The Chevrolet Corvette C4's 'Doug Nash 4+3' Manual Transmission Is Mechanical Wizardry

Illustration for article titled The Chevrolet Corvette C4s Doug Nash 4+3 Manual Transmission Is Mechanical Wizardry

The 1980s weren’t a great time for sports cars, with engines choked with smog equipment, tall final drive ratios killing acceleration, and other questionable fuel-saving equipment making cars suck more than they needed to. The “Doug Nash 4+3,” a four-speed manual on the Chevrolet Corvette with push-button overdrive in the top three gears, is considered by many to be one of those bits of questionable equipment. But it’s still fascinating.

Illustration for article titled The Chevrolet Corvette C4s Doug Nash 4+3 Manual Transmission Is Mechanical Wizardry
Photo: vette333! (Ebay)

Doug Nash was a famous Detroit hot-rodder in the 1960s, who, after retiring from racing, got into the engine and transmission business, eventually developing a manual transmission for General Motors’ mighty Corvette for the C4's 1984 to 1988 model years.


Called the “Doug Nash 4+3,” the transmission was essentially just a Borg-Warner Super T-10 four-speed with an overdrive bolted to the ass-end. That electro-hydraulically controlled two-speed overdrive was automatic, engaging when the driver let off the gas (i.e. under low load), and could be turned on and off via a button on the top of the shifter. Here’s a look at the mysterious Doug Nash 4+3 in operation:

Search the car forums, and you’ll find Chevrolet Corvette C4 owners ranting about their Doug Nash 4+3s losing overdrive, while a few will defend the thing to the death, saying basic fluid and filter maintenance will keep the bolt-on unit in decent shape.

In any case, the 4+3s are fascinating units. Just look at the secrets that YouTuber GearboxVideo reveals in the clip above, including an awesome miniature valve body.

This second video gives an explanation of how the clutches work with a planetary gear set to yield an overdrive ratio for your fuel-saving needs in second, third and fourth gear.

This weirdo manual/automatic hybrid may not go down in history as the greatest trans, but like the the vacuum-activated Auburn two-speed rear axle I wrote about a few months ago, it’s still an intriguing and unique bit of old-school engineering. And I’ve got to respect it for that.

Sr. Tech Editor, Jalopnik. Always interested in hearing from engineers—email me. Cars: Jeep J10 ('85), Jeep Cherokee ('79, '91, '92, '00), Jeep Grand Cherokee 5spd ('94), Chrysler Voyager Diesel ('94)

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Mick Molte

Volvo put a few variations of the same idea in their cars from the mid 60s to the early 90s. They were pretty common in MGs and Triumphs as well, and turned up in some Euro Fords, and I think some Italian makes as well, possibly even Ferrari?

The Volvos had a fourth gear sensor that prevented use in 1-3. Some of the MGs were set up so you could use the OD in 3rd or 4th.