Using a lower compression ratio, with higher nitro fuel has a double negative effect, but also a positive effect.
As a rule of thermodynamics, the higher the compression ratio, the higher the power and the higher the efficiency.
This would mean you must always run the highest possible compression ratio your fuel will tolerate.
In full size engine terms, Diesels have an efficiency level of about 40% and gasoline engines about 32%, because Diesels use compression ratios of around 20:1 and gasoline engines about 9.5:1.
A high compression model engine, designed to run on methanol/oil mix, with no nitro at all, may develop as much power as a lower compression engine, run on methanol/oil/nitro mix. This because it is not only the BTUs the fuel has, but also the ability of the engine to utilize all those BTUs, converting them into horsepower.
High compression engines do it better.
Also, higher nitro fuel does not only cost more. Because of its significantly lower stoichiometric ratio, you must use a lot more fuel when it is a nitro mix, than when it is not. The main needle must be open 1/2-1 more turn, when using 15% nitro, than when using 0%.
But there is also a down side, a definite one.
High compression engines are much harder to adjust properly, on methanol, then lower compression engines are, on methanol/nitro.
Some of us are unusually spoiled while others can tolerate a little more finicky engine.
In Europe and other places, most people fly their planes on 0-5% nitro.
In the North American continent live the 'spoiled brats' (
) that need the easy adjustments possible with 15% nitro.
While MVVS engines were originally designed for 0-5% nitro, adding ONE head shim allows this engine too to be run very well on 15% nitro, with no ill effects (besides higher fuel costs). MVVS acknowledged this just lately.