Hi to all. I read about how MAP works and have a question. Do constant speed planes like the c182 or pa44 have an additional Venturi (apart from the one in the carburetor) in the induction system past the throttle plate?
No. It's just a tap-in to the intake passage on the downstream side of the carb. It's just measuring the absolute pressure in the duct, which is always less than atmospheric in the absence of turbocharging or supercharging. Really, it's literally just a barometer, and is graduated as one. You can take a MP gauge and use it as a barometer at home.
RPM changes with a CS prop cause a small manifold pressure change due to the change in pumping rate of the piston with RPM and thus the velocity of the intake stream. A drop in RPM from a change in governor setting causes a small manifold pressure rise as the flow rate drops, and vice versa.
You anticipate this when making RPM reductions, where you are normally making the throttle change first. For example if reducing power from 24" and 2400 RPM to 22" and 2200 RPM, I might throttle back to 21.5", anticipating it will pick up half an inch of MP when I reduce the prop RPM to 2200.