ATK HP105 Chevy 383 Stroker Marine Base Engine 380HP

The ATK HP105 is a remanufactured marine base engine built from a Mercruiser 377-cubic-inch MPFI block, fitted with an upgraded Scat 4340 forged crankshaft and forged pistons under the original-style Mercruiser camshaft. ATK rates it the way Mercruiser rates marine power: 405 hp at the crankshaft, 380 at the prop.

Who this engine suits

It ships bare of a fuel system, so it is meant to be finished with the Mercruiser MPFI setup it came from, an aftermarket EFI system, or a carburetor conversion, whichever matches your boat. Dart Vortec cast-iron heads with 2.02/1.60 valves and a Melling oil pump round out the build, with forged pistons running 9.1:1 compression.

What to check before you buy

• No intake, oil pan, timing cover, harmonic balancer, flexplate, or flywheel ships with this base marine engine; plan your fuel system and accessory drive separately.
• A single-roller timing chain and stamped-steel 1.5-ratio rocker arms are used in the valvetrain.
• Confirm with us whether your boat's wiring and controls suit a carburetor conversion if you are not keeping the factory MPFI system.

Common questions

Why two horsepower numbers? Marine engines are commonly rated both at the crankshaft and at the prop; this one is 405 at the crank and 380 at the prop.

Is this new or rebuilt? It is ATK's remanufactured version of the Mercruiser 377 MPFI engine.

What's the compression ratio? 9.1:1, mild on purpose for a marine engine meant to run reliably at the prop. California requires BAR compliance for engine swap installations.