Sorry to say but I entirely disagree with advice to run 8% oil fuel in small engines. We're not talking about car engines with low shaft loads here - I doubt you will find anyone willing to risk a 1/2A aircraft engine on 8% oil fuel. Since when are 1/2A's tough to needle because of sufficient
oil content? Too low or high nitro content, yes. Over compressed, yes. Wrong or bad plug, yes. Maybe if one got silly and ran 40% oil, but I think viscous effects might prevail. 18-23% oil in 1/2A's has never been the source of needling problems to my knowledge.
I've observed that some folks think that adding oil to the fuel somehow reduces the amount of energy developed per revolution in direct proportion to the percentage change. This is not true - the intake volume of the engine is predominantly occupied by air, with a very small volume per stroke occupied by atomized fuel - which is still in liquid form. Compare the density of gaseous air to liquid fuel for an idea of the proportional effect of adding oil. In other words, increasing the oil content of the fuel by 5% does not reduce power output by 5%. Open the needle to admit the same amount of methanol and nitromethane, and you get it nearly all back. A small amount of intake volume is stolen by the additional oil content, but it is a small fraction of the oil percentage change in the fuel.
For giggles I attempted to work out the volumetric ratio of fuel to air in a fuel blend of 20% castor, 25% nitromethane. Please check my math if you wish, age takes it's toll.
Stoichiometric air:fuel ratio 6:1
s.g. = 1.137g/cc
Stoichiometric air:fuel ratio 1.7:1
Castor oil: s.g. = 0.956 g/cc
Stoichiometry N/A as we shouldn't be burning the oil
The stoichiometry of this fuel blend works out to about 3.73 fuel:air by weight
Specific gravity of fuel blend = 0.91 g/cc
Specific gravity of air = 1.204 kg/m^3 = 0.001204 g/cc
Therefore the volumetric ratio of liquid fuel to gaseous air is 1:2,815
That means that roughly for every percentage increase on oil content, you lose 1/2815% of potential power output.
If you translate this to a 0.8cc engine running at 16k rpm with 85% volumetric efficiency and running at optimum fuel/air and no wasted fuel:
0.8cc x 16,000rpm x 0.9 = 10,880cc/minute of mixture
Of those 10,880cc, 1/2816 parts are fuel, or 3.86cc/minute.
A Babe Bee has a 5cc tank, so this translates to 5/3.86 minutes running time = 1m 18s running time. While this is not bang on, it is in the ballpark so the numbers can't be all that far off. Can someone remind me how long a Babe Bee on a 6-3 runs on 25% nitro fuel? I think it is a bit longer than that, maybe 1:30, 1:40?
Long and short is, how much does oil percentage change affect the power output due to fuel dilution? Nearly bugger all.