The R-985 engine will run rather well on true 91 to 93 octane unleaded gasoline. The problem is more of a quality control issue than it is the engine’s ability or inability to effectively burn unleaded automotive fuel. It is true that the engine was designed and tested using leaded gasoline of various octane ratings.
However, tetraethyl lead is principally an octane booster. 93 octane rated unleaded car gas does not perform much different than the 80/87 octane aviation fuels of yesterday. Aviation fuels have much more quality control applied to the production, storage and delivery than automotive gasoline. Your supplier of automotive fuels is not nearly so restricted (beyond hazardous materials limitations) on the methods of transporting, storage, and delivery of gasoline as is his aviation fuel supplier counterpart! I am alluding to differences all the way from refining to the fill spout on your aircraft. Aviation fuels cannot legally be transported in pipelines as that can result in contamination of the fuel by the “plugs” used to separate bulk purchases of different types of product transported in the line. The lack of “quality control” on automotive results in a much wider range of effective anti-knock ratings in that fuel, an unacceptable circumstance when it comes to aircraft fuel!
I do not think the aviation industry will have much difficulty utilizing the unleaded gasoline that is coming to be in aviation’s future as long as the quality control that exists today remains in effect. Unscrupulous fuel suppliers attempting to pass unleaded car gas off as aviation fuel may cause some problems but the fuel itself (91 to 93 octane unleaded aviation fuel) will work out. The most likely problems will surface in supercharged, high altitude operation. Some adjustments will likely have to be made to ignition timing (possibly retarded to some degree) or possibly utilizing some ceramic coatings on combustion chamber or exhaust components. The valve seats used in today’s aircraft engines are already harder and of better quality than those used in automotive engines. Water cooled engines as a rule does not face the same extremes in temperature variations that air-cooled aircraft engines do.
I do not recommend utilizing automotive gasoline in any aircraft due to the lack of quality control in its production, handling and storage. I do not believe it is possible to provide this Quality Control at the user end and I do not believe any supplier would be willing to cooperate with anyone on the user end to do so.
This prospect of an aircraft engine being designed to burn automotive gasoline may become reality with the advent of computer ignition and fuel injection similar to the automotive engine of today. That will indeed be a certification hurtle!