In a nutshell, Mogas which is otherwise known as motor gasoline, is used by ground vehicles, while Avgas is specifically developed for aircraft use.
Part of the process of supplying a dedicated aviation fuel involves strict specification and quality control measures. These have been developed by the Aviation Industry for Avgas to ensure that it is fit for flight operations and reaches the aircraft in good condition. For example, these measures include filtration standards for removing dirt and free water which could block filters, cause corrosion or impact engine performance.
And, while additives may be used to enhance both Avgas and Mogas performance, the aviation industry has stringent procedures for approval of additives with a focus on aircraft engine and fuel system reliability. Only a limited number of chemical types are approved and carefully regulated. Approval procedures for Mogas are very different and its performance in an aviation application is often untested and unknown.
In addition, Mogas regulations may also permit the use of oxygenated fuel components which can be a potential hazard to aviation. A common oxygenate, ethanol, can cause damage to fuel system hardware and has a low energy content which can reduce aircraft range.
Other factors to consider include the fuel octane quality – if the fuel is of poor combustion quality it may start to explode uncontrollably, which could damage the engine. Avgas ‘Motor Octane’ quality is 91 and 99.6 minimum for Grades UL91 and 100LL respectively, significantly higher than Mogas which is typically 85 minimum (EN228). There’s also fuel volatility and carburettor icing, which can affect engines operating on both Avgas and Mogas, but which are more tightly controlled for Avgas based on aviation experience.
The bottom line comes down to flight safety and performance; it is vital to follow the manufacturer’s fuel recommendations for your aircraft. If Avgas is your recommended fuel, make sure it’s Avgas in your tank.