I have a twin (stock) 1998 502 Mag MPI engine application. One of the engine's water pressure indicates 22psi at 3300rpm and goes up to 27 psi at 5000RPM. The other seems to stay under 20psi from 3K and up. Can someone provide some insight here? How does the cooling system regulate this pressure and is this pressure normal or too high at 27psi?

They should read the same.. Swap the gauges to make sure you dont have a bad gauge.... If the pressure stays high then look for a restriction...(kinked hose or some sort of bolckage)

Merc says not over 35 psi, But most engine builders on here say between 22-27 psi

18 lbs is enough, thay way you don' t worry about blowing
a head gasket or hose.

OK thanks for the input.