Currently, there are three 8100 Series (496ci) Vortec marine engines, (derived from the fuel-injected 8.1L big-block used in Silverado pickups) identified as HP1, HP2, and HP3, and rated at 375, 420, and 525-plus horsepower respectively. The HP3 is especially impressive considering its 9.1:1 compression ratio and the minimal number of upgrades. CNC-ported cylinder heads, port-matched intake manifold, high-lift hydraulic roller cam, special valvesprings, aluminum roller rockers, machined-steel camshaft sprocket, windage tray, headers, and welded oil-pump pickup are the only major differences between the HP1 and HP3. Final horsepower and torque figures for the HP3 hadn't been decided upon at the time of the preview, but at 525 hp and 560 lb-ft of torque, the engine is clearly not at its full potential. Manifold pressure readings of an HP3 being run on the dyno at the GM Tech Center indicated the simple addition of a larger throttle body would put a nice bump in the power curve.