BMW CEO Norbert Reithofer has revealed in the past that future BMW powertrains will be downsizedin an effort to improve fuel-efficiency and reduce emissions, and that even the hallowed ‘M’ division could also be affected by the new wave of environmental consciousness sweeping the carmaker. BMW has always stood by the high-rev philosophy for delivering power from its M engines but that may be about to change.

Inside sources have revealed that the next-generation M5, due in 2011, will be powered by a high-performance twin-turbo V8 engine instead of a naturally aspirated V10. The source also revealed that initial tests of variations of the 400hp (298kW) turbocharged 4.4L V8 found in the X6 crossover is showing promising results, reports Automotive News.

Additionally, BMW’s M engineers are also finding good results from the award-winning 3.0L straight-six turbo. It comes “very close to a large-displacement, naturally aspirated engine,” the source told reporters.

Prototypes of the current M5 have been seen sporting front-mount intercoolers and additional air-intakes, confirming that a high-performance turbocharged powertrain is being developed. And, while purists may scoff at the idea of a turbocharged M car, the decision by rivals to launch forced-induction competitors such as the new twin-turbo RS6 and upcoming SL Black-Series, which is expected to get a twin-turbo V12, may leave BMW with little choice.