After reading about Volkswagen's Direct Shift Gearbox (DSG) transmissions, which claim shift speeds as fast as 8 milliseconds, I decided to buy a Golf GTI.
Having driven it for some time now, I noticed that the upshifts are fast, almost instantaneous, but downshifts are much slower - just under one second.
I have tested this under various conditions and it seems to be the case all the time, whether I choose Manual or Sport mode.
Is this difference in shift times because of rev-matching? Or is it because of some artificial delay imposed by the manufacturer, as suggested by some tuning companies? If it is the latter, is it safe to electronically tune the transmission control unit to achieve faster downshift times?
There is no denying that in the Volkswagen DSG, the downshifts are not as immediate as shifting up.
Being a dual-clutch transmission, it has the ability to select a gear in preparation for a gear change. Hence, one clutch is engaged for first gear when starting off from rest and, during this time, the second gear is pre-selected. During the changeover, clutch for gear one disengages almost simultaneously as the clutch for gear two is engaged.
When accelerating through the gears, priority for pre-selection is always for the next higher gear. This is why upshifts are completed in milliseconds.
The DSG obviously cannot pre-select one higher and one lower gear at the same time. At any moment, a downshift would require the actuators to first de-select the higher gear before a lower gear is selected in preparation for the downshift.
This naturally takes a little longer than just swopping clutches, which is what happens when accelerating.
Finally, it is much easier to predict when to change to the next higher gear during acceleration, based purely on throttle input, than for the control algorithm to evaluate the downshift sequence during deceleration.