Bob:

There are some really neat little tricks--rules of thumb, so to speak, about how to compute HP when ROP and LOP. First, it is important to understand that when ROP, the only variables in HP output are those which control mass air flow. Mass airflow is controlled only by MP and RPM. By definittion there is more fuel than can be burned, so the FF is NOT an issue in these calculations.

OTOH, when LOP, HP is controlled ONLY by fuel flow. There is extra, unused air, so ONLY FF is an issue. RPM and MP are NOT an issue.

For ROP HP calculations:

For every inch of MP below max, subtract 3.5%. For every 100 rpm below max, subtract 2.5%. Therefore:

Max MP - Existing MP x 3.5 = %HP reduction.
Max rpm/100 - existing rpm/100 x 2.5 = %HP reduction

Example on a turbo'd IO-520: (I'd use a 360 in the example, but I don't have the numbers handy or memorized)
at 2500/24.

29-24 x 3.5 = 17.5%
2700-2500 x 2.5 = 5%
Hence the power reduction is 17.5% (TCM manual says 78%--pretty close for a rule of thumb.)

On a 285 HP engine that relates to 221 HP.

Now, LOP:

for each engine there is a constant calculated by using the BSFC for that engine. No need to do the whole MATH ALERT thing... Most all TCM engines are in the .38 BSFC range, so we'll use that.
On the turbo'd engines, the multiplier is 13.75.

13.75 x FF = HP

Let's say we are burning 16 gph.

13.75 x 16 = 221 HP (rounded up) on the same 285 HP engine that would be the same 77.5%

Shameless advertising follows! <g>

We are offering an intense and facinating two-day engine management course which discusses these concepts as well as other fascinating topics and includes an extensive session in the most advanced engine test stand in the world. We use data from TCM, Lycoming, P&W and Wright as well as from the engine test stand.

The purposes of the Seminars are to teach pilots, based on hard engineering data, how to get their engines to TBO with the lowest possible maint. costs along the way. That can be done ROP or LOP if good, sound principles are followed. It is clear that 50dF ROP may well be the absolute WORST place to run an internal combustion engine of ANY kind... aircraft engines included. We can PROVE that there is good, sound engineering data supporting that statement--from the engine manufacturers.

Our web site will be up in a few days <www.advancedpilot.com> or anyone interested in this can contact me at my e-mail, ace@walteratkinson.com. I will be at OSH and will be camping around row 14 in the North 40 at my Twin Beech and can be located at the JPI and GAMI booths. My cell number is (225) 939-7508.

BTW, we have numerous quotes from Cessna POHs and TCM and Lycoming Operating manuals which clearly allow LOP operation. We have hard data which is quite convincing that if you choose to run ROP, you should be running MUCH richer than 50dF ROP at any power setting above 65%.

I don't check in here too often, so it is best to get me by e-mail.

You're all invited to join us.

Walter Atkinson
Advanced Pilot Seminars