I like to use fuel flow to determine HP. It doesn't matter if the prop is fixed pitch or constant speed.
The BSFC of any Lycoming is about .435 pounds of fuel burn per hour per HP.
So if the engine is burning 8 gph figure 6 pounds per gallon which is 48 pounds of fuel burned per hour.
In this case divide 48 pounds by .435 and you are producing about 110 HP.
The number probably is not perfect but it sure is in the ball park. Do the math and make a chart.
6 gallons per hour = 82.8 HP
12 gallons per hour = 165.6 HP
14 gallons per hour = 193.1 HP
This assumes proper leaning which of course you should not do at more than 75% power which will result in a much higher burn rate at take off and initial climb power than the formula indicates.
I have the formula set up on a spread sheet and all I do is plug in the burn and up comes the HP. The percent of HP can also be calculated at the same time if that is important. With 180 Hp engine, 9.8 gph is about 75%.
