Tested engine performance at sea level in ISA conditions:
Full throttle in the F13 gives about 28.5 inHg manifold pressure while the Cessna 152 gives about 29.9 inHg. Values were read via SimVar watcher. From my understanding, an engine without supercharger should deliver the outside air pressure (in this case about 29.9 inHg) at max throttle.
I might have to cross-check this with FS2020 though to see if there is a difference between the sims but I have already spotted problems with engine performance in other planes.