The planetary design of Overdrive Unit is still in use. A unit is available
to install behind the 5 speed in Dodge Diesel pickups. It uses the exact
same design and patents.
The OD allows the use of a low geared rear end for pulling (acceleration)
and still allows adequate unloaded top speed.
I have not researched the question of 4th only OD that deeply. A comparison
with UK spec. cars would be illuminating. All Fed. late MGBs used what was
the OD switch to control the spark advance solenoid. The switch operating
plate was modified to allow the switch to close only in 4th gear, no-longer
3rd and 4th. This allowed vacuum to go to the distributor only in 4th gear.
On cars equipped with OD this meant that you only had OD in 4th. I would
imagine that the operating plate could be replaced with an earlier one, but
I havn't investigated.
Kelvin.
> Good explanation.
>
> As for the previous, broader question, look at it this way --
> can anyone
> name any cars in 1962 that had 5-speed transmissions? Ferraris? No.
> Porsches? Don't think so, though I may be wrong. The reason MGs had 4
> speeds is that 5 speed technology was not widely available,
> at least not
> as a BMC parts-bin component. The reason they had overdrive
> instead is
> that OD technology had been common since the '30s, if not
> before. Someone
> commented recently that they didn't think OD saved any wear
> and tear on
> the engine or driveline components. That may be true, but I always
> assumed that the motivation was passenger comfort (reduced noise and
> vibration) and fuel economy, not wear reduction.
>
> BTW, the late OD only operated in 4th gear, for some reason.
> I've always
> wondered why they changed it.
>
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