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Technical Curiosities:
The Laycock de Normanville Overdrive

October 29, 2011 by Matt

Laycock de Normanville Overdrive OD Gearbox Transmission

This one was used on a pretty wide variety of cars, but I had to include it because of the name. I mean, c’mon. How I could I not write a post about an automotive component with a name like that? It’s awesome.

Invented by a Briton, one Edgar de Normanville, and manufactured by automotive supplier Laycock Products, the unit basically consisted of a solenoid-activated planetary gearset residing between the standard manual transmission and driveshaft, offering a reduction in gear ratio at the driver’s command. At the press of a button or flip of a switch (depending on the car) in the cockpit, the overdrive would engage and lower the engine speed relative to the driveshaft, markedly improving fuel economy.

Advantages? Transmission development has always been pricey, and as fuel economy began to become a priority for automakers, the idea of a fuel-saving external add-on to an existing 3- or 4-speed manual transmission was an appealing one. Not only that, but the nature of the Laycock Overdrive meant that it could be engaged at any time, even in the lower gears, effectively doubling the number of ratios at a driver’s disposal. A 4-speed tranny became an 8-speed, for instance.

Downsides? Added complexity, mainly. The solenoid in particular could be finicky on higher mileage cars, and frequently the overdrive unit had its own fill and drain plug separate from the “main” transmission, requiring a unique maintenance interval. As all-in-one 5-speeds with integrated overdrive became de rigeur, the Laycock Overdrive faded from the scene.

Laycock de Normanville Overdrive OD Gearbox Transmission Diagram Schematic

I first learned about it from its presence on the Volvo M46 4-speed + overdrive transmission. Installed from the factory on Volvo 240s and later 740s, it’s one of only two manual transmissions (the proper 5-speed M47 being the other) available stateside for those of us interested in a bolt-in manual swap for a Volvo 200-, 700- or 900-series. Remarkably, up until the late ’80s, the Laycock Overdrive was fitted to a wide variety of cars, not just Volvos: Jaguars, MGs, Austin Healeys, Alpines and Triumphs, among others, were available with the unit. Not too shabby for an obscure bit of engineering with a funny name.

Editor’s note: This post is part of an ongoing series spotlighting obscure automotive engineering solutions. Read the other installments here:

Filed under: Technical, Technical Curiosities

5 comments

  1. Joel S. says:

    First car was an ’84 Volvo 240 Diesel wagon w/ this transmission. It was one of those with the square push-button right on top of the shifter. Never knew how it worked, and didn’t know I could use it below 4th gear while cruising long city streets. Didn’t like the car at the time because it was slow, but would love it’s practicality now. Diesel-stick-overdrive = great fuel mileage.

  2. Chris says:

    For the record, I have read that de Normanville was an American who gave the idea for a planetary OD to Laycock, a British engineer, during a conversation in a pub after WWII. The model shown in the photograph looks like a J type which superseded the D type fitted to Volvo 1800s during the ‘sixties. It was matched with the M41 gearbox in the early ‘seventies. Over a million Volvo’s are said to have been fitted with these ODs. Other European car makers that used them include Ferrari and Bentley.

  3. Ken says:

    He was a Brit, he was born in Coventry !

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