Was that fan pressure measured with the hose pointed direct into the fan wind? Normally a mower isn't mowing at maximum rated power. The mower will bog down below governed rpm when cutting tall grass. That simulates a prop in climb which doesn't need a governor because it is bogged down through out the climb. I suspect the fan cooling rate is about the same as free air cooling at say 60 mph. At 90 mph dynamic pressure in a pressure cowl would exceed the fan cooling. Using both fan and ram air pressure cowl would cool even more for those running high power and PSRU.