[TR] Controversial subject...

[John Macartney]

Thanks for that, Randall. As always a clear and well-reasoned commentary
from someone who is a dependable and authoritative mine of information -
much of which I've usually forgotten :)

Cheers, John

-----Original Message-----
From: triumphs-bounces at autox.team.net
[mailto:triumphs-bounces at autox.team.net] On Behalf Of Randall
Sent: 26 March 2014 23:52
To: triumphs at autox.team.net
Subject: Re: [TR] Controversial subject...

---- John Macartney <john.macartney at ukpips.org.uk> wrote: 
> Laycock themselves set the primary pump pressure appropriate to the 
> make and model of car to which the O/D would be fitted.

I believe that is true, tied mostly to engine displacement (hence maximum
torque) and gear ratio in the lowest gear with overdrive available.  There
is even a S-T service note that says "Pressures vary according to
installation".

> Allen Hess says his unit engages
> with a 'bang' and Allen, it shouldn't.

IMO it depends on which OD is involved, and what throttle position.  The
early A-types did have a rather harsh engagement, when engaged at part or
trailing throttle, and I believe that was by design.  Somewhere, I've even
got an advertisement that describes it as "kicks like a mule".  Later
A-types, for cars with IRS, were modified to ensure a more gradual
engagement, apparently to reduce the shock on the differential mountings.
Even later than that, the J-type (like the one you drove in Uncle Jack) does
shift more like an autobox.

> Once the pressure was adjusted to suit the oil we were using, 
> everything was fine.

The actual viscosity (resistance to flow) of a "straight weight" oil varies
a lot with temperature; typically 20:1 or more over the range from 20C (a
cold start on a mild day) to 100C (roughly normal operating temperature).
The overdrive's pressure regulation system has to be able to deal with this
wide variation; which is much more than the difference (at any given
temperature) between say 30 weight and 90 weight (which is only roughly
1.5:1).

So, while I don't doubt that low pressure was your problem, I doubt that the
oil used was to blame for the problem.  Much more likely that the spring in
the pressure control dashpot was weak from heat and old age, IMO.
Thankfully it's much easier (and safer) to adjust on a J-type than an
A-type.

Randall

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