> Randall
>
> I put this to my friend in the UK, who has responded thus:
>
> I think I've amassed a reasonable understanding of the workings of the A, D
> and J type over the years. The principle difference with the J type is the
> operating pressure is constantly present
Well, I guess we'll have to agree to disagree then. When the J-type is not
engaged, the pressure in the entire system is relieved to approximately 20 psi,
even on the Stag. Pages 40.00.09 and 40.00.10 of the later Stag ROM have a good
description of the process; or you can verify it yourself with a pressure gauge.
When the solenoid is engaged, it allows the 20 psi pressure to move a shuttle
and effectively readjust the relief valve to the operating pressure, so the
relief valve is closed until the pressure rises to the set point. While the
pressure is rising to the set point is when the pistons move and the clutch
engages ... the relief valve should still be closed at that point so how fast
the clutch moves depends entirely on the pump speed and pump/piston
displacement. The clutch should be fully locked by the time the relief valve
opens (or it will always slip).
> A and D types only pressurise when the solenoid is
> engaged, hence the delay in engagement.
Perhaps this is a "separated by a common language" thing, but that's not the way
I would describe it. The A and D type hold full operating pressure in the
accumulator all the time. When the solenoid activates, it dumps the contents of
the accumulator directly into the operating pistons. Later Triumph A-types had
the volume of the accumulator reduced, so that the pressure would drop and
soften the engagement (for IRS cars) but on early ones the accumulator is enough
larger than the pistons that the pressure drops very little and the clutch slams
into engagement under full operating pressure.
If you think about it in terms of the gearbox clutch, the difference is between
letting the pedal up slowly, and just moving your foot to the side. The
ultimate force is the same, but the engagement is much harsher because the
pressure plate is moving faster.
> The point I was trying to make was that
> a common
> "mod" is to fit an "uprated" PRV usually from a Stag
Perhaps I should mention that my J-type experiences are with a Stag unit, and
Stag pressures. My A-type has a lower operating pressure but definitely bangs
into OD.
Someone asked about how to modify the pressure ... I'm not certain of the
details since I've not made any mods (my J-type is a factory Stag unit). But I
would guess that either a stiffer spring (for the main relief spring, not the 20
psi one) or shims under the existing spring, would do.
Randall
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