The 2006 BMW M5 is a unique sport sedan with an engine that will appeal to speed racers. It's fast with plenty of advanced features to go with it. Its five-liter, 40-valve, normally aspirated V-10 produces an outstanding 507 horses and 384 lb-ft of torque and feeds it to rear wheels through a seven-speed sequential manual gearbox. The M5's engine and transmission are so sophisticated that it is hard to believe this big sedan can produce such a potent blend of performance and refinement. To get those horses to the ground, BMW's engine control unit (ECU) contains three 32-bit processors that process up to 200 million calculations per second. It controls all engine functions, including the throttle valves, bi-VANOS and fuel mixture, as well as governing the various systems that manage the car's brakes, stability control and the gearbox. To cope with the heat and combustion pressure that a high-rpm load can generate, the M5's crankcase incorporates cast-iron inserts into its aluminum structure. Its bi-VANOS variable valve timing and electronic throttle-by-wire system combine to keep the M5's power flowing smoothly and efficiently, with response that peaks around 8,000 rpm. It's a joy to put this car through its paces on the track. The M5's rear end is incredibly communicative, while the chassis's 50/50 weight distribution and stiffened suspension make it a safe machine, especially in dry weather. Steering is an easy affair, aided by BMW's Active Steering system that enables the M5 to read and adjust its yaw angle quickly and accurately.  The steering wheel is mounted low and is set at a slightly forward angle, the M5 feels perfectly balanced in either of its two modes: comfort and sport.