FfF,
As you rightly said, the problems of valve seat recession can be overcome with modification & expense. That is the easy bit. There are more, lesser known issues, such as the possibility of increased valve guide wear caused by the removal of TEL (tetra-ethyl lead) compounds from fuel, and the poor compatibility of aviation lubrication oils with MOGAS, resulting in an increase in general wear rates of all the engine parts. Yet another problem is the possible incompatibility of MOGAS and rubber parts such as fuel pump diaphragms and fuel lines etc.
Vapour lock is definitely a problem affecting unleaded fuels. It may or may not be cured by simply changing the fuel pump, it's an individual problem depending on the design of the whole aircraft fuel system. Vapour lock is not caused by changing the octane rating per se, but by the requirement to change the composition of the fuel recipe in order to regain an octane rating without the use of lead compounds. Adding lead was obviouly an easy answer!
Unleaded fuels may also be health damaging, they now contain higher levels of benzene (poisonous/carcinogenic) than leaded fuels. Benzene, a fuel in itself, has a high octane rating so the content in unleaded fuels has been increased. Unfortunately it is also very volatile, giving the increased vapour lock problem. BTW, this is the reason why unleaded fuel goes "off" quicker than leaded fuels. If a tank of MOGAS fuel is left standing for a period of time, the benzene content reduces through evaporation resulting in a reduction in fuel quality. This may be seen as a difficulty in engine starting and octane rating. This has obvious implications for aircraft not flown often, especially if the MOGAS they are filled with has come from a garage with a low turnover of fuel. It may result in pilots complaining of a "bad tank" of fuel.
Some areas of the USA have complex vapour recycling pumps at gas stations to reduce the possibility of human ingestion of benzene vapour. So even sniffing a whiff of this stuff may be dangerous! TEL might be less harmful overall; I don't know the pros and cons but have my suspicions as we don't seem to have been given much information on the long-term health issues, especially regarding Lead Replacement Petrol (LRP) contaning other metal compounds such as potassium.
The one engine problem that cannot be overcome at the moment on existing engines designed for high octane leaded fuels is the loss of performance caused by reduced compression ratios required if using a lower octane rated fuel. Again, it's an individual problem depending on engine design. Some engines may be affected very little but conversely, some GA aircraft were only marginally powered in the first place!
An engine using any form of forced induction actually needs a higher quality fuel because forced induction = higher compression pressures. This is also why intercoolers are fitted to forced induction engines, the charge is already heated by compression and is more likely to suffer from detonation once inside the combustion chamber due to the increased temperatures. Forced induction engines are designed with lower compression ratios (larger combustion chamber volumes) to compensate. They do tend to be less fuel efficient than a conventionally tuned engine producing the same power as the secret to high fuel efficiency is a high CR, like diesel engines...(aha - that's definitely the way to go, so watch this space we'll all want one soon!)
Aircraft engines are not generally highly "tuned" and are generally slow-revving but they DO have large cylinder and combustion chamber volumes which still makes them prone to detonation. What needs to be understood is that fuel air mixture burns very rapidly on a "flame front" out and away from the spark plug/s but it does NOT explode as some people seem to imagine. A piston can only transfer energy to the crankshaft in the form of a steady push, not in the form of a "knock". If the mixture does explode, that is detonation and the piston cannot tranfer this energy to the crankshaft which is why it is so damaging (imagine someone tranferring energy to yourself in the form of a firm push and then the same amount of energy as a kick. The kick hurts!). As combustion takes place on the flame front, the combustion chamber pressure increases very markedly so that unburned mixture may detonate, just like inside a diesel engine. Large cylinders, having more volume, take longer to burn a charge and the proportionate increase in compression chamber pressures is very high compared to smaller cylinders. This is why engines such as those used in F1 cars have very small cylinders, they burn the charge much quicker and there is less chance of detonation taking place, which allows a higher state of tune to be used. TEL works to increase the octane (anti-knock) rating because it forms incombustible dust particles which act as a buffer inside the combustion chamber, preventing detonation.
Go diesel!
ShyT