The problem with our engines is the wide variation in fuels we can buy, meaning nitro content. Methanol by itself can withstand very high compression before it detonates and high compression makes for efficient combustion. Nitro will only tolerate a much lower compression before it detonates. The problem then comes when you mix various percentages of nitro with the methanol. The more nitro you have the lower the compression before it detonates.

Most engines have a compression ratio that's optimised for a fuel with something like 15-20% nitro so if you want to use more nitro then the compression has to be reduced by adding shims. Conversely, if you want to get the best out of the engine by using less or zero nitro you have to remove shims or even machine the head to raise the compression enough.

But there's a complication here. Raising compression also advances the ignition timing which in itself can lead to pre-ignition (and of course, lowering compression retards ignition timing). Fortunately for us, fine adjustment to ignition timing can be made by using different heat range plugs. A hot plug advances timing and a cold plug retards it.

The only way to know what's best for any engine is trial and error (and a good tach ). Decide what fuel you want to use and check max revs before doing any mods then add a shim and check again. If the revs go down then start removing shims. If you want to leave the shims as is then try different amounts of nitro until you find the mix giving the highest revs and then play with plugs. Going to a cooler plug may then allow you to use slightly more nitro.

What you're then doing is optimising the fuel, compression and plug for that engine and no other but by then it'll be so critical that even changing the brand of prop (let alone size) will need at least a change in plug used.

As a guide though, in another thread you mentioned the Enya 60X. I run mine on zero nitro but have raised the compression to 13.5:1 which gave a 19% increase in HP although I haven't bothered experimenting with plugs.