<<OK, OK, I withdraw the formula---

Scott Krueger>>

Hey, at least you can't say no one listens, er, reads.

I find it handy to find the "squared" setting that gives 75% power as that
is the setting at which the POH usually allows leaning (I know, LOP and all
that).  For my old TR182 it was something like 23 inches MAP (Manifold
Absolute Pressure), 2300 rpm.  As per Scott's rule of thumb I could then
trade an inch of MAP for a hundred rpm.  In other words 25/2100, 24/2200 and
22/2400 all produced approximately 75% power, at least close enough.  That
way I didn't have to keep looking up actual numbers.  Looking in more detail
at the POH it was obvious that there was more to the numbers in the book
than that as they did take into account the reduced back pressure and lower
ambient temperatures at altitude.  Yes, horsepower since the very first
surrogate horse has been standardized at 33,000 ft-lb/min, converted to
rotational terms as lb-ft-rpm/5252.  I think it was Watt that first measured
the output of a standard horse.

As for different ratings SAE publishes two different test methods, "gross"
and "net."  Gross power is obtained with essentially no accessories, unless
electrical power is required to run the engine and then an alternator (or
equivalent load) is required.  There is no air filter, exhaust system, etc.
required.  No cooling power is required.  The mixture must be set to
"reflect intended in-service operation."  Best power?  Full rich?  Maybe
subject to interpretation.  The readings are corrected to 100 kPa (14.5
psia, 29.52 in.Hg.) and 99 kPa dry air pressure.  Inlet air temperature is
25C (59F).

For SAE net power the conditions are different.  The atmospheric conditions
are the same, but the complete inlet and exhaust system are required.
Automotive OEM's used to advertise the gross reading, but after the first
"fuel crisis" decided it was more responsible to advertise the net rating.
The GM engine test code as listed a number of other tests besides the two
basic SAE standards.  Since both Lycoming and Continental sell engines
without the air filter and exhaust systems one could assume they are using
the SAE "gross" test standard, although I've not read that in any of their
literature.  I think at least Lycoming claims that their engines will
produce at least the rated power; the average engine must then be able to
produce a little more than the rating.

Gary Casey