The Supercharged Engine

At the time Alpina began development of the B7 engine, BMW’s V-8 engine with Valvetronic – the unique system that regulates power output by varying intake-valve lift – was at 4.4-liter displacement, not the 4.8 liters of today’s 750i. From this basis, which delivered 325 bhp at 6100 rpm and 330 lb-ft of torque at 3600 rpm, Alpina took on the challenge of supercharging a Valvetronic engine, something that had never been done before. To tell something of the ending before the story, the achievement is impressive: 500 bhp at 5500 rpm and 516 lb-ft at 4250 rpm. In German, such figures qualify as bullig and no translation is necessary to get the meaning.Both supercharging (in German, a supercharger is a Kompressor) and turbocharging have their attractions as power-enhancers for combustion engines. A supercharger is mechanically driven by the engine; a turbocharger is driven by streaming exhaust gas. In this case the supercharger’s salient points include lag-free response, relative quietness, low exhaust back pressure and the ability to run the engine at part load (i.e. most of the time) just as it runs without the supercharger: with Valvetronic regulating its output. These last two factors speak for good efficiency and reasonable fuel economy in everyday driving.The supercharger is a new development. It’s of the Radial or Nautilus type, not a brand-new concept as such but now appearing on an automotive engine for the first time. It had been patented by ZF in the 1980s, but only recently have metallurgical advances made this actual vehicle application possible. (In connection with the B7, Alpina has several patents pending.) At full boost, the supercharger delivers a positive induction pressure of 0.8 bar.There is a clear subjective difference between an M-type exhaust note and that of the B7. Being high-revving, M engines emit a relatively high-pitched exhaust sound. The Alpina B7 engine, with its lower-rpm range and emphasis on big torque, just as naturally – and despite the measures to keep low-frequency sound in check – puts out a lower-pitched sound. Commenting on this sound in everyday driving, the April 4, ’04 issue of Germany’s auto motor und sport magazine called it “Gently murmuring, acoustically not unlike a costly Riva boat, the V-8 pulls away and shows with great refinement that there’s also driving pleasure at less than 500 bhp.”