It's also noteworthy that octane increase actually reduces specific power output, but provides a net power gain due to other modifications like boost increase and etc. The lower the octane, the more explosive the fuel, so the better power. Problem is of course if the octane is too low you get pinging (predetonation). If it is too high for your engine, what happens is you actually lose power.
I am not sure that this is so. Gasoline can be made-up from a combination of several hundred different chemical molecules. It is quite possiable to get fuel with the same octane rating with widely different chemical compostion. THe BTU rating per lb can vary a bit but has little to do with the octane rating. Octane helps mesure the temperature at which a fuel will auto-ignite. THe lower the octane the lower temperature at which it will ignite with no spark or other source of ignition. Aircraft fuel was blended to have a lower volatility than motor gasoline. The higher the volitility the easier (lower pressure and temperature) the fuel will vaporise and form an explosive mixture. In aircraft they were worried about higher rates of evaporation at higher altitudes.
At the point where an octane increase can only provide a couple of pounds extra boost, you're probably losing out by using it .
I don't believe that this is so.
1. In the case of the Mercury it doesn't agree with the published perfomance figures.
2. It doesn't seem to follow logic. The Mercury with 87 octane fuel was limited to about 4 1/2lb of boost for a total manifold pressure of 19.5 lbs. Using 6 1/2 lbs of boost gives a maniflold pressure of 21.5. just over a 10% increase. This means that the engine is burning 10% more fuel in a given time period. Which should lead to 10% more gross HP. THe Idea that the 100 octane fuel is 10% or more less powerful ( or that much lower in btus per pound) doesn't seem to make sense. Becasue the power losses (internal friction, pump drives, supercharger drive power,etc) stay the same the net power increase should be more than 10%.
EDIT>I would imagine that the BTU per pound of fuel was in the fuel specifications. IT certainly was post-war, otherwise you have to reset the carburator/fuel injection for each different batch of fuel. Using a constant BTU per pound requirement for most fuels would make things easier for everbody; engine designers, fuel refiners and suppliers, mechanics, etc.<edit
Not to mention higher octane tends to oil plugs and corrode valves more quickly, which may reduce serviceability, and the modifications associated with higher octane can otherwise dramatically increase the stresses on the engine.
I beleive you ment "foul" the plugs and not "Oil"
The extra stress on an engine doesn't come from the octane rating but from the extra "boost" when used which dramaticly increases the pressures inside the cylinder which results in higher loads on on the pistons, connecting rods, crankshaft, bearings, crankcase, cylinder hold down bolts etc. and the fact that if you are burrning 10% more fuel you are creating 10% more heat which the cooling system (fins or liquid) might not be able to get rid of.
In something as small and light as a Gladiator you would apparently really feel what is probably a marginal increase in engine output for some loss in serviceability, but in this case survivability was much worse if you didn't take some measures.
While a 10% increase in HP probably didn't make much difference in level speed (5% or less) it probably helped the climb performance a bit more.
Also noteworthy is that octane and boost increase tends to lower the critical altitude of a given engine. In the Daimlers you lose up to 1km of critical altitude by going from B4 to C3 with an associate boost increase on what is otherwise a fairly identical engine. I'm not sure if this holds true for radial engines, as far as I know the critical altitude of a B4 running 801 is the same as running on C3, but most of the engine/supercharger settings between these two fuel types didn't change. The greatest reason for the fuel change was probably the hot running of the engine in its tight installation, so bench performance wasn't being achieved on B4. Here is the example that cylinder temperatures also have something to say about suitable octane irrespective of other factors.
Actually when going to the higher octane fuel the original power rating of the engine doesn't change. See the Merlin III in the Spitfire. On 87 octane it gave 1030 HP at 16,250ft using 6lb of boost. The engine was limited by the supercharger. The supercharger could only supply enough air at that altitude to produce 1030 hp no matter what kind of gasoline was in the tank. When using 100 octane fuel the power at 16,250 ft and above was the same as when using 87 octane fuel. At lower altitudes where the supercharger could supply higher boost the power ratings did go up. THe lower the altitude the more air (boost) the supercharger could supply and the more power could be made until the engine reached either it's cooling limit or it's structural strength limit.
Try graphing the power ratings of Diamler engines in question. Plot their power ratings at altitudes with the end point ( 0 HP) at about 55-56,000ft and see if the HP curves either match or are very close to each other.
AIr cooled engines didn' behave like liquied cooled engines for the Allies either
A bit counter intuitively air cooled engines were known as severe duty engines while liquied cooled engines with much higher boost pressures were known as mild engines.