Problems with MOGAS for aviation:
1. Detonation / knocking. Due to the lower octane rating of MOGAS it may be impracical for use in some types of aircraft engine. Engines with a high compression ratio or large cylinder volumes may suffer from explosion, rather than smooth combustion of the fuel/air charge, irrespective of ignition timing adjustments. Piston overheating and severe damage (melted & holed pistons) may result. The debris thus caused may mean scrapping the engine due to bearing wear or worse (if it survives the engine-out landing). Unlike in car engines, knocking/pinging/pinking is difficult to detect in aircraft engines.
2. Vapour lock. MOGAS is more volatile than AVGAS. This means that it is more likely to form vapour in the fuel lines, resulting in engine fuel starvation with the associated rough running or engine failure. That is why the regs allowing use of MOGAS include an altitude limit.
3. Long term damage to valves and/or valve seats on 4-stroke engines. Lead compounds released from the combustion process of leaded fuels (AVGAS) coat the exhaust valves and valve seats. This acts as a protective buffer between them. Older engines relied on this (softer and cheaper materials could be used for valve seats). Use of a fuel without lead additives means that direct metal to metal contact occurs at exhaust gas temperatures, possibly resulting in partial welding together of the crystalline structure between valve and valve seat, only for them to be ripped apart again when the valve is forced off its seat on the next exhaust stroke. This results in valve seat recession, leakage and then burning out of the exhaust valves. Loss of compression and performance may result.
I have never actually seen any documented proof of public health improvements since the wider use of leaded petrol has declined. Bearing in mind the carcinogenic properties of unleaded fuels, I think we may have been duped into swapping one lot of health issues for another. I hope I am wrong because I would dearly like to buy 4 or 5 star petrol again without having to drive 25 miles each way to the pump.
ShyT