What is the maximum engine displacement allowed in NHRA Pro Stock?

The maximum engine displacement allowed in NHRA Pro Stock is 500 cubic inches.

The limit is part of NHRA’s technical formula for Pro Stock, a naturally aspirated class often called “factory hot rods.” The restriction helps keep engine size within a defined range while teams pursue power through cylinder-head design, high engine speeds, fuel injection and careful tuning.

NHRA introduced a uniform 500-cubic-inch maximum in 1982, replacing an earlier weight-break system. The change also distinguished NHRA Pro Stock from “Mountain Motor” categories, whose larger engines can exceed 800 cubic inches and follow different rules. That distinction is the main source of confusion: 572-cubic-inch and larger engines may appear in other drag-racing classes, but they are not legal under the standard NHRA Pro Stock formula.

Modern NHRA Pro Stock engines remain single-camshaft, 90-degree V8s and use electronic fuel injection. The class’s 500-cubic-inch limit is therefore an engine-displacement rule, not a statement about horsepower; competitive engines can still produce well over 1,000 horsepower.