A restrictor plate is a thin metal plate with four holes that restrict air flow from the carburetor into the engine. The size of the holes can determine the amount of horsepower and thus reduce speeds.
They are used only for races at Daytona and Talladega Superspeedway, the two longest and fastest high-banked tracks on the Nextel Cup and Busch series schedules. They are not used on Craftsman Trucks.
Restrictor plates were introduced in 1989 after a horrendous accident two years earlier at Talladega, when Bobby Allison's car got airborne and flew into a protective fence along the front straightaway. Speeds at the time were more than 212 mph. Since then, the use of restrictor plates has kept them in the 190-mph range.
-- Shav Glick