Many of you younger car enthusiasts scarcely remember the era of the overdrive transmission, but in the 1950’s and 60’s its use was ubiquitous in a number of American cars, dating as early as 1934 for Chrysler’s Airflow, and extending to 1972 as installed in Ford trucks. Other makes included Nash, Rambler, Studebaker, Hudson, Lincoln, and Mercury.
To my knowledge there is no vehicle in production today that offers a transmission similar to the Borg Warner. I personally have owned 3 different vehicles with that same transmission (a 1951 Hudson Hornet, a 1950 Studebaker truck, and a 1953 Nash-Healey Lemans Coupe), and have found them to be a delightful enhancement over a standard 3-speed transmission.
These units are virtually bullet proof, and incredibly simple in operation. The overdrive unit is bolted to the back of the actual transmission, forming an integral and compact unit.
In the photo above, the transmission itself is bolted directly to the aluminum bell housing, and the overdrive unit, identified by the single lever, is bolted to the rear of the transmission.
The entire system consists of the overdrive unit itself, a usually-firewall-mounted relay to provide power to the solenoid unit integrated in the overdrive unit, a speed sensing governor (also part of the OD unit), and a throttle kickdown switch to allow the operator to “downshift” from the overdrive (essentially 4th gear), back to the native 3rd gear in the transmission.
From the driver’s standpoint, operation couldn’t be simpler:
Starting from a standstill, the unit remains disengaged until reaching a certain speed, usually around 30 mph, as determined by the governor. Above that speed, the transmission is “ready” to shift into overdrive, however, it requires the driver to momentarily relieve the engine torque on the unit by lifting off the gas pedal. Immediately, the unit will then automatically upshift into the overdrive mode, and will remain there until either the car’s speed falls below 30 mph, OR the driver presses the pedal all the way to the floor, activating the kickdown switch, and shifting back to 3rd gear.
Unlike most modern day transmissions, once the overdrive unit is disengaged (i.e., downshifted), as long as the driver maintains engine thrust by keeping his/her foot on the throttle, the transmission will NOT upshift back to overdrive.
It is interesting to note that, in essence, there are 2 drive speeds for EACH of the 3 gears of the standard transmission… one with the overdrive engaged, and one while disengaged, so technically, this is a SIX speed transmission system. However, reaching overdrive upshift speed in first gear is a bit impractical, so for all intents and purposes the driver has a choice of 5 forward gears.. 1st, 2nd primary, 2nd overdrive, 3rd primary, and 3rd overdrive. In fact, for around town cruising, many drivers just use the two second gear options, which provides a nice range, and eliminates the need for any physical gear shifting.
The “automatic” up and down shifting of the overdrive unit is sufficiently fast enough to have been used on the Nash-Healeys at Lemans, where an entry in 1950 finished 4th overall, and in 1951 took 4th in class and 6th overall behind a Jaguar, two Talbot-Lagos, and two Aston Martins.
Then in 1952 the Nash Healey took first in class, and third overall at Lemans finishing only behind two Mercedes 300SLs!
These units are virtually indestructible, and quite simple to maintain and repair, most operational problems arising from old wiring, or crusty contact points in the relays. Fortunately there is a wealth of information about these units on the internet, including full overhaul manuals, and detailed wiring schematics, which are simple and straightforward to follow.
One interesting aspect of these transmissions is the fact that when the unit is in the disengaged mode (i.e., vehicle speed below approximately 30 mph, OR overdrive function is locked out by pulling out the Overdrive knob under the dash), the entire unit operates in a freewheeling mode. Thus, if the driver lifts off on the gas pedal, the engine goes to idle, allowing the car to “freewheel” on its own momentum, and contributing to better fuel economy (The 1952 LeMans winner actually logged an overall fuel consumption of 13 mpg while achieving an overall average of nearly 90 mph over the entire 24 hour race!
One benefit of the freewheeling transmission is that the transmission can be shifted from one gear to another WITHOUT having to depress the clutch! In fact, although the Borg Warner tranny was a typical non-synchromesh first gear , the freewheeling aspect allows a downshift to first gear with negligible gear clashing (most transmission without first gear synchro would have to come to a complete stop before engaging first gear).
The simple and easy to follow electrical connections of the Borg Warner allows easy modification to its operation. For example, on the Nash Healey, it was decided that the downshift switch would be the button in the center of the steering wheel (typically the horn button), allowing the driver to downshift without having to press the throttle all the way to the floor for a balls-to-the-wall downshift.
The “O” button in horn ring center is the Overdrive kickdown switch
Following that example, I have fabricated a similar auxiliary downshift switch (machined and polished from a solid block of billet aluminum) on my Hudson Hornet, right on the steering column, allowing partial throttle downshifts, as show below:
Yes, the Borg Warner overdrive transmission is now long gone, but never forgotten!
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