Automotive History: From Powerglide to 4-speed – The History of Transmissions on Early Corvettes

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.