Although announced at the 1979 Frankfurt Show, the 745 Turbo was not launched until the middle of 1980. BMW had taken a little time to get their high performance flagship into shape, but its emergence certainly gave a boost (in more ways than one) to the power/prestige race between Munich and Stuttgart.
The basis of the 745i was a turbocharged version of the 3210 cc in-line six-cylinder ohc taken from the 732 and 633 CS models. For use with turbocharger the engine's compression ratio was lowered from 9.3 to 7:1, but although the cylinder head was modified to accept different valves, a great many of the normal unsupercharged engine's components were retained.
Pistons and crankshaft were modified to withstand the higher pressures that supercharging brings at high rpm. A KKK (Kuhnle, Kopp & Kausch
) turbocharger was used, and to obtain good results BMW mounted an air/air intercooler on the inlet side of the engine, dropping air temperature from 120 deg. to 80 deg. C. and condensing it.
Controlled by a wastegate, the circuit also included a butterfly valve, which when shut, presented a closed circuit for the gas, thus avoiding a dramatic pressure fall, and a lag in response. Pressure was quantified at +0.6 bar (8.70 psi) at 2600 rpm, the mixture being supplied via Bosch L-Jetronic equipment, with two cut-off devices.
One was a rev-limiter set at 6300 rpm on rising revs, the other operating under deceleration with extra injection in the interests of fuel consumption. It was notable that BMW did not incorporate the Bosch Motronic computerised engine 'management box' as they had previously used on some unsupercharged 7-class models, finding a simple contactless transistorised ignition system quite satisfactory.
Certainly the turbocharged engine delivered impressive outputs, maximum power being 252 bhp (188 kW) DIN at 5200 rpm, with maximum torque at 280 Ib ft (38.70 mkg/375.48 Nm) at 2600 rpm. Remarkably, the motor, although delivering around 80 bhp per liter (60 kW/lit) also developed more than 239 Ib ft (33 mkg/323.52 Nm) between 1800 and 5500 rpm. On the debit side the 745 power-unit consumed between 10 and 20 per cent more fuel than its unsupercharged counterpart.
The 745 Turbo model was supplied only with ZF automatic transmission, was unusually well-equipped with, according to marketing material produced at the time, air-conditioning, Bosch ABS anti-lock braking, Michelin TRX 220/55 VR 390 tires, and there was even a Siemens computer which would automatically turn on the heating system, under the influence of low temperatures - while the owner was still having breakfast!
In 1984 the engine grew to 3.4 liters (BMW engine code M106), produced 6 psi intake pressure, and produced the same power output as the M102. Again these cars were all built with automatic transmissions. The water buffalo interior model was the Executive trim option, while the High Line trim package offered ivory leather interior trim including a leather dashboard, sun visors, headliner, and other leather bits not offered on the Executive.