BMW M70

The BMW S70/2 engine, largely unrelated to the M70 and S70B56 engines, is a naturally-aspirated, DOHC, V12 petrol engine, which powered the 1993 to 1998 McLaren F1.

Design

The M70's design is similar to that of two 2.5 L M20 straight-six engines joined at a 60-degree angle, due to the following features: single overhead camshaft valvetrain, bore spacing of 91 mm (3.6 in), bore of 84 mm (3.31 in), stroke of 75 mm (2.95 in), and a compression ratio of 8.8:1.

The M70 has the following differences with the M20 engine:

• Aluminium alloy engine block (AluSil) instead of cast-iron (both engines have an aluminium cylinder head), to reduce weight.
• Airflow measurement using Mass air flow sensors (MAFs) instead of Air flow meters (AFMs) to improve fuel economy.
• Electronic Throttle Control instead of a mechanical throttle cable.
• A timing chain was used instead of a timing belt, to reduce servicing requirements.
• Hydraulic valve lifters instead of mechanically adjusted tappets, to reduce servicing requirements.

The M70 has two Motronic 1.3 ECUs (one for each cylinder bank). To provide redundancy, the M70 also has two fuel pumps, fuel rails, distributors, mass air flow sensors, crankshaft position sensors, coolant temperature sensors and throttle bodies.

Some M70 engines (such as fitted to the E32 750iL Highline) are fitted with two alternators. The second alternator is smaller and is used to charge an auxiliary battery and power equipment in the rear passenger compartment, such as a telephone, fax machine, wine cooler, independent climate control and power sun shields.