Re: 1939 Six/1700 Overdrive Issue?

Posted by 39SixSedanMan On 2023/1/17 14:58:47

West is correct; think of the transition into OD as another gear.

As with any manual gear shift, a shift from one gear to another is smoothest when the driver has perfectly matched the engine output speed with the transmission input corresponding to the vehicle speed. When done perfectly, there is no speed and very little torque difference between the engine output and transmission input. To make it a little more challenging (or fun if you are like me and love to drive manual transmission cars) is that the engine speed isn't constant. It will be slowing from whatever speed it was at before you changed gears; therefore, the challenge is multifaceted. Wonderful things like clutches and synchronizers make this much nicer in a manual transmission.

So, for an overdrive, there is also such a sweet spot, or range. Unlike a transmission, there are no syncros damping the transition. Nor is there a clutch, however, at the moment before OD engagement, the transmission is free wheeling and allows for zero torque in order to complete the OD engagement. The clunk you may hear or actually feel is the sudden coupling of wheels to engine, in which now instead of free wheeling, the engine is slowing the car. You feel the change from just coasting to engine decel.

So, with all that, I have found that cars with different engines, rear end ratios have diffrerent sweet spots. The suggestions I have for you, below can both be summed up as "Get Out and Play!"

•Play with different speeds at which you allow engagement. I expect you find a speed that is almost silent as well as nearly unnoticeable.My experience is that if you wind up the engine and delay allowing it to shift into OD, the degree of sound and rate of decel change is much more.

•Play with letting off the gas pedal gently, Do it across several speeds to get a feel for it. At higher speeds, this will lessen the sound as well as the abrupt change in decel since slowly backing off will allow for OD transition while the engine speed is still higher.
All of the Borg Warner ODs Packard used, the R6, R9 and R11 exhibit this, but are very robust.

Get out and play.