The race motors such as the 2.5OS or XR2 run about 15 PSI because they dont have any thermostats or a poppet (pressure release) valve. The non race (fishing) motors do have these so run more water pressure, up in the 20 to 30 psi range. 0 to 35 PSI seems to be the norm for a water pressure gauge on mercury outboards.
A slider/shutter fronty end will give more power on a standard engine but will be even more beneficial on a tuned motor.
The 1 inch intake spacer is a good easy to install cheap mod if your still running the center horn front end.
__________________
|