This is a good story about the ZF ecotronic VT1F Continuously Variable Transmission equipped MINI Cooper CVT. We had it "working" on Friday, glorious fun, and impressive for those who watched or later rode with me on PIR in Portland at a lapping day. Needless to say, this car generates talk. "It's a MINI, with an automatic ... a CVT!"
Portland International Raceway is an open, flat, technically interesting, 1.97mile 12-turn course, actually a City park, used for car club track days, testing, sports car and motorcycle events, and one CART race each year. A graphic with the old turn numbers for reference is at:
http://www.portlandraceway.com/graphics/BigMap2.gif
We focused on driving techniques for this car under SD (SportDrive).
Acceleration
From turn-in at Turn 7 (new #, was 5) to braking at Turn 10 (new #, was 7), the effective "back straight", our test was simply achieved speed at the braking point. SportDrive (SD), with the e-throttle held just shy of the Kickdown detent so that the CVT didn't "shift", was the fastest - to 104mph at Turn 10. These selected CVT and throttle positions produced completely stable engine revs (5500rpm) through the entire back straight -- for turbine smooth "CVT" acceleration. The transmission was obviously doing all the adjustments.
Edit: I attached a .gif to show these corners. Turn 7 is lower left, Turrn 10 is behind the trees at the end of the back straight upper left.
Pushing the e-throttle to the floor (past the detent) generates a "shifting" behavior that runs the motor up to 6000rpm, then back to about 5200rpm, then to 6000rpm again as acceleration continues. We think this is in software, not the transmission. By a small margin, it's not as quick as the "CVT" behavior above. D is not as quick as SD in either pedal position, nor is Steptronic.
We are "driving software" in this MINI, with inputs from e-throttle, e-transmission (ZF VT1F CVT), even an e-brakes overlay (ABS, CBC, ASC,
DSC) to and from the Siemens EMS2000 computer onboard.
Braking
Using a driving style of left-foot-braking/right-foot-gas may be the most effective in the Cooper CVT. This clearly worked on Friday. I'm was driving the MINI like a clutch kart!
Maintenance Throttle
Driving through a sweeping corner may have several techniques that work well. One, in SD, is to use very light left-foot-braking to balance the car against the throttle after trail-braking at turn-in.
Or, flicking the lever Forward once puts it into the "appropriate" (maybe one higher than I would pick) Steptronic gear that will "hold" in gear (after BMW/MINI CD#33.1 was installed to the car). It works to throttle steer the car in Steptronic. Exceeding 5900rpm or dropping to about 1600rpm shifts gears up or down automatically, but it's definitely possible to use this mode to manage significant weight transfer - throttle steering!
Apex
How to get the car rotated, and how to get the motor and transmission "staged" (revved) to accelerate is the biggest challenge in the MINI CVT in SD.
I could apply e-throttle (to establish a "target speed" in the computer and bring up revs) while trail braking to rotate and balance the car well before the apex, and then "release" the car to accelerate by getting off the brakes onto Exit and the straight.
This got easier to do with more practice. I applied more throttle, earlier, which also seemed to work better.
The key with the throttle seems to be setting a position (ie. just at the kickdown detent) and leaving it there to set, and NOT change, the "target speed" in the computer. We're talking a tricky concept here that I'm not telling you we've perfected!
Ideas like "Feather" the throttle, "roll on" throttle, or "apply more" throttle upset the computer mapping that's going on in SD, producing unwanted adjustment attempts in the transmission and motor revving changes. Each of these throttle foot skills seem to be a poor choice in the Cooper CVT, if using SD or D. Changing throttle position at all makes the transmission search for new ratios and wastes time. The "set max throttle and forget it" idea was much less "busy" under actual driving conditions.
High Speed
We didn't get to play with switching over to D as a way to boost the very top-end speed at the end of the straight (would it act as an "overdrive"?). I'm thinking it might help wind another little bit from the last several hundred feet? We'd need to switch back to SD while under braking.
Problems
I really didn't have any. Getting all this to work together needs lots more time and effort on my skills and coordination. But, our MINI is working, it's great fun to drive, and it got everyone's attention because this little car isn't supposed to keep up with Ferraris or Audis or Mazdas or Subarus or Porsches, or even MINI Cooper S. But, I did that Friday with the MINI CVT in the twisties, and I had second fastest time of the day in the autocross school session a week before this event.
These other cars could all out accelerate our 115bhp on the straight, but the rest of the track was an big equalizer.
This CVT system seems near bulletproof on the track and street. I haven't had ANY mechanical strangeness or symptoms that would say "don't do that." It provides wonderful flexibility to have an appropriate transmission mode choice for any street situation.
Would I like the floored throttle to give the fastest acceleration? Yes, but knowing it doesn't means I can carefully change my driving technique. Would I like Steptronic to shift more quickly and smoothly, and without "shifting"? Yes, but the CVT using Steptronic is much more capable and responsive in the corners than I expected. Am I happy with it overall? Yes.
I'd sure like to know what's going on in the transmission, in its computer, in the EMS2000 computer, and how inputs to them generate behaviors. I want to understand how the system works so I can drive it more effectively.
Do these observations seem rational, and moving in the right direction to understand how to drive the car well?
