BMW M70 and S70 Engines (1987-96)

BMW M70 Engine

M70B50 engine

Available from 1987, the M70 was a 5.0-litre V12 petrol engine that was first used in the E32 750i and 750iL. The first post-war twelve-cylinder German engine, the M70 had a 60-degree ‘V’ angle, a 75.0 mm stroke and an 84.0 mm bore (the same as the related six-cylinder M20 engine). Total capacity for the M70 engine was 4987.5 cc.

The M70 was superseded by BMW’s M73 V12 engine.

The M70 engine had an aluminium/silicon alloy (Alusil) block, a die-cast aluminium cylinder head with a cross-flow design, a single overhead camshaft (per cylinder bank), a drop-forged steel crankshaft with twelve counterweights, sintered connecting rods, two valves per cylinder (including sodium-filled exhaust valves), hydraulic tappets and knock sensors that were mounted to the cylinder heads.

Due to the M70 engine’s offset of the cylinder heads, the camshafts were of unequal length and the longer shaft was used for the left cylinder head. The camshafts were driven by a single-row chain which had plastic guide tubes and tensioning rails, while drive chain lash was controlled with an automatic hydraulic tensioner.

The M70 engine used hot wire air flow sensors in conjunction with Digital Motor Electronics for measurement of physical air mass (Bosch Motronic M 1.2/7 for the E32 750i/750iL and Bosch Motronic M 1.7/5.2 for the E31 850i and 850Ci). The engine management system treated the M70 as two six-cylinder engines as each cylinder bank had its own ECU and could operate in isolation should the other shut down. Furthermore, each cylinder bank had its own secondary ignition distributor and high tension cables. The EML electronic throttle control system facilitated synchronization of the cylinder banks.

The M70 engine had aluminium alloy pistons which were coated with a 0.1 mm ferrous metal coating and operated at a compression ratio of 8.8:1; the firing order was: 1-7-5-11-3-9-6-12-2-8-4-10. Furthermore, idle speed was 700rpm and maximum engine speed 6000rpm.

The M70 engine was mated to either four-speed ZF 4HP24 automatic or six-speed Getrag 560G manual transmissions.

BMW S70 Engine

BMW S70B56 engine

Available from 1992 to 1996, the S70B56 engine was an increased capacity version of the M70 which had an increased bore (by 2 mm to 86.0 mm) and stroke (by 5 mm to 80.0 mm) for a capacity of 5576 cc. Compared to the M70, changes for the S70 included:

• New aluminium alloy pistons with a Ferrostan coating;
• An increase in compression ratio to 9.8:1;
• Revised camshaft timing;
• Maximum engine speed was 6000 to 6400 (gear-dependent);
• Idle speed was 750 rpm;
• A double-flow exhaust system; and,
• The use of premium unleaded fuel.

The S70B56 engine was solely available for the E31 850CSi and only 1510 units produced – the lowest production volume for any BMW engine.