I also have the altitude compensating fuel pump. I found this explanation and I keep a copy of it saved:
The altitude compensating mechanism basically adjusts full rich based off of altitude. As you increase altitude and there’s less air, the same full rich air/fuel ratio will require less fuel. Also as a matter of clarification, calling it Cirrus’s fuel injection system is a bit inaccurate - it’s just a standard Continental fuel injection system and the same altitude compensating fuel pump (or same style) is used on plenty of other naturally aspirated aircraft, and has been for decades. Basically it’s just a fuel pump for a turbo (which references boost to adjust max fuel flow), but since there is no turbo it just makes fuel flow go down with altitude and the associated reduction in atmospheric pressure. Those fuel systems have the following inputs:
- Engine RPM (which determines how much fuel the pump will flow)
- Throttle position (which acts as an orifice/restrictor)
- Mixture position (acts same as throttle from a fuel system perspective
- Atmospheric pressure or upper deck pressure (pressure before the throttle butterfly)
- Turbo engines often have a fuel limiter which just acts as a ceiling on fuel flow because 1-4 will result in more fuel than what is desired for full rich.
Also, there is this pictorial and explanation from Cirrus (photo included below)