Guessing numbers

It still looks like a curve fitting/regression type problem. As I indicated earlier, I do not readily see a function that will fit the data.

Perhaps if we could look at this from a theoretical perpective. I'm not any kind of automotive/mechanical engineer, so I'm mostly speculating here. Assuming a 4 stroke engine and that we know the displacement, I believe we can readily say that an engine processes a volume d (d=displacement) of air + fuel every 2 rotations. From the given engine speed (RPM), we should be able to calculate how much air+fuel passes through the engine at a given engine speed. It has never been clear to me what the units are on that 14.6 ratio (mass/mass, mole/mole, vol/vol, other??). From the known engine speed and discplacement, and assuming values for manifold temperature and pressure, we should be able to calculate how much air and fuel the engine would theoretically use. By figuring out how manifold pressure and temperature are related to engine rpm and throttle position, we could further estimate how these parameters figure into the quantity we are calculating.

Recognizing that this procedure is probably grossly oversimplified, perhaps it will give us some sense of what form we might use for f in our regression equation.

Can I ask -- what is your background? Are you an engineer (or engineer in training) who would know the equations that are used in describing engine operation, or are you a hobbyist trying to understand better how an engine works?