Shifting tuning is what user would spend most time playing with. Having proper shift is what we all want and need. Proper shift is quick, precise, and firm, but just in that sweet spot we all love – not too punchy, but nice and clear 😉 And we guess with that nice DSG fart upshift or sexy throttle blip on downshifts 😀
Let’s first understand shifting phases that GCU performs:
- Prefill phase
- Shifting phase
- Post-shift phase
- Regular run (shift finished)
Phase 1. – Prefill phase is the first one after shift input occurs and is accepted by the GCU (meets required tresholds, and is within accepted ranges by limiters). During that phase GCU still has previous (actual driven gear’s) clutch fully engaged as in Normal run mode at final Clutch Pressure value, and the next gear’s clutch is turned on at Upshift/Downshift Prefill Pressure value for Up/Downshift Prefill Pressure Time. Idea of prefill pressure tuning is to have upcoming clutch as close touchpoint as possible to make next shifting phase fully proportional by taking out clutch deadzone. Too low value will cause i.eg. RPM flares, while too high value will cause clutch collision resulting in short period locking of the drivetrain – find nice sweet spot in between 😉
This is also moment when GCU starts sending Gear Cut / Blip Request.
Phase 2. – Shifting phase is the actual clutch transition happens. Previous clutch is disengaged over Fall Time in ms, and new one is engaged over Rise Time in ms with Spread (crossover) defined in ms (0 means they meet halfway). Spread time table adjust feeling of the shift greatly. New clutch reaches then phase 3.
Phase 3. – Post shift phase is moment when new clutch after Rise Time reaches Shift Pressure value and holds it for Shift Pressure Time. Idea of shift pressure is to close slip nicely and precisely so change is smooth and no flare on new gear ratio occurs. Too low value will cause RPM flares, too high value will cause harsh shifting. Again, find sweet spot in between 🙂
As soon as clutch slip reaches 0% GCU closes transmit of Cut/Blip request.
Frequent issue #1: I have already lowered prefill and shift pressures and it’s still harsh.: It’s most likely scenario when prefill and shift pressures are too low to do anything – they do not even try to close clutch slip and are insufficient to even maintain drivetrain delivery. Then once total shift time is over GCU applies pressures according to Clutch Pressure tables causing shock.