Restrictor plate

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 to provide equal level of competition, and to lower costs; insurance purposes have also factored in for motorsports.

A few top classes like Formula One limit only the displacement and air intake mouth dimension. However, in 2006 air restrictors (as well as rev limiters) were used by Scuderia Toro Rosso to facilitate the transition to a new engine formula.

Many other racing series use additional air restrictors.

After Group B cars were outlawed from rallying because they were too powerful (rumored to have reached 600 hp), too fast and too dangerous, the FISA decided that rally cars should not have more than 300 hp (220 kW). For a while no special restrictions were needed for that (e.g. the Group A Lancia Delta HF 4WD had about 250 hp in 1987). But with development in the 1990s, Group A cars were rumored to have reached 405 hp or more. So the FIA mandated restrictors for supercharged and turbocharged engines in all categories (World Rally Car, Group A and Group N).

This means that the rally version of a car like the Mitsubishi Lancer Evolution can have less power than the street version (the "280" hp Evo VII was believed to have more than 300 hp, and in some markets the FQ-320, FQ-340, FQ-360, FQ-400 versions were sold, with the number representing the total horsepower).

It also means that the torque and power curves of the engine are unusual. The engine produces peak torque and almost maximum power at a relatively low RPM, and from there to the rev limiter the torque drops and the power does not increase much.

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Wikipedia