Restrictor Plate

A restrictor plate is a flat metal plate with holes in it that is installed in a race car's engine to reduce its power and speed by limiting the amount of air and fuel that can flow into the engine.

Think of a restrictor plate like putting your hand partially over a vacuum cleaner hose—it restricts the airflow and reduces the suction power. In racing, this device sits between the carburetor (the part that mixes air and fuel) and the intake manifold (where that mixture enters the engine). By controlling how much air and fuel can enter, the plate effectively limits how much horsepower the engine can produce.

Racing organizations like NASCAR use restrictor plates primarily for safety reasons. On very fast tracks called superspeedways, race cars can reach dangerously high speeds—over 200 miles per hour. At these speeds, if a car crashes and becomes airborne, it can fly into the fencing and potentially injure spectators. The restrictor plate keeps speeds lower and more manageable.

NASCAR first mandated restrictor plates in 1988 after a frightening incident at Talladega Superspeedway. Driver Bobby Allison's car became airborne during a crash and hit the catch fence at approximately 210 mph, tearing a large hole in the protective barrier and endangering fans in the grandstands. This near-tragedy prompted NASCAR to take immediate action to reduce speeds.

The restrictor plate typically reduces engine horsepower by about 300 horsepower and drops top speeds by roughly 10 mph. While this might not sound like much, it makes a significant difference in safety. The size of the holes in the plate determines how much restriction occurs—NASCAR has used plates with holes measuring 57/64ths of an inch in diameter.

Restrictor plates have been primarily used at NASCAR's two largest and fastest tracks: Daytona International Speedway and Talladega Superspeedway. In 2022, NASCAR also implemented them at Atlanta Motor Speedway after the track was reconfigured in a way that increased speeds and raised safety concerns.

An interesting side effect of restrictor plates is that they create what's called "pack racing." Since all cars have similar reduced power, they tend to stay bunched together in large groups rather than spreading out around the track. This often leads to dramatic, close racing with frequent lead changes, though it can also result in massive multi-car crashes when something goes wrong in the tight pack.

In 2019, NASCAR moved away from the traditional restrictor plate design. After the Daytona 500 that year, the series switched to a tapered spacer with a variable opening size. This device serves a similar purpose to the restrictor plate but offers more precise control over engine performance. Despite this change, many fans and commentators still use the term "restrictor plate racing" to describe racing at superspeedways where engine power is limited for safety.