Avgas quality concerns ... helicopters

[rutan around]

Bendy asks:-

Just state at what part of the cycle (rough degrees 0-360) that this comment of yours is possible.

My answer is "I don't know" However if you supply all the factors listed below and shout me a weekend in the USA with George Braly I'll have a stab.
I will guess it wouldn't take too many degrees of advance from the optimum setting to cause trouble because pressure and heat have a big effect on flame speed. It becomes a vicious circle. The spark is early so the fire starts early and the pressure builds sooner than it should. The higher pressure speeds up the flame front and so peak pressure occurs many degrees before the desirable 10* to 15* after top dead centre.

Krishnamani Selvaraj, M.E Internal Combustion Engines, College of Engineering Guindy, Tamil Nadu, India (2007
The combustion characteristics of an I.C engine(petrol and diesel) significantly depends on the ignition delay. In the petrol engine ignition delay or lag is the time between initiation of spark and the time of ignition. Whereas in the diesel engine, it is the time between start of fuel injection and the time of ignition. Since the combustion is heterogeneous in the diesel engine, the delay period is long as compared to the petrol engine.The delay period is the main factor influences the flame speed in the I.C. engine. some of the factors determine the flame speed are:

1. compression ratio 2. Engine speed 3. Engine load 4. Spark timing in petrol engine and Fuel injection timing in diesel engine. 5. Fuel injection pressure in diesel engine 6. properties of fuel like viscosity, volatility, octane number for petrol ,cetane number for diesel and surface tension 7. type of combustion chamber in the engine. 8. properties of coolant. The engine knocking mainly occurs due the presence of higher flame speed. The delay period should be as long as possible for petrol engine for smooth engine operation and for diesel engine the delay period should be as short as possible.