Here's a formula that is helpful in determining an engine's horsepower.
It is based on an MEP of 150 psi., sort of typical of an 8.5:1 CR. CDI
X RPM / 5280. An O-320 at rated 2700 rpm, using this, gives 163.6 hp.
Of course, this 5280 constant only works for normally aspirated
engines. The 60 hp 800cc/49ci Hexadyne, featured in the just-out
Contact! issue #81, calcs to 53 hp at 5750 rpm, but it's 9:1 CR, so the
published 60 hp seems reasonable.