A restrictor plate is a device that limits engine power to slow a vehicle down. They may be used in racing for safety or in an effort to even the field. In addition, they are installed on some vehicles for insurance purposes, to restrict a driver from exceeding a safe rate of speed. Vehicle manufacturers may install them on custom models by request and they can also be fitted by mechanics on finished vehicles, in a modification to the original design.
This device consists of a flat metal plate with a series of drilled holes. It is placed over the engine's air intake to limit the amount of air that enters, and by extension, to control the level of power available to the vehicle. The size of the holes determines the amount of control offered by the restrictor plate. Other measures can be taken to limit speed as well, such as adjusting cooling systems to force vehicles to overheat if they exceed a given speed.
In racing, officials can mandate the use of a restrictor plate. Their guidelines for the race dictate the size of the holes as well, to ensure that all vehicles are fitted with consistent equipment. Drivers may need to change plates between events, as some may use more permissive regulations than others. Safety officials can inspect a vehicle to confirm the device is installed and measure the holes to make sure they fall within regulation.
As race car development progresses, cars go faster and faster, and are tuned for very high performance. In meets, this can translate into safety issues, as the higher the speeds, the more dangerous the potential crashes. A restrictor plate can keep cars within a certain speed range, which can make racing much safer. They can also provide some equalizing forces by ensuring that groups with substantial funds to invest in development cannot outstrip smaller teams that may have equally skilled drivers but lack the technology for high speeds.
For insurance purposes, a restrictor plate cuts top speed, which can reduce the premium on a policy. This can be a particular concern with sports cars and motorcycles designed to be driven at high speed. The insurance company may have concerns about liabilities associated with such vehicles, and can refuse to cover them if a restrictor plate is not fitted, or may charge very high policy fees to encourage drivers to consider limiting engine power.