Wrote this in another thread recently, fits here also:
These are the two commonly used equations that most people use to figure up PSI.
(GPM x PSI) / 1100 = HP
(GPM x PSI) / 1450 x 1.5 = HP
According to Russ, he prefers the second, giving the reason that the first pushes the engine to the limit constantly.
A GX390 is about 13 HP, and if you use 5.5 for GPM to solve for PSI, you get:
5.5 x PSI / 1450 x 1.5 = 13
PSI = 2285
The first equation, results in 2600.
And yeah, John, a gear driven will bolt right on easily. Just did it with my GX390.