Three-time DTM champion Rast will compete for BMW in a variety of GT3 races and championships this year, having ended a decade-long association with rival German marque Audi in the wake of its decision to scrap its LMDh programme.
The 36-year-old got his first experience of BMW’s GT3 contender as part of a two-day private test at Oschersleben last month and logged more miles in the M4 in the official DTM test at Spielberg a fortnight ago.
Rast said after his first outing at Oschersleben that he was having to completely change his driving style to adapt to the M4, which is a completely different car to the Audi R8 LMS GT3 he is accustomed to from his time with the Ingolstadt-based manufacturer.
Now having had two more days of running in the car, Rast said he still needs to put in more work to be able to match the pace of reigning champion Sheldon van der Linde, who will be his team-mate at Schubert Motorsport this year.
"You can never tell if the point will come that you really drive on the same level as Sheldon," he says in an interview with Autosport’s sister title. Motorsport-Total.com.
"I hope that at some point the BMW will fit me like a glove, but it's not certain that that will happen. I'll do everything I can, but unfortunately there's no guarantee.”
Rast was seventh and three tenths off the ultimate pace in the Spielberg test, while van der Linde finished a close second to pacesetter and Mercedes driver Maro Engel despite missing the opening day of action to take part in an NLS race.
He was also fractionally slower than two-time DTM champion Marco Wittmann, who will drive for the new Project 1 team this year following Walkenhorst’s withdrawal from the series.
Both van der Linde and Wittmann have one season of racing with the M4 GT3 under their belt, with the former also getting more mileage when he drove the car to victory in the Kyalami 9 Hours earlier this year.
Rast explained that he is still having to make changes in his own driving to get the maximum out of the BMW, which is radically different to the Audi with which he finished third in the championship last year.
The mid-engined R8 LMS is stronger in cornering speed and on the brakes, while the M4 is superior in acceleration out of the corner.
"Driving style is still the issue," said Rast. "With Sheldon you can see that he slows down the car and then accelerates early again.
“I still see a tendency in myself to try to force the car into corners too much. The synapses in my head still have to be linked and the others need to be deleted. I'm sure that will take a while.
"So you have to put all your focus on being at full throttle early and using the engine to bring all the power to the straight. It's a bit different than the Audi."
Further explaining the need to change his driving style, Rast added that foregoing corner speed to get a better exit makes him think that he is not pushing the car to the limit.
"You think to yourself that you can actually drive faster, but you have to force yourself not to so that you get the good exit that suits the BMW," he said.
"It's still weird. And I often catch myself pushing too much in the corner and then realising at the end of the corner that I was too fast again and then didn't get a good exit. It's not always easy.
"You can't try to reinvent the wheel now. You have to give it time and trust that your driving style will eventually take over and that it will work."
Rast will get another day’s worth of running in the M4 GT3 at Oschersleben ahead of the DTM season opener at the same venue on May 27-28.
