Basically, an engine generates power by burning gasoline. In order for a car to produce more horsepower, it has to burn more gasoline, which requires more air.
The corollary is also true, meaning that if an engine is using more air, it will generate more horsepower.
The amount of air flow required is directly related to the amount of horsepower generated. If you know the air flow you can calculate the horsepower and vice versa.
This calculator allows you to choose between either of those two calculations.
For practice use an air flow of 80 cubic feet per minute and it will equal 115.27 horsepower and use this for your hosepower to air flow calculation.