Every time I pick up the Sky Ranch Engineering manual  I run across another
     chart pertinent to the subject of the month. This is for a run of the mill
     air cooled aircraft engine.

     Here is the difference in BSFC and HP of an engine running on a dyno
     with full load throughout the RPM range verses an engine driving a fixed
     pitch prop.

     The upper most curve shows a drop off in intake manifold pressure with
     max load (dyno load) due to breathing losses expressed as pressure
     drops.

     Next curve down is prop load affect on the intake manifold pressure.
     Prop load is way less than dyno load up until the two curves come
     together.

     The third curve down is HP verses RPM full load (dyno load). Note the BSFC is
     roughly .65 at 1700 RPM. This is what you might have on climb out
     with a variable pitch prop set to max pitch. In other words you are lugging
     the engine. BSFC reaches a full load minimum of about .51 and then climbs
     back up to about .55.

     Finally the fixed pitch prop loads are shown as the bottom set of curves.
     At moderate cruise RPMs  and HP BSFC is a minimum at about .47.
     Just about what we are seeing with the rotary at full lean.

     What we need and don't have is a BSFC map for the current Mazda rotary
     tuned to our needs. Perhaps with a p-port. Here are some old such maps.

     The final story is MPG. Not shown in these charts. The drag and HP
     requirements drop off drastically with speed so best MPG is not necessarily
     at best BSFC. Fuel consumption in pounds per hour is primarily depended on
     HP needed and generated. BSFC is a secondary factor.

     Paul Lamar

Just thought I'd add a few comments.

1. I agree with everything you have said, and it is exactly the same situation in marine applications.

2. The full load power curve you have shown may be typical of an aviation engine, but it is very different from most other types of engines. Most engines have a power curve that is more convex. That
is, the power vs rpm is increasing more than linearly up to some speed, then starts to increase less than linearly, then finally starts decreasing with speed. Mazda rotary engines tend to have a
more linear curve than most engines, and the power doesn't start to decrease until very high rpm. Some engines reach redline before the point where the power starts to decrease.

3. The reasons you don't see a lot of full range sfc maps for automotive engines are: 

a) the manufacturers have no incentive to publish this --it will only create confusion and raise questions that aren't particularly relevant to selling cars. 

b) most dyno tests that are performed in the aftermarket are aimed at showing full load performance as a way to sell modifications. 

These dyno curves should always be taken with a little grain of salt. A full load dyno curve is often generated in less than a minute. To obtain accurate fuel consumption data at enough points to
generate a decent map takes many hours of dyno time (and many gallons of fuel, too).

Jon Lauter

An interesting aside on air cooled aviation engines. They are not breathing limited right
up to red line. That is why they have a linear relationship between HP generated and RPM.
It is also why you can get enormous increases in HP for a short period of time before they
destroy themselves by installing a shorter prop and over revving the engine. All the air 
racers do it.

The aircraft engine evolved over the last hundred years by becoming very large in
displacement for a given HP generated. This was done so the HP was generated at 2700 engine
RPM limited by the prop and the engine weighed a minimum. Every ounce of unneeded 
weight was removed from these very large displacement engines.

Normally the typical 360 cubic inch 150 HP air craft engine would be generating at least
360 HP at 6000 RPM if it were a light duty car engine. Breathing would be limited
at max RPM. It would also weigh more. If it were a heavy duty gas truck engine it would 
be generating 150 HP at 4500 RPM but it would weigh 1000 pounds.

Paul Lamar
 
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