ATK HP55M Chevy 383 Stroker Mid Dress Engine 500HP

The HP55M is ATK's 383 cubic-inch stroker built for serious street power, rated at 500 hp and 505 lb-ft of torque. It ships mid-dress: the harmonic balancer is installed, but you supply your own flexplate or flywheel and finish the induction and ignition to your own spec.

What makes this build different

Dart SHP aluminum heads with large intake and exhaust valves sit atop the long block, built on a bore of 4.040 inches and a stroke of 3.750 inches. Forged pistons hold 10.2:1 compression, matched to a hydraulic roller camshaft and a double roller timing chain for durability under load. This is a new, assembled long block from ATK's Chevy stroker lineup, not a remanufactured core, and it is tuned to run hard rather than idle gently.

Before you order

Because it arrives mid-dress, plan on adding your own carburetor or EFI setup, distributor, and exhaust. ATK's own recommendation for this combination is a 750 CFM carburetor, and the build is set up to run on 91 octane fuel or better. Confirm your intake, ignition, and accessory brackets clear the larger aluminum heads before committing to the swap, since the extra casting width can crowd some factory brackets on older chassis.

Q: Does it include a flywheel or flexplate? No, this mid-dress build omits both; source one to match your transmission.

Q: What fuel does it need? ATK specifies 91 octane or higher for this combination.