I don't believe it Marc. 75% of 210HP is  157.5 HP at 8.5 gal per
hour or 51.85 pounds per hour is a BSFC of 0.329.   You must be joking.
Which liar/incompetent ran the dyno? Who needs diesel engines?

Paul,

I doubt very much that Marc is running at 2700 at altitude(note that altitude was undefined), which he would need to get 75% of 210HP.  RPM decay needs to be taken into account (note that at low altitudes, you could get 75% at WOT if you had an aggressive propeller that slowed RPM).  In addition, he is probably giving fuel specs for an altitude where 75% is not achievable (i.e., 9-12K).

It is easy to get a relatively accurate fuel flow at altitude, but it is very difficult to obtain a horsepower value.  As we've seen, taking a swag at horsepower can lead to wildly unrealistic BSFCs.  That's why the only BSFCs I really count on are dyno-generated where both fuel flow and shaft horsepower are measured directly.  I think there are two reasonable methods for determining (guessing?) horsepower at altitude:

1.  Calculating the power required by looking at airplane drag for a certain speed.  This is the method you've been using.  The two variables you have to estimate are airplane drag and propeller efficiency.  I noticed that you and Bill came up with very different BSFC numbers for Herb's parameters based only upon a difference in the airplane drag assumption.  I would be uncomfortable relying on "guesstimates" of drag and prop efficiency.

We are still working on the right drag number. Neither Bill nor I have that nailed
down yet. We await Herb's top speed and altitude.

2.  Another method is to use the "Altitude Performance" charts provided by the engine manufacturers.  The charts I have (from Lycoming) are for best power mixture, so an adjustment (which I would base on the dynanometer power and BSFC curves) would have to be made for running at mixtures other than best power.  The inputs to the charts I have are:  MAP, RPM, pressure altitude and inlet air temperature.  The main assumption here is that the engine in the plane is performing like the one used for the charts, which is probably a fair assumption.  I would think that this method might yield more consistent resuts than method 1 above.

For 360 series engines, I can give you these readings.  I think I can also swag from these charts for 320 series engines to confirm your earlier calculations.  Note that one important variable that is often not recorded is inlet air temperature.  This can make a big difference in power.
Rick Price

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