ATK HP98M Chevy 350 Mid Dress Engine 345HP

Splitting the difference between ATK's bare and complete 350 long blocks, the HP98M arrives with its oil pan, timing cover, harmonic balancer, and intake manifold already fastened down, while the flexplate or flywheel choice stays with you. Output matches the rest of this 350 family at 345 horsepower and 400 foot-pounds.

What is already on, what is not

ATK bolts on a dual-plane intake and an HEI distributor here, leaving the carburetor for you to pick; a Quick Fuel 680 CFM unit and a 110-gallon-per-hour Holley mechanical pump are ATK's own suggestion. Neither ships with this engine, so add those plus a flexplate or flywheel before installation day.

Inside the long block

Bore and stroke are 4.040 and 3.480 inches, the standard dimensions behind this 350. ATK cast iron heads use stamped-steel 1.5-ratio rockers over hypereutectic pistons dialed to 9.1:1 compression, a double roller chain handling timing duties. The break-in call here is 3,000 miles on conventional 10W30 oil mixed with a zinc additive, then 91 octane fuel going forward.

Q: What is missing compared to the complete version? The carburetor, fuel pump, and flexplate or flywheel are not included on this mid-dress build.

Choosing mid-dress over the complete version makes sense if you already have a carburetor or fuel pump on hand, or simply want to pick your own rather than accept ATK's recommendation for them.