6.21.4 All Cars may install a restrictor between the air filter and intake manifold to reduce horsepower and torque. The diameter and thickness of the restrictor plate orifice(s) shall be noted on the Dyno Sheet and must match at all times. Carbureted cars may install a mechanical throttle stop on the carburetor linkage for the same purpose. Throttle stop dimensions must be noted on the Dyno Sheet for inspection purposes and must match at all times.

6.24.3 Any suitable exhaust system may be used after the manifolds (or headers on cars that are allowed headers). The exhaust must exit behind the driver and be directed away from the car. Mufflers may be required to meet sound regulations depending on track location.


Was/is there a reason that an exhaust restrictor is not specifically mentioned in the rules?