How Much Horsepower Did the 426 Hemi Make in NASCAR?

The Chrysler 426 Hemi, in period NASCAR race trim, typically produced about 575–600 horsepower, depending on the tune, track, and rule package; this is notably higher than the factory “street Hemi” rating of 425 gross horsepower and reflects the engine’s competition-focused build and development. Below is a closer look at how and why the figure varies and what context shapes the number most often cited by historians, teams, and engine builders.

What the 426 Hemi Was in NASCAR

Introduced for racing in 1964, the 426-cubic-inch “Race Hemi” was engineered for high-speed oval competition, featuring hemispherical combustion chambers, big valves, robust bottom-end hardware, and a competition cam and induction. It quickly became the benchmark powerplant for Chrysler-backed teams, driving landmark wins at Daytona and other superspeedways before rules changes and displacement limits reshaped the playing field.

Why the Reported Horsepower Varies

Horsepower figures for the 426 Hemi in NASCAR depend on the specific year, track type (short track vs. superspeedway), carburetion, fuel, compression, and the sanctioning body’s rule changes. In unrestricted or lightly restricted late-1960s setups, many period accounts and modern rebuild dynos cluster in the high-500s, with well-developed engines touching around 600 horsepower. When displacement or intake restrictions were imposed in the early 1970s, output fell accordingly.

Typical Output by Era and Setup

The following points summarize commonly reported horsepower ranges for the 426 Hemi in NASCAR, reflecting period practices and later documented engine builds.

• Mid-to-late 1960s (unrestricted or lightly restricted): approximately 575–600 hp, as used by top teams on superspeedways.
• Short-track tune (same era): generally similar peak power, with cam and gearing favoring drivability and midrange over top-end speed.
• Early 1970s rules shifts (e.g., displacement limits for aero packages): reduced output relative to the 426-ci setups due to smaller cubic inches and other restrictions.
• Street Hemi comparison: factory-rated at 425 gross hp—substantially less than race-prepared engines due to milder cams, lower compression, exhaust, and emissions equipment.

Taken together, the consensus landing spot for the classic full 426-ci NASCAR race Hemi is roughly the high-500s, with standout builds reaching about 600 hp under period-correct conditions.

Street vs. Race: Understanding the Numbers

The “425 hp” many fans know refers to the showroom 426 Hemi’s gross rating under 1960s test standards—not the track-tuned race engines. NASCAR Hemis benefited from higher compression, aggressive cam profiles, carefully matched induction and ignition, hand-fitted internals, and frequent tear-downs between events. Those differences, paired with race fuel and long-WOT operation, account for the sizable gap between street and NASCAR horsepower figures.

Rules and Restrictions Shaped the Ceiling

NASCAR’s evolving rulebook was designed to control speeds and encourage parity. While the late-1960s 426 Hemi powered a wave of dominance with near-600-hp potential, subsequent measures—including displacement limits for certain body styles and, in later eras, intake restrictions at specific tracks—curbed peak numbers. As a result, any single horsepower figure must be tied to the exact rules and setup of the time.

Key Takeaway

When enthusiasts ask how much horsepower a 426 Hemi had “in NASCAR,” they’re usually referring to the late-1960s, full-displacement race builds that delivered about 575–600 hp—engines that cemented the Hemi’s reputation and legacy in American stock car history.

Summary

The NASCAR-spec 426 Hemi commonly produced roughly 575–600 horsepower in its late-1960s prime, significantly more than the 425-hp factory street rating. Output varied with track type and rules, but the high-500s to around 600 hp is the historically supported range most associated with the engine’s dominance in stock car racing.