[QUOTE=Huskymaniac;683800]
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I still believe it is possible that torque, sufficient for damage, can arrive before the line pressure rises whenever the throttle is rapidly increased (pedal is stomped). The solenoid may respond fast enough to the signal that the throttle is higher and the tranny pump is tied to the engine RPMs so, yes, the pump will probably start pumping at or even before the torque hits the tranny. But the line pressure may still have a delay. In any enclosed system, the pressure will build over time once the flow of liquid into the system is increased. How fast this happens depends on the volume and flow rate. The flow rate depends on the pressure and flow resistance. The improved valve body should help decrease that flow resistance, increase the flow rate and increase the rate at which the line pressure builds. What the line pressure is at a given point in time should depend on that rate as well as the initial pressure at time t=0. Mike's new TCU code should result in that initial pressure being higher. Thoughts?
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But the line pressure may still have a delay. In any enclosed system, the pressure will build over time once the flow of liquid into the system is increased”.
Question, how long does it take, for your brakes to work when you press the pedal, to increase the pressure in the system?
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Finally, I am left wondering what happens during a shift. Doesn't the solenoid back off the line pressure during the shift? If so, and if the line pressure delay or buildup hypothesis is correct, then I would think that any stored energy built up in the TC during the shift could be a problem as the line pressure is ramped back up. That would make the signal to cut the engine throttle, and its timing, pretty important.
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You are looking at the results, instead of the causes.
Why did they do all this TCU modifications to the box in the first place? If we look at the legacy RS Turbo, that does not use these TCU modifications, on the 1st to 2nd change we can see why they did the mods to the SVX and what effects, the mods have.
When the 1st 2nd change is to take place, the shift solenoid shuts its drain to send pressure to the band servo, to lock the front Sun gear, changing the overall gear ratio from 9.74:1 to 5.40:1, if the engine was revving at 5000 rpm in first, it has to slow down to about 3000 rpm for 2nd, This will happen as soon as the band applies to brake the engine speed down to the speed of the box’s new ratio. This causes a ‘bump’ that is felt as the braking happens.
To soften this ‘engagement bump’ the line to the band servo, has an Accumulator ‘T’ into it, so that the pressure rise is slowed to allow the band to slip, softening the engagement. The High clutch also has an Accumulator to allow it to slip on engagement.
To prevent excessive slipping the Accumulators have their piston spring backed-up by a line pressure that is varied by the throttle pressure, so that the band/clutch engagement pressure ‘bump’ is varied to suit the engines torque.
Every auto box has used this system for over 30 years, so why did they change the system. In short they did not think the ‘bump’ was fitting for such an expensive car. To lessen the engagement bump even more, they reduce the line pressure during the change to allow the band/clutch, to slowly slip into engagement. Also because the line pressure is reduced, the accumulator backing pressure is also reduced, to delay the pressure rise and increase the softening, slipping time.
This results in a change that is very hard to feel, but at the expense of the early demise of the band/clutch facings, and the generation of more heat. To get some sanity into the mod, they turn off every second fuel injection, to reduce the engine torque by 50% so that the linings will survive longer, and this produces its own problems.
The Torque Converter plays an important part in the change. Going back to the Legacy RS, Turbo box, when the 1st /2nd change, changes the gear ratio, the engine driven Impeller is spinning at 5000 rpm, the Turbine connected to the gearbox, is braked to about 3000 rpm. Because we have a speed difference of 2000 rpm, the engine’s torque drives the converter down to a reduction speed of 1.9:0, and a torque multiplication of 1.9:1. So the ratio change is not a direct 1st of 2.785, to 2nd of 1.545 as the converter will match the 1st ratio, and go through every ratio up to the 2nd ratio, to give a smooth reduction of the engine speed. As the engine braking is geared down over a longer time, the loading on the band/clutch engagement is reduced.
As the SVX TCU reduces the engine torque before the change, the lack of torque means that the converter is not driven down to match the new ratio, spinning on till the band/clutch engagement slows it, increasing the slipping that the elements suffer, and wasting the energy that would have been transferred during the change.
So you don’t need to look too far to see why, this box has such a high failure rate, when controlled by the SVX TCU.
Harvey.