The engineers at Coates International lament the days before unleaded fuel turned high-compression engines into gas-fired barbecue grills. Now that 10.0:1 is on the high side of acceptable, the boys put their pocket protectors together and came up with a new engine design that eliminates the poppet valve, replacing it with a spherical rotary valve. According to the Coats site, "The valves rotate away from the combustion chamber and are vented and charged on the opposite side of each sphere; this creates a lower combustion chamber temperature, allowing for higher compression ratios to be used, thus creating an extremely efficient engine. Some of the Coates Spherical Rotary Combustion Engines are at 12 to 1, 13 to 1, 14 to 1 and 15 to 1 compression ratios depending on the application." If you want us, we'll be at the library.
The Coates Spherical Rotary Valve Engine [Coates International]
Air-Hybrid Engine Developer Taking it on the Run - to Stuttgart [internal]