BMW’s vast experience making inline-six engines gave the company a leg up when it came to designing and producing a V-12. In many ways, the M70 V-12 engine is a pair of M20 six-cylinder engines joined at a 60-degree vee angle and sharing a common crank. The M70 shared bore and stroke dimensions, used a pair of ECUs along with two distributors, two sets of six-cylinder plug wires and even two air filters from the smaller engine. However, they shared very few other parts and no part of the rotating assembly. BMW cast the V-12’s block in Alusil, a hypereutectic aluminum-silicon alloy, as opposed to the M20’s cast-iron affair. BMW also introduced throttle-by-wire (rendering the throttle cable obsolete) and a mass airflow sensor with the M70, features that found their way into nearly every BMW within a few years. In factory spec, the M70 made an even 300 ps (metric hp; the SAE rating was 296 hp) and torque was a robust 332-lb.ft.
Alpina does not replace the factory blue-and-white BMW roundel on the hood with its own logo–despite what you may have seen out in the wild, Alpina recognizes that some pretty sound engineering goes into the production of BMW engines and automobiles, so it works very carefully to not throw out the proverbial baby with the bath water. And so it was with Alpina’s version of the 5.0-liter V-12.
Alpina left alone the M70’s engine block, crankshaft, connecting rods and the intake and exhaust manifolds. However, they did fit higher-compression Mahle pistons, larger intake valves and a set of worked-over cams along with commensurate work to the cylinder heads to improve breathing. A new exhaust system and catalytic converter rounded out the hard changes to the engines, and a reprogrammed set of Motronic ECUs tied it all together.
Alpina’s reengineering of the M70 for the B12 5.0 resulted in 49 additional horsepower (345 vs. 296) and 15 additional pound-feet of torque (347 vs. 332) without any displacement increase or forced induction. Though buyers of the standard 850i had an optional six-speed manual available to them, the B12 5.0 Coupe, as it was known, came fitted only with the four-speed automatic. With the redline raised from 6,000 to 6,400 RPM, Alpina also reprogrammed the car’s four-speed automatic transmission’s electronics so that full-throttle downshifts made the most of that rev range to keep the lower gear until redline forced an upshift. In Alpina guise, the big 8 reached the 0 to 100 KMPH (62 MPH) sprint in under seven seconds on the way up to a top speed over 170 MPH.
Of course, along with the driveline upgrades, Alpina cars also came with suspensions designed for improved handling without any ride detriment. On the B12, this consisted of fitting different springs and Bilstein shocks, along with signature Alpina 20-spoke wheels.
Alpina built just 97 B12 5.0 Coupes and a further 57 B12 5.0 Coupes based on the 850CSi and with a larger 5.7-liter engine.