I think Leon is correct, the pressure is constant in a J and the oil is
diverted by the solenoid, which is why a good unit will change instantly
- any J type that has a delay of a few seconds is not as it should be. A
D type has to pump pressure up which is why even the best units have a
delay in engaging, Anyway if you want to fiddle with the pressure of a J
it depends what sort of relief valve is fitted, early ones are difficult
to do much with and even the addition of extra washers doesn't do much.
Later ones are very easy to adjust but care must be taken as even the
thin shims have a large effect. Ideally the pressure is matched to the
torque (or bhp), as standard the Spitfire is lowest with about 330 lbs
and the Stag highest with 550 (?) lbs, high pressures with little torque
tend to give an uncomfortable change as the there is little to stop the
overdrive clutch engaging violently, lots of torque is basically trying
to make the overdrive not engage which is why higher pressures are needed.
John Kipping.
Randall wrote:
>>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
|