Some actual testing that has been done on fuel has shown the alcohol content to usually be closer to 5% than 10%. The octain will still be 93 no mater 100% gas or a mix. Maybe the shell fuel was just better than the pump with 100% no name gas!!
Our 'gasoline' is a mix of refined petroleum and other related fuels even when there is no eth added. Agree the Shell fuel might be better due to any number of things. Agree that the alcohol content is an unknown. Disagree that the R+M/2 octane will be the same when eth is added to the mix. Eth can raise the octane although obviously there are other ways to do it. When I was upstate NY all of the stations with an eth free option - its 91. Had to get premium with eth to get the 93. My undertanding is that the eth is mixed in close to the final distribution point. And I suspect most of the eth free is a result of not mixing it in.
Because of its vapor pressure and boiling point, I suspect that ethanol mostly effects the 'front end' of the gasoline and the Reid Vapor Pressure. Fuel companies are allowed to supply higher RVP gasoline in the winter months. Have that winter fuel in a carb on a hot day and its miserable because the easy to light portion is evaporating before it gets to the cylinders. I've got a datalog showing that.
The Shell fuel also might have an overall better fuel curve than the other gasoline for our engines. Most pump fuels today have components that require a lot more heat to ignite than the fuel of 1970 (or race fuel and av gas). These effect how the burn completes.
