by Vin Stross

Created on: August 15, 2008

A Restrictor plate or air restrictor is a device installed at the intake of an engine to limit its power. This kind of system is occasionally used in road vehicles (e.g., motorcycles) for insurance purposes, but mainly in automobile racing, to limit top speed and thus increase safety, to provide equal level of competition, and to lower costs.

The first use came in 1971 as part of NASCAR's plans to reduce the size of engines from 427 cubic inches (7.0 L) to 358 cubic inches (5.8 L). In order to allow teams with smaller budgets to race the larger engines, NASCAR made mandatory the use of a restrictor plate to be placed on larger engines to equalize performance with smaller engines. The transition ended in 1974, when NASCAR banned the larger engines, and went to the 358 cubic inch engine (a compression limit would be implemented in 1996). This was a transitional process and, as not every car used restrictor plates, this is not what most fans call "restrictor plate racing."

The second use came following the terrifying crash of Bobby Allison at the 1987 Winston 500 at Talladega Superspeedway. Allison's Buick LeSabre blew a tire going into the tri-oval and flew tail-first into catch fencing early in the event, injuring spectators (although not actually entering into the grandstands). After a summer where the two subsequent superspeedway races were run with aids to prevent cars from flying, and smaller carburetors (390 cubic feet per minute instead of 750 cubic feet per minute) proved to be inadequate to sufficiently slow the cars, NASCAR imposed restrictor plates again, this time at the two fastest circuits, both superspeedways: Daytona for all NASCAR-sanctioned races and Talladega for Cup races. The Automobile Racing Club of America also enforced restrictor plates at their events at the two tracks. In 1992, when the Busch Series began racing at Talladega, the plates were implemented.

In some tracks, NASCAR's concerns with speeds because of power-to-weight ratios result in restrictor plates at other tracks. The Goody's Dash Series (known now as the ISCARS series with its new ownership) used restrictor plates at Bristol during at least the last years of the series' existence when the cars were using six-cylinder engines (compared to the traditional four cylinder engines), in addition to their Daytona races.

However, restrictor plates are not used for Craftsman Truck Series trucks. Rather, air intake, aerodynamic, and, eventually, a tapered carburetor spacer were implemented

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