On an automatic car you can "feel" the Power Circuit happening. Just put it in gear (observing all cautions, don't run over your washer nd dryer, don't overheat your transmission, and slowly depress the accelerator. When the Power Circuit opens the engine will noticeably respond, if you have a tach you will see it. That's the PV/PC opening. Of course this only works if the valve is working and the mixture is reasonably close. If you look up at the power chart think of it this way. I am at 1/2 throttle and making 75HP and then the Power circuit opens and voila 90!