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.