Inside the 1940 GM Hydra-Matic, The First Modern Automatic Transmission

When Hydra-Matic was introduced by Oldsmobile in 1940, General Motors called it “the most important automotive advancement since the self-starter.” Here’s a closer look.

 

We want to choose our terms with care here. When we describe the 1940 Oldsmobile Hydra-Matic as the first truly automatic transmission in volume production—the first modern automatic, if you will—we also recognize all the worthy attempts that came before it. The Model T Ford, with its two-speed planetary transmission, was hard to beat for ease of operation, though it wasn’t a real automatic as we know it. Preselector transmissions, including Hudson and Cord in the USA, Wilson in the UK, and Cotal in France, came closer still.

But in the Hydra-Matic, there was no gear shifting required once the car was under way, and there was no clutch pedal. It was fully and truly automatic in operation, just like the automatic transmissions of today: Just place the selector in “Drive” and off you go. One difference worth mentioning: On the original Hydra-Matics, there was no Park position as such on the gear selector. The column indicator was marked “N-Hi-Lo-R” or similar.

 

In its internal operation as well, the Hydra-Matic was much like the automatics of the current day. There were three planetary gearsets to provide four forward speeds and reverse, with the gearchanges accomplished via clutches, bands, and hydraulic pressure. One key difference: While contemporary transmissions use a torque converter coupling to supply mechanical advantage for low-speed acceleration, the Hydra-Matic employs a straight fluid coupling with an effective ratio of 1:1—hence the need for four forward speeds. On the original Olds Hydra-Matic, the four ratios, first through fourth gears, were 3.66:1, 2.53:1, 1.44:1, and 1.00:1. Naturally, a variety of gearsets were used through the years to suit a wide range of specific applications.

 

Hydra-Matic Drive, as it was originally named, was developed by a team led by Earl Thompson (the creator of Synchromesh) that began as a Cadillac program in 1932 and was then moved to GM Central Research in 1936. An early semi-automatic version of the unit, using a similar planetary gear unit but with a standard friction-plate clutch instead of the fluid coupling, was offered in Oldsmobile and Buick cars in 1937-38 as the Automatic Safety Transmission.

In October of 1939 the full Hydra-Matic was introduced as a $57 option on the 1940 Oldsmobile, and Cadillac followed in 1941. Versions of the Hydra-Matic saw action in U.S. fighting vehicles in World War II, including the M5 Stuart and M24 Chafee tanks. Pontiac (photo above) adopted the Hydra-Matic in 1948, but the Buick and Chevrolet divisions opted to develop their own automatics. (See our features on the Buick Dynaflow here and the Chevrolet Powerglide here.)

As the first automatic that was widely available to the public, Hydra-Matic caught the Kleenex effect and for a time, became a popular generic name for any automatic gearbox. And indeed, GM made the Hydra-Matic transmission available to a number of automakers, including Hudson, Nash, Lincoln, Kaiser-Frazer, Willys, and even Rolls-Royce. While both Oldsmobile and Cadillac claimed the innovation in their advertising, or at least implied it, Hydra-Matic was essentially a GM corporate development. However, both divisions participated in the vehicle integration and production process.

Hydra-matic saw continual development through the years, including a major internal redesign for 1956 to soften the unit’s harsh gear changes, especially on the 1-2 and 2-3 upshifts. Rugged and reliable, the unit was widely offered in light trucks and was even popular in drag racing in the ’60s. But on the downside, the Hydra-Matic was expensive to produce, extremely heavy due in part to its cast-iron case, and lacked a torque converter. After 24 years in production, the original Hydra-Matic was phased out in 1964, but the Hydramatic name (with the hypen removed) has lived on at GM into the 21st century, but on a series of entirely different transmission families.

9 thoughts on “Inside the 1940 GM Hydra-Matic, The First Modern Automatic Transmission

  1. When I was in high school circa 1971, our local bus line provided busses to the school. They were usually 1948 GMC coaches with standard transmissions. There was one exception. One of the busses on a slightly shorter wheelbase, was equipped with a hydra-matic. A lot of the drivers fought over it! It appeared to me to be a newer model GMC bus more like a mid-50s model.

  2. Here in Oz on our domestic cars they were behind the times. Holden got the hydramatic in 62 on the EK Holden. All 138ci of engine/ That trans was used until late 64 on the EH Holden with the 179ci red motor which was a vast improvement.
    On these models they used a sprung carbon splined flex plate which were very common to start rattling as the springs got loose and eventually fell out. Then ofcourse the splines stripped. I must have replaced 20 flexplates on those models. My recollection is an alloy body?? They were heavy, more so on your back lifting them in on ramps.
    In 65 the HD model they went powerglide which is well,, a powerglide which they used on 6s to early 70 and V8s similar.
    Ford had their 2 speed auto behind their weak 6s in 62 from memory but Chrysler had the torqueflite in late 60.With the then huge and powerfull 225 slant 6. About the only time Chrysler lead the pack!
    We got pov pack Chevs here in limited numbers. But while the US had V8s and autos we got 6s and 3 on the tree until around 1960 with 283s and cast iron powerglides

    • The Oz Hydramatics were a completely different unit; properly known as Roto Hydramatic, it was a more compact light duty alloy-cased 3-speed, also used in various Opels & Vauxhalls, and some lower powered Pontiacs & Oldsmobiles. Similarly, Ford had a variety of different transmissions here that they called Fordomatics (I’ve got one in a ’54 Ford and it’s a different animal to the various Falcon 6 & V8 units) and Chrysler had some Valiants with Borg Warner 35s that still wore Torqueflite badges, The car companies weren’t averse to being a tad creative in their marketing!

  3. The Hydramatic was an adaptation of the ALLISON transmission; which was originally developed for tanks, then adapted for over the road trucking. The integrated planetary set ( one set’s sun drives another’s planets) was developed by a man who’s name is actually Homer Simpson!!!! European vehicles with planetary transmission are not Simpson gear; but, “power shift”, where each planetary set is responsible for a single speed; this is an example of how licensing can effect technology. Even with modern engines, chassis; over the road trucking would not possible without the Allison transmission.

    • As mcguireb posted, Allison trans appeared long after Hydramatic. Allisons are used on city buses, garbage trucks and other applications that call for frequent stops. I’ve never one used for long haul. The automated trans now popular for long haul are totally different.

  4. The preselector used by Hudson, Cord, and Tucker was the Bendix Electric Hand

  5. Worked on a few of these back in the day. Cadillac and Pontiac were the most common. I heard that International actually installed some Hydramatic transmissions in its R-100 pickups in late ’53 before offering the Borg-Warner automatic in ’54 and up. I actually saw a truck for sale that showed the shift pattern of the Hydramatic but since there is only information on the BW unit I wondered if I was seening things…

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