ATK Engines HP89 Small Block Chevy 350 Base Engine, 390HP

Rated at 390 HP and 420 TQ, the HP89 is ATK's other 350CI long block offering, pitched as very streetable with lots of power rather than a track-only build. Like the rest of ATK's 350 long block range, it starts from a seasoned OE roller block, here fitted with a 4 bolt main and 1pc seal, and ATK calls out a noticeable idle as part of the character.

What's turning inside

Bore and stroke measure 4.040” x 3.480”, with an OE reground crank running OE powder metal 5.7” rods and compression set at 10:1 via hypereutectic pistons. Valve timing comes from a hydraulic roller grind specced 220/224 at .050, lift-wise .528 in. on the intake side and .536 in. on the exhaust side, with a 110 lobe separation angle. On top sit ATK's own high performance aluminum heads, running 2.020 in. intake valves and 1.600 in. exhaust valves, actuated through billet aluminum roller rockers at a 1.6 ratio.

What to check before buying

Its 5-quart rear sump oil pan is not universal: ATK calls out certain Chevelles with steering behind the crossmember, a handful of Corvettes, and early Chevy II models as applications where it won't clear. For the rest of the drivetrain, ATK points toward a 2200-2800 rpm stall converter and headers in the 1 5/8 size range to complement this build.

Common questions

How much power does this engine make? 390 HP and 420 TQ.

Will the stock oil pan clear my chassis? Not always; ATK names specific Chevelle, Corvette, and Chevy II applications where the included pan is too tall.