Restrictors
Restrictors are devices installed in a race car's engine air intake system that deliberately limit the amount of air flowing into the engine, which reduces the engine's power output and maximum speed.
Think of a restrictor like partially covering your mouth while trying to breathe heavily during exercise. Just as you can't get as much air into your lungs, a restricted engine can't pull in as much air as it normally would. Since engines need air to burn fuel and create power, less air means less fuel can be burned, resulting in lower horsepower.
The most common type of restrictor is called a restrictor plate, which is a flat piece of metal (usually aluminum) with precisely measured holes drilled into it. This plate sits between parts of the engine's air intake system, acting as a barrier that controls how much air can pass through. Some racing series also use tapered spacers, which are cone-shaped restrictors that serve the same basic purpose.
Racing organizations use restrictors primarily for two reasons: safety and fair competition. On the safety side, restrictors prevent cars from reaching extremely dangerous speeds, particularly on large oval tracks where vehicles might otherwise exceed safe limits. NASCAR famously requires restrictor plates at superspeedways like Daytona and Talladega, where cars could theoretically reach speeds well over 200 miles per hour without them.
For competition purposes, restrictors help level the playing field between racing teams. Without them, teams with bigger budgets could simply build much more powerful engines and dominate races purely through horsepower advantage. By capping engine power, racing series ensure that driver skill, car setup, and race strategy matter more than just raw engine performance.
When restrictors are in use, they create unique racing dynamics. Cars typically have good acceleration at lower speeds but hit a power ceiling at higher speeds and engine RPMs. This limitation means drivers can't simply pull away from competitors using engine power alone. Instead, racing becomes more about aerodynamic drafting, where drivers position their cars behind others to reduce air resistance and maintain speed.
The reduced power from air restrictors often causes cars to race in tight packs rather than spreading out across the track. While this can create exciting, close racing action for spectators, it has also sparked controversy. Critics argue that these tight packs increase the risk of massive multi-car accidents, since one mistake can quickly involve many vehicles racing closely together.
Different motorsport series use restrictors in various ways. NASCAR has mandated their use at specific tracks since 1988. Rally racing employs restrictors to control the power of rally cars competing on public roads and challenging terrain. Formula racing and sports car racing series also implement various forms of air restriction systems as part of their technical regulations, adjusting specifications based on car class and competition requirements.