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The key is the monohead, an assembly consisting of the cylinder head and the cylinder liners in one rigid unit. This hinges within the engine block, pivoting along one side while hydraulically-powered rams force the other side of the monohead up or down as needed. This movement - a total hinging arc of 4 degrees - changes the distance between the top of the combustion chamber and the crankshaft's centre line, so altering the chamber's volume and therefore the compression ratio.
A rubber bellows-type gasket seals the gap between monohead and block, so crankcase fumes can't get out and dirt can't get in. And instead of Saab's usual turbocharger, the SVC engine uses a supercharger which can deliver up to 1.8 bar of boost - that's nearly twice as much as a regular Saab turbo manages - for a quicker response. This gets round the problem that a small engine doesn't produce enough exhaust gas at low engine speeds to get a big, high-boost turbo spinning.
The low compression, and smaller cylinders' better ability at shifting heat, are why the engine can run at such crazy boost without going bang. The supercharger's drive clutch disengages at light loads, to save the energy that would otherwise be needed to turn it. Its throttle closes at the same time, and a by-pass throttle activates instead.
Making a small engine work hard is more efficient than making a big one work lazily, which is why just 1.6 litres of prototype SVC engine can propel a hefty Saab 9-5. As a 20-valve five-cylinder, with small pistons and valves, it can run more smoothly than a four-cylinder and the high boost puts back the torque that small-cylinder engines usually lack.
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