Wolfgang gave a correct partial answer - as opposed to a partially-correct answer :-).  The climb rate you are looking for is the DIFFERENCE between the "climb" rates at full power and at no power.  At the same airspeed, climb at full power and then glide with no power (at the same altitude, of course).  Say you weigh 1800 pounds, can climb at 1000 ft/min and have a descent rate of 1000 ft/min at no power (so I can do the math in my head).  The difference is 2000 ft/min, giving an engine power (no, let's call it "thrust horsepower") of 109 hp.  The propeller efficiency might be 80 percent, so the crank power would be 136 hp (109 divided by 0.8).  Let's say you did the test at about 8,000 feet - as an approximation, the correction to sea level power is 133%, so the corrected sea level hp is then 180 hp.  Yes, there are too many corrections and approximations to make it very accurate, but it can give you a rough idea of the actual power of your engine.  Somebody out there should give it a try and report the results.

Gary

Sure you can.

Wt x V / 33000 = HP

Wt = weight in Lbs

V = climb rate in Ft/Min

33000 = Ft-Lbs/Min per HP

1800 x 1000 / 33000 = 54.5 HP

Keep in mind that's the NET HP . . . after engine efficiency, propeller efficiency and drag.

Wolfgang