OK, I have received the following from my engineer friend.
Please note that these are NOT my words, I merely post these words for
the interest on any FOTers still interested enough to read them. I
will not be posting anything else on the subject, but I do hope that
this input will help some of you to form your own opinions about the
differences between J and A type Overdrives.
My friend wrote:
>>>I'm surprised at how a small aside concerning improving the cone
clutch 'grip' by using better friction materials is preferable to the
more common modification recommended by TTune and some others, of
increasing the line pressure has sparked such an issue. I don't
advocate increasing the operating pressure as an upgrade and this is
based on experiences with a BL ST Dolomite Sprint modified J type unit
which as supplied, operated at 620 PSI+. The engagement was both noisy
and abrupt and was obviously intended for track use only.
Reducing the operating pressure to approx 450+ with a different PRV
assembly made engagement far more civilised for road use. It no doubt
increased the longevity of the unit as well. The purpose of the spring
in the dash pot of the PRV assembly is to smooth out the engagement /
disengagement cycles so the PRV assembly does and can be modified to
influence the rate of engagement. The dash pot spring of the BL ST
PRV assembly had been modified to provide a quick(er) engagement and
the 620+ psi pressure did the rest.
Clearly your friend (Randall) has a sound understanding of overdrives
and I agree with all that he says concerning how the different
operating systems work. I've no experience of the early A type with
the large accumulator referred to but with the more common A and D
types, there is a delay in engagement whilst the pressure builds up in
the pistons and oil galleries even with the 'help' of the pressure
present in the accumulator. The J type has no accumulator as such. Oil
is constantly circulated around the pistons at residual pressure
(approx 20 psi) so the system is primed at all times. The rise from
residual to operating pressure and subsequent engagement is virtually
instant as basically, there are no empty spaces to fill and the whole
system is already partially pressurised. Whilst my technical
description (or lack off) perhaps left something to be desired and
some also appears to have been lost in interpretation. The net
principle difference is an engagement delay in A & D types whist the
pistons and galleries fill with oil and subsequently reach operating
pressure whilst the J type operating system is already filled with oil
and partially (residual) pressurised which in essence is what I
originally said.
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He then followed the above with:
I've just checked the J type PRV references in the BL Spitfire 1500
workshop manual. The PRV dash pot spring purpose is mentioned in the
pump housing section and the manual states that the PRV dash pot
spring is fitted to provide a smooth overdrive engagement. This would
suggest my view in the original email to you some years ago that a
modified PRV assembly, particularly a poorly modified one can give
rise to harsh engagement was correct. And they do from my own
experience.
Having re-read the comments of your friend, (Randall), he says that he
agrees to disagree but from his own description you can deduce that in
the A & D type overdrives the pistons and operating system (other than
the accumulator) are not pressurised until the solenoid is
energised. In the J type, the operating system is always pressurised
at 20 psi (with the mainshaft turning) and at 350-550 psi (depending
on application) with the solenoid activated. Perhaps I'm missing
something here but does this not mean the operating system is
constantly under pressure which I think you'll find is what I
originally said.
This is now getting rather anal.
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The end.
With Best Regards to all.
Leon
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