Borg-Warner Electric Overdrive
What is overdrive? Simply stated, any overdrive transmission turns the driveshaft faster than the engine’s crankshaft when overdrive is engaged.
To get underway, the crankshaft must turn faster than the driveshaft; the driveshaft is underdriven. This gives the engine a mechanical advantage over the driveshaft. Once cruising speed is reached, however, if the engine could turn a lower rpm than the driveshaft, through overdrive, it would reduce engine wear and yield better fuel mileage.
Both automatic and manual transmissions may have overdrive. With today’s manuals, though, the driver must depress the clutch and physically shift into overdrive. It wasn’t always that way.
Engineer William B. Barnes is credited with designing and mating a secondary overdrive unit to a common three-speed manual transmission, using a planetary gearset to increase the output shaft speed. He shopped it to Borg-Warner’s Muncie Gear Division in 1932, where it was further developed and presented to Chrysler Corporation. Chrysler first made it available in 1934 Chrysler and De Soto Airflows to augment their gas mileage.
Barnes’ original design required that the unit’s sun gear be held in place with a manual control to engage overdrive. Subsequent development added electric components to control that function, resulting in what became known as “automatic electric” overdrive. Simply known as “overdrive” thereafter, it became a popular option on many cars and trucks during the 1930s and beyond.