ShyTorque
I didn't say a carb measures mass flow (of the air). I said it measures some combination of volume flow and mass flow. It doesn't measure pure velocity either - I am sure air at a given velocity at 100,000ft will suck up a lot less fuel in the venturi than air at 10,000ft at the same velocity would.
If a carb (or a fuel injection system) measured pure air mass flow, that would be great because you could set up a particular engine operating point, e.g. peak EGT, during cruise climb, and just sit there all the way to the aircraft's operating ceiling (as the MP drops off) without any further leaning, and the engine would be running at peak EGT the whole time.
Altitude compensated carbs are an attempt to achieve this.
Car engines that operate at peak EGT have to have a mass flow sensor. This can be done by conventional air flow measurement aided by measurement of pressure and temperature. I don't think any old-style aircraft engine does this though.
The reference to 75% power comes from GAMI's tests on IO540 type engines. I would have expected it to be roughly right for other engines of similar design. It won't be easy to verify because it is said that one cannot tell detonation from the cockpit (pre-ignition is something very different). Yes it will depend on the engine load but that is what power is, at a given RPM. Can you give an example of a current-model engine which can be made to enter detonation at say 50% power?