Road Test: BMW 750iL

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Key construction features

The short and rigid aluminium crankcase houses a forged steel crankshaft that spins in seven bearings, with balance inherent in the 120° cranks. Cylinder liners provide a bore of 84mm whilst the 325i’s connecting rods (“modified on the outer surfaces” to suit the relative position of V12 rod angles) allow a stroke of just 74mm.

The recessed crown pistons run an 8.8:1 cr that is compatible with unleaded petrol, whilst the cylinder heads cling to older traditions in the use of sohc per bank (chain driven) and two valves per cylinder. By far the majority of combustion space is contained in the crown of each piston, their design visually reminding one of the Heron heads that were so fashionable in the British industry of the Sixties and Seventies.

The valve-gear is actuated by rocker arms with hydraulically-maintained clearances which require no service attention. Ancillary power-unit drives, for items such as the air conditioning compressor/engine fan and the alternator/hydraulic pump, are allowed the Eighties drive system of poly-vee belts. Fuel induction is by equal-length aluminium rnanifolding of irresistible allure to engine enthusiasts, each six-branch bank bearing the BMW logo. To reduce noise, all intake manifolding is mounted on elasticated joints on the inner face of each (identical) cylinder head.

A third generation of Bosch Motronic engine management is sub-divided into one Motronic computer unit per cylinder-bank. All the usual functions of a fully-mapped ignition and injection sequence are provided, plus compensated idle speeds which account even for engine wear.

The Motronic has also learned to speak with the electronically-managed four-speed automatic to assist in cushioning each gear change, and will also be asked to function in association with ASC (Automatic Stability Control), an automatic power reduction taking place when the rear wheels are in danger of spinning. We have experienced this spoil-sport device in Germany, and it does avoid the worst excess of oversteer from tight corners, but provides Audi engineers with ammunition in their “traction is more important than power” argument.

That leaves the exhaust system to trail, largely unheralded, through a series of collector pipes which culiminate in twin down-pipes for each bank. The system is united at the catalytic convertor, separated briefly before a combined exhaust box allows egress through distinctive rectangular stubs.

BMW jumped ahead of its home market opposition with the bold move into V12 power, and a V8 is confidently expected to be added to the Bavarian arsenal before 1990. In response, Mercedes is credited with a 48-valve V12 of 350 bhp for 1990 to make its forthcoming 600 the “strongest and fastest” limousine in the world.

I hope Jaguar has the answer to all that under development. Do not everlook both Audi with a V8 and the Japanese, who will also be joining the luxury power game in the closing years of this decade. JW

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