ramair is only effective at a specific velocity the opening is designed for... below that speed, it's not "ramming", and above that speed, air is being diverted away due to resistance and aerodynamics.

basically, your motor is only going to take in as much air as it can, and the only way to increase the volume of air going in is with forced induction.

as you become faster, you'll need redesigns to the funnel system to increase the amount of air you'll be able to "ram" in.

lastly, this is why you see very few NA ramair motors... WS-6 TransAm. all other ram air designs out on the street now combine forced induction with it.