Couldn't agree more.Short answer: The same as any other 4 stroke in the same airframe at the same speed, ignoring differences in cooling drag if going from air cooled to water cooled.
The practical, real world answer is that they are 4 stroke, internal combustion piston engines. For all practical purposes, 4 stroke IC engines will burn somewhere between 0.4 & 0.5 pounds of gasoline, per horsepower, per hour. Perfectly tuned water cooled engines under low to moderate load will be at the low end, and closer to 0.5 at full power. Air cooled engines under moderate to moderately high load *might* get close to the low end, if they're running optimized mixture and electronic ignition which can advance/retard timing automatically. Air cooled engines running at near full power typically consume at 0.55-0.6, because they use excess fuel to cool the exhaust valve.
Above is driven by the basic physics & poor thermal efficiency of IC engines.
To do the math:
For easy round numbers & typical midpoint efficiency, let's assume 100 HP at cruise.
100*0.45=45 pounds of gas per hour
Gas weighs ~6 lbs per gallon, so,
45/6=7.5 gallons per hour.
If you're burning less fuel than that, you're almost certainly making less power than 100 HP.
If you're burning more fuel than that, you *might* be making more power, but only if mixture and ignition timing are optimized.
Claims that exceed those parameters are almost without exception 'fish stories'. Example: Cessna 172 drivers are fond of claiming that their 160 HP Skyhawk 'burns 6.5 gallons per hour at 75% power.' It doesn't. If they looked at the operational manual, or Lycoming's operator's manual, they'd see that it can't. What's happening is that they're running at well under 75% power, because pushing the throttle open any further at cruise doesn't net any speed increase in a C172 (airframe drag). 75% power in that engine will be right at 9 gallons per hour; slightly lower if fuel injected with electronic ignition.
Or I could be wrong.
I'm in the process of reaching out to Teal Jenkins to get some good data of these engines; I know he has mentioned 10k rpm at full power, but saw this on a Fakebook post the other day (I posted this in another thread):If the interwebs tell the truth, the Apex has a 2.29" stroke. The source for that info failed to disclose rpm, but IIRC, the output shaft is 7500 rpm with ~1.2-1 reduction within the case, meaning about 9,000 rpm @ full power. So, doing the math, that's 3435'/min. A Lyc 360 has a 4.375" stroke, so doing the math at 2700 rpm, 1968'/min. But...
Most Lycs 'live' at around 2400 rpm (conservative operators), so, 1750'/min
Where will the Yamaha live? I don't know, but I'll bet they live below 6000 rpm; once the typical STOL plane gets off the ground good, it's aero drag means that power has little affect on speed. At 6000 rpm, 2290'/min. Not that far from your number. (A clean, fast airframe might well result in different numbers.)
Question: I don't know enough about the BSFC/speed relationship. Is it driven purely by piston speed, or does stroke length figure in the equation? Is it a friction thing, or a combustion time vs stroke thing, or...?
Same data appeared in another thread. No MAP, leaning, or rpm included, so no way to even wildly estimate power from engine info. What would the same airframe do in the same conditions with a different engine?