Performance air filters claim to increase horsepower and improve acceleration, but do they actually work? I took four air filters, included dirty & clean OE filters, a low cost aftermarket filter, and an expensive performance filter and put them to the test. To measure differences in horsepower and torque, each filter was put on a Mustang dynamometer, with multiple runs. A VBOX was used to measure real world acceleration differences. The idea behind performance air filters is that a less restrictive air filter means the engine doesn't have to work as hard to bring in air. That means it's more efficient, and makes more power. It could also mean the engine pulls in more air, and with more air, the engine can inject more fuel to make more power.