Originally Posted by
apalrd
Yes and no.
The clutches are slipping during the 'torque phase' and 'speed phase' of the shift. During a heavily loaded upshift the torque phase will happen first. In this phase, the outgoing (release) clutch is slowly released, and the oncoming (apply) clutch is slipped to handle the torque, but slipping to hold the old gear ratio. For RFE, the release clutch is just vented (solenoid turned off) and the apply clutch is PWM controlled to hold constant gear ratio. Then comes the speed phase. At this point, the engine is still at the old RPM but the transmission has finished the torque handoff. the PWM duty cycle of the apply clutch is increased to pull the engine speed down to the new gear ratio. Due to the inertia of the engine, the engine produces more torque than it had previously (from inertia of decelerating the engine!). This torque has to go somewhere.
Option 1:
If the Apply clutch is ramped up to decelerate the engine before the Release clutched is fully vented, the two clutches will overlap. In this scenario, the two clutches will fight and the extra torque from the engine will go into heating both clutches. This is bad for the clutches, but the rest of the transmission is fine.
Option 2:
If we ignore the torque from acceleration, it will be passed through the transmission to the driveline as a bump. Depending on how fast the apply clutch is ramped up (the target turbine acceleration for the shift), this bump may be huge, may break traction on the rear wheels in a 1->2 shift, and may break or stress driveline parts since the torque can well exceed the normal engine output torque. This can be a benefit in high gears, as long as you don't break traction, but in low gears it's useless and stressful on all of the components.
Option 3:
If we slow down the shift (ramp up the PWM duty cycle slower), the magnitude of torque will go down, so you might not feel it. The apply clutch is still slipping for the duration of the shift, and will heat up, but can handle it for a short period of time.
Option 4:
The engine reduces its torque to match the torque which is gained from engine acceleration. In this case, the torque going into the transmission is the same as it will be at the end of the shift, no stress on components, and the shift can be completed more quickly with less heating of the apply clutch.
The RFE transmission has hydraulic accumulators on all of the clutch circuits which limit the speed of the shift, but the 8HP does not. Try driving an 8HP Ram 1500 in tow/haul mode to feel an incredibly fast but bumpless shift. The 8HP requests a very large torque reduction from the engine, so it can complete the speed change very quickly without the driver feeling it.
During the entire duration of the shift, the transmission is handling the full engine power while slipping. The torque reduction combats the extra torque from engine acceleration only and will always make the shift faster. There is also an engine-side limit to how much torque reduction you can request. I would request as much of it as you can, to speed up the shift as much as possible.