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Typical compression ration is approximately 8:1, but this varies. The fuel requirement for the engine is spelled out in the type certificate data sheet. Some smaller lower compresion engines have supplemental type certificates that allow the use of lower octate, specifically the use of auto fuel.
The 0-360 is 8:1, the 0-540 is 8.7:1. The turbocharged version is 7.3:1. The smaller engines, such as the 0-235, are lower in compression ration, that engine being 6.75:1.
Remember that the requirement for antiknock additives isn't just a function of compression ratio.
For many years, 80/87 was available, but the demand for avgas is minute compared to other fuels. The profitability of fuels other than 100LL right now is zippo, hence the availability is also zippo...the availability of 100LL isn't far behind.
Thus, the use of 100LL in your aircraft engine is driven economically, and is also a function of legality...as spelled out in the TCDS.
Of course, there's always the issue of the use of auto fuel vs. avgas...no true uniform standard exists for the manufacture of fuel that ensures all fuels are produced the same...wide variances exist in fuel batches, and auto fuel is far less stable. Avgas is a higher grade fuel, not only in terms of antiknock characteristics and additives, but in refinement and quality. It's not merely an issue of "octane."
Most engines don't require it. Its that its the only avgas readily available. Thats one reason that the O-320 thats in the Cherokee 140 is eligable for an auto gas STC. I'm not condoning the use of auto fuel in aircraft, but it is an option for the aircraft owner.