncies, wide and close ratios as it had for the past 10 plus years. The Super T-10 came in two ratios though too but it was significantly different than the past options. They were 2.64:1, 1.75:1, 1.33:1 for the wide ratio and direct or 2.43:1, 1.61:1, 1.23:1 and direct for the close ratio. The close ratio transmission was only available with the L82.
“[… A] contemporary 3-speed manual, which in 1953 was the only manual transmission available in American made cars.”
So the 4-speed eventually used would be analogous to the 7-speed used in the C7 in how irregular/novel it was?
Although there are many reasons we could consider the Powerglide to be “just as good” as a manual transmission in a sports car, I think Zora Arkus-Duntov put that idea to bed. I think the old timers like Maurice Olley who were wowed by GM’s early prowess with automatics just could not understand how people who wanted to drive sports cars wanted the control of shifting their own gears.
The T-10 was also used by Studebaker starting in the 1961 Hawk and in their performance-oriented stuff from there out. I have read that there were both wide ratio and close ratio versions offered, which makes me wonder if Chevrolet offered those choices as well.
The collaboration with BW and Chevrolet was probably not unlike that between BW’s Detroit Gear Division and Studebaker in the development of Stude’s 1950-55 Automatic Drive. Chevrolet’s production people probably figured that it would be a low-volume proposition, not worth building in house. How different would the performance car world have looked after 1958 if the T-10 had been a Chevrolet-built unit – would it have been used in anywhere near the applications it eventually was?
After GM’s exclusivity of the T-10 ended in 1960, BW built versions with various gear ratios for all the other companies that then bought it.
Actually, there was little or no engineering/design collaboration between GM and B/W on the design of the T-10. A GM engineer took out the patent on a four speed transmission back in 1957, and because of certain intrinsic design elements of the BW T-85, he based it on that transmission, as it had been around since 1935 and was almost bullet-proof. Therefore every T-10 has that GM patent # stamped on the side of the case. It’s 100% a GM design, and GM collected royalties from BW.
And after GM lost its exclusivity on it in starting in 1960, they simply built a better T-10 in the form of the Muncie four speed.
There’s an excellent article on it here: https://www.chevydiy.com/muncie-4-speed-the-complete-history/
As to the Automatic Drive, not to be a Studebaker skeptic, but I would love to know just how much actual engineering input/work Studebaker did on that. I ask that because Studebaker apparently had zero engineering capabilities when it came to transmissions, having been a very long time BW customer.
I suspect is might well have been some (input), and certainly it would have required coordination, but lets face it, BW was a giant in the field at the time, and in much better shape to do the R&D on a new automatic (or two).
Do you have any info on how that collaboration actually went?
“Do you have any info on how that collaboration actually went?
I will confess that it has been awhile since I dove into this area. I can say that Studebaker was probably more capable in engineering than any of the independents (save Packard, which was definitely next-level) and was at the peak of its power when that transmission was being designed, so if there were ever a time when its engineers would have had their thumbs in a project such as that, it would have been then.
But that is something I should try to chase down.
Therefore every T-10 has that GM patent # stamped on the side of the case. It’s 100% a GM design, and GM collected royalties from BW.
Actually it’s the Muncies that are all stamped with the Patent number. Some of the early 60’s Chevrolet T-10s are stamped with “Patent Pending” but the Ford, AMC and other T-10s have no patent number stamped on them. The revised Super T-10 doesn’t have a patent number either. It seems that James Fodrea made the original patent application in 1957 and then he follow-ed up with another application 1960. The patent wasn’t granted until 1963. This coincides with the M20 and M21 introduction. Without reading his actual patent, I’d guess that there were some changes made to the original patent application that were related to the updates to the Muncie 4-speed. So I’d estimate that that’s why the patent applied to Muncies but seemingly not the T-10s.
” I have read that there were both wide ratio and close ratio versions offered, which makes me wonder if Chevrolet offered those choices as well.”
As I posted below and I think Paul also mentioned, a wide ratio T-10 was available in Chevrolet cars for 1961, but only the fullsize models, not the Corvette. In 1963 the Corvette offered a wide and close ratio, and the T10 was replaced by the Muncie during the model year.
A minor addition: the standard three speed had revised ratios starting in 1961, with a 2.47:1 first and 1.53:1 second. I’m guessing that the 2.21 first gear was a bit steep for everyday driving.
And the choice of wide or narrow ratio 4-speed boxes started in 1962, according to the brochure. And apparently those were both T-10s, as that was what the Corvette used until 1963. But then the Muncie 4 speed was really just GM building a better T-10, for which the had the original patents and collected royalties for.
Yes, I didn’t mention the ratio change in 1961, but I did mentioned in one of the the captions the ratios were unchanged from 1955-60. The change in ratios to the three speed was across the board, Corvettes and other Chevrolet cars. I suspect while the 2.21 ratio was okay in a Corvette, especially in a sport car type event, by this time the 4-speed had become mainstream so anyone competing with a Vette likely ordered it instead. Further, it allowed Chevrolet to consolidate and have one set of ratios for the Vette and the other cars.
The wide ratio T-10 was first offered in the 1961 Chevrolet sedans, but the Corvette used the close ratio T-10 exclusively until 1963. For the Chevrolet sedans, the transmission ratios depended on the engine choice. For 1961 only the 409 got the close ratio (same as Vette), while all others got the wide ratio which had 2.54:1, 1.92:1, 1:51:1 and direct.
The Muncie M20 and M21 (and later M22) continued with very similar ratios, the M20 being the wide ratio and the M21 and M22 the close ratios. The wide ratio was 2.54:1, 1:89:1, 1:51:1 and 1:1, and the close ratio was 2.20:1, 1.64:1 1.31:1 and 1:1. The Muncies are stronger than the original T-10, in particular the heavy duty M22. Ford also moved away from the T-10 to the Toploader, which was probably even stronger than the Muncies.
Vince, the ’62 Corvette brochure clearly shows both the wide and close ratio four-speeds “tailored to engine choice” . Image below:
Sorry about that Paul, I stand corrected. I am usually more thorough on my fact checking, I didn’t see the little asterisks on the data sheet (see below) when I checked my source for the article (I didn’t use the brochure, rather the vehicle information package). So yes, the Corvette offered a wide ratio T-10, with the same ratios offered in the Chevy sedans for 1962.
I updated the article to reflect these changes.
Terrific post, Vince. Like a chapter from some high-quality (and expensive) hardback tome.
One thing I’ve never quite grasped is torque multiplication in autos. I get (roughly) HOW it’s done via the angle of fluid through the stator (I think!), but I don’t get why it doesn’t seem to work quite as promised. That is, in theory, if the “gearing” through the whole fluid coupling is almost infinite up to lock-up speed, why are physical gears needed? Perhaps to put it in the context of this article, why didn’t the Powerglide work at least as well as a four speed or more?
(I have confidence you’re someone able to explain this – there’s a great calmness and clarity about your style!)
It’s not “infinite”. The degree of torque multiplication depends completely on the design of the torque converter. As Vince pointed out here, the Powerglide had a stall ratio of 2.1:1. Stall ratio is the same as maximum torque multiplication. Meaning, if you put your foot on the brakes in Drive, and open the throttle, the engine will only rev up to a certain point, which is also called the stall speed. At that speed, which might be 2000-2500rpm or so (higher with special torque converters for racing) maximum torque multiplication is in effect. Which is why it’s common practice of course to do just that at the drag strip: brake, and rev engine in gear to max stall speed, and then let go of brake and go.
So the effective maximum gearing equals the max. stall ratio + the gearing in Low gear, which in the PG was 1.82:1, so the effective max gearing at take off was 3.92:1. Lower than the first gear on a manual, which explains why PG cars accelerated as fast or faster in the lower speed ranges. Lots of torque multiplication at take off.
It’s not possible to have infinite torque multiplication, otherwise you’d be able to turn the world with a go kart. Or maybe it is, theoretically, but not practically.
Torque converters were originally conceived with the idea that they could replace pesky gears entirely. Which explains why the original Dynaflow and Powerglide started in direct gear; essentially a one-gear automatic. Low gear was only to be used in exceptional situations: very steep hills, up or down.
And gearless torque converters have been widely used in applications to replace gears totally, as in trains and other heavy equipment. It solved the problem of having pesky gears to change either manually or automatically. GM transit buses revolutionized the field with their torque converter transmission, which had no gears per se; it just locked up into direct drive above a certain speed.
But the wider the stall ratio, the less efficient the torque converter becomes. That’s just inherent in the design. Fluid losses increase. Which explains why torque converters increasingly had reduced stall ratios as more mechanical gears were added. I don’t have the numbers, but I’m willing to guess modern torque converters have pretty modest stall ratios, given all the gears. And they constantly lock up between gear changes to increase efficiency.
Hope that helps.
I sort of thought that you meant that the torque converter’s torque ratio was infinitely variable from the 2.1:1 max down to 1:1, which it is. However, for the Corvette, the Powerglide’s stall speed is probably about 1500 RPMs, possibly a bit more. What this means is that by 10 MPH the torque converters torque ratio has decreased to about 1.5:1 times the low gear ratio of 1.82. By 20 MPH, the ratio is about 1.2:1 and at 25 MPH fully coupled. If the transmission up shifts to high at about 45 MPH, the torque converter will be turning over at 2000 RPMs and wil remain coupled.
I am basing this on the twin turbine dynaflow’s first design with a fixed stator. With the variable pitch stator the performance stall speed was 2500 RPMs.
I should add that if you could start off with only the torque converter, which is not possible, then the torque ratio would drop to about 1.5:1 by 15 to maybe 18 MPH. By 35 MPH the ratio would be 1.2:1. Note that the Corvette’s power glide would upshift at nearly 60 MPH, and the torque ratio is at least 1.82:1 all the way to the upshift.
Ah! I have now sort-of got the general idea, (he said with unwarranted confidence). It’s all to do with efficiency and usefulness (and by “infinite variation”, I did mean it within the limits the previous reply mentions). A train doesn’t need to zip away from the line: a car does, answered by the point ” …the wider the stall ratio, the less efficient it becomes”. The torque is multiplied till the stator begins to move with the turbine, after which, it’s essentially direct drive, and since there’s a limit on that multiplication (and big friction and heat losses), actual gears are necessary to help the virtual gears of the torque converter.
Clever, aren’t I?
With your kind assistance, for which I’m grateful, ofcourse. (I’m still not gonna get above a Pass but a Pass is not a fail, and can I leave detention now please sir?)
Thank you for the kind words Justy. It sounds like you have a grasp of things for the most part. As Paul stated, the concept of the original Dynaflow and the Powerglide were that they were essentially a torque converter only transmission, meaning no gear reduction was normally used. The problem was, even the Dynaflow, only had a stall ratio of 2.25:1, meaning that was the maximum torque multiplication that was available. This was a steeper ratio than Buick’s three speed manual (2.67:1) and far steeper than the Hydramatic. It was infinitely variable from that point until coupling, but even at coupling there is still some slippage without a mechanical lock-up.
Not only is the wider the stall ratio the less efficient the torque converter is, but also the higher the stall speed, the more slippage and less efficient it is. Many drag racers use a high stall torque converter, which doesn’t hydraulically couple until a much higher RPM than normal. This allows the engine to spin up to its power band more quickly and launch harder. It is especially ideal for engines with aggressive camshafts that make little low end power. However, under normal driving, these torque converters slip much more than usual, which offers some extra torque multiplication, but also produces more heat due to the slippage, meaning it’s far less efficient than a low-stall speed torque converter. This is especially inefficient when driving around at low speed, low RPM, such as in city driving.
There were torque converter transmissions that had greater stall ratios of around 4:1, but these weren’t effective in automobile applications. These were used in heavy applications where the engines ran at full throttle more often than not, which meant there was minimized slippage. A car that runs about at lower speeds, with light throttle and accelerates and stops, would have far too much slippage with a torque converter like that.
Great article as always Vince!
Not transmission related, but I don’t know if I’ve ever seen a Corvette from this era with a non-contrasting color in the fender scallop. Looks really sharp.
Thank you and I agree it looks better without the contrasting colour, which is why I picked that photo.