Tuning the 850s M70 Engine
For anyone lucky enough to own one of these classic BMWs the thought may have crossed your mind about whether it is possible to drag more power out of its mighty 5 litres? After all similar size engines in modern supercars churn out at least 100bhp per litre, the M70 is only around 60bhp per litre.
The fact is with 2+tons to move the engine is no barnstormer and would be better with nearer the 400bhp of the Csi. Unfortunately its not a good platform with poorly designed heads etc which were fine at the time.It seems other than twin turbos a la Dinan the is very little can be done at a realistic price to improve matters.
Engines with over 500hp are available but at an astronomical cost and it would be more realistic just to go out and buy a car which already has the power and performance.The following is an article written by Ed at MWrench which explains more fully:
BMW 850i M70 Engine Tuning Information
I am not writing this to rain on anyone’s parade but more to help the new comers to the E31 community and also hope to bust some of the myths that have been floating around.
Performance chips generally do help a bit to increase the HP and torque of the V-12 but nowhere near many published claims. Dyno test can be and are misleading. The V-12 does not go “open loop” until 4200 RPM at full throttle so no fuel enhancement may be made as the loop is closed around sensors that feedback stoichiometric conditions, that is, “perfect” air/fuel mixture. Best power mixture cannot be obtained until 4200RPM and higher, BUT, the torque peak on most stock V-12 is right around this RPM range and declines quickly after that. Cutoff is at 6000 RPM (6400 for CSI in some gears) but after 5000 both torque and HP numbers are declining. DMEs are adaptive to some small degree i.e. minor changes are made to compensate for driving habits and conditions.
If a vehicle is put on a dyno after thousands of miles of driving, torque and the derived HP numbers will be the result of the DMEs modified during usage. Then when new chips are installed, power has been removed from the DMEs and all the data points are erased giving the new chips a “virgin DME” baseline plus whatever mapping changes that were made by the chips author. Torque surely will be higher but probably by a higher amount then if the DMEs with the original chips were powered down for a period of time to baseline the DMEs before the dyno testing commenced.
To get a better estimate of power gain, the original chips should be returned and dyno tested again. Many vehicles will show higher numbers on consecutive tests because many ofthe lubricating fluids thin during testing and in many cases the real operating temperature of the driveline hasn’t been reached during the first or second dyno run.