% BHP
The Rule of 48
This is an old rule, I have copied this from the Van's site.
For normally aspirated engines this gives a good approximation of % BHP
For those unfamiliar with the "rule" it goes like this... take your rpm in 100s and your manifold pressure in inches and add them together. A sum of 48 equals 75% power (such as 24" MAP and 2400 rpm), each reduction of 3 is a reduction of 10% power, so 45 would be 65% and 42 would be 55%.
A table listing all the combinations and settings in between that is used in the cockpit is a handy tool for cruise power management.
All that said, does anyone know how accurate this is? Any empirical data to compare with it?
Charlie