ATK Engines ATK HP291PM Chevy 350 Mid Dress Engine 325HP

The ATK HP291PM is the mid-dress version of ATK's classic Chevy 350, rated at 325 hp on the same flat-tappet cam and hypereutectic pistons at 9.1:1 compression used across this engine family, which keeps the vacuum signal a roller-cam stroker build would lack. What mid-dress means here The balancer, intake manifold, and HEI distributor are installed, along with the OE-style water pump and cast-iron heads opening to 1.94/1.50, but ATK leaves the carburetor, fuel pump, and ignition hardware, the plugs and wires, for the installer to source. Two things that differ from its siblings This version does not include a dipstick at all; ATK recommends sourcing a ZZ4 250-style dipstick separately rather than reusing the location note from its siblings. Otherwise, break-in follows the same family routine: conventional oil with a zinc additive for roughly the first 3,000 miles of driving. The balancer, intake, and distributor are bolted on here too, along with the water pump; carburetor and ignition consumables are not. Common questions Is a dipstick included? No, ATK notes this engine ships without one and recommends a ZZ4 250-style dipstick. Why no dipstick on this version? ATK does not explain the omission; it is simply listed as not included on this mid-dress build. Will it still pull vacuum for brakes? Yes, the flat-tappet cam used here is milder than ATK's roller-cam stroker builds, so vacuum accessories work fine. California requires BAR compliance for engine swap installations.