A restrictor is something that limits or controls the flow or movement of something. It keeps it within specific boundaries or prevents it from going too far or too fast. Full definition
Series officials mandated a reduction in the size of the Corvettes» intake air restrictors for this race, an adjustment that cut engine output by 35 horsepower. (corvetteracing.com)
The car dominated racing so much that NASCAR eventually placed restrictions on the HEMI engine by forcing the use of carburetor restrictor plates on Chrysler models featuring the HEMI engine and limiting engine displacement to a maximum of 305 cubic inches on Charger Daytona and Plymouth Superbird models. (topspeed.com)
In contrast, the Aston Martins are permitted to use larger 31.2 mm restrictors on their V - 12 engines and weigh 199 pounds less than the Corvettes. (corvetteracing.com)