I want an overdrive in a 1940 sedan

Question:

I have a 1940 Oldsmobile 90 Series, L40 4-door sedan with a straight eight flathead engine. I had the engine, which delivers 110 HP@3600 rpm, totally rebuilt by a professional. I would like to know if I change the Hydra-Matic transmission to a more modern transmission with overdrive, what late model transmission would fit? Could the rear end handle a more modern transmission? How would the linkage from the carburetor to the transmission work? Could the transmission perform with a 6-volt electrical system? Would the transmission work with the engine’s present vacuum system?

Answer:

Most late model overdrive automatics will physically fit into your car without major structural changes. You will, however, need an adapter in order to attach a modern overdrive transmission to your 1940 Oldsmobile engine. Try contacting Bendtsen’s (transmissionadapters.com) or Speedway Motors (speedwaymotors.com) for an adapter that will work for you.

You will have a problem with any electronically controlled transmission because the controls for all of them are 12-volt. To overcome this you will need a power converter to convert the car’s 6-volt electrical system to 12-volts or you’ll need to convert the entire car to a 12-volt setup.

The performance and longevity of the original rear end in your car will not be significantly impacted if you swap your original transmission for a more modern unit.

A new transmission’s linkage to the carburetor and/or gear selector will need to be fabricated from scratch or adapted from an existing linkage.

If your replacement transmission requires a vacuum source, the original engine’s vacuum will work fine as long as the engine is in good operating condition.