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Re: Why OD on 4spd and not just 5spd?

To: Barney Gaylord <barneymg@ntsource.com>
Subject: Re: Why OD on 4spd and not just 5spd?
From: Tab Julius <tab@penworks.com>
Date: Mon, 21 Feb 2000 16:57:11 -0500
Thanks!


At 01:46 PM 2/21/00 -0600, Barney Gaylord wrote:
>At 02:07 PM 2/21/2000 -0500, Tab Julius wrote:
> >
> >.... is OD different than 5th gear in ways other than the fact that it
>simply doesn't have a stick position?  Is pushing/sliding the little OD
>button effectively the same as choosing 5th gear or does it do something
>very different?
>
>In function the OD is just another gear like any 5th gear, designed to slow
>the engine down.  In hardware it is very much different than a 5-speed
>stick shift.  At the time of production of these cars the standard gearbox
>was a 4-speed unit, and retooling to make a 5-speed box would have been
>quite expensive.  The overdrive unit was available off the shelf (from
>another supplier I think) and could be bolted right onto the back of the
>original gearbox with as little change as different mainshaft for the
>gearbox, and maybe a change of propshaft length.
>
>As there would be no direct mechanical shift link from the original gearbox
>to the OD unit, the new gear needed to be shiftable by remote means, or
>hydraulically in this case, with electrical control switching.  Internally
>it works much like part of an automatic transmission, with a planetary gear
>set for speed reduction and a hydraulicly operated clutch for actuation.
>Because of the complexity and expense of the OD unit, they have generally
>been replaced in recent times by 5-speed gearboxes.
>
>Barney Gaylord
>1958 MGA with an attitude (but no OD)
>     http://www.ntsource.com/~barneymg


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