Water-Cooled VW Performance Handbook
Horsepower and air flow
The gasoline motor is basically an air pump: The more air that flows through it, the more horsepower it makes.
Estimating air flow by horsepower
If you know how much horsepower your motor makes, you can estimate its air flow in cubic feet per minute (CFM).
For example, a 1,600 cc motor in a good state of tune requires about 54 cubic feet per minute to make 78 horsepower.
This calculator uses the formula:
Air flow (CFM) = (Horsepower × 0.625) × 1.1104
Estimating horsepower by air flow
If you have access to a flow bench, you can measure how much air flow you get through your intake and cylinder head. From this figure, you can calculate horsepower.
For example, a stock 1,600 head flows about 61 CFM. Therefore, the most you could expect with bolt-on modifications would be about 88 horsepower.
This calculator makes use of the formula:
Horsepower = (Air flow × 1.6) × 0.9