Another question. How do other aircraft engines that allow both types of fuel handle the issue? Im assuming the engine manufacturers have similar or dare I say it, better knowledge about these problems?
Generally the engines approved for lower octane fuels have relatively low compression and were originally designed for low octane fuel. Apart from lead polution higher octane 100LL does no harm as the slightly slower combustion event finishes a few degrees later safely after top dead centre. Peak pressure is well after TDC making it lower and easy on the valves and cylinder heads. As peak pressure occurs in a good place mechanically very little power is lost.
When lower octane UL fuel is used in engines designed for high octane fuel bad things happen.The spark occurs where the factory set it. The time from spark to completed combustion is shorter with UL so peak pressure occurs earlier in the cycle sometimes before the piston reaches TDC. This is very hard on the engine and it's a credit they hang together as well as they do.
One way to help the PP to occur later is to use higher revs. Now this is all very well for Jabba with his fire breathing monster engine fitted with a constant speed prop but for us mere mortals the only option is to lower the nose. It's unfortunate that engines fitted with fixed pitch props reduce revs just when they are working hardest ie in climb.The cooling air decreases and the PP occurs in the worst place creating more heat. The combination creates excellent conditions for detonation but by using low octane fuel detonation is not very well resisted and can quickly get out of hand.
Automobile engines get round the problem by having knock sensors which retard the spark in the above scenario.
Don't ask me past this point as the rest of the seminar was after the long liquid lunch and Jabba's dulcet tones put me to sleep.
Cheers RA