I know we've got petroleum industry guys who are going to cringe at this explanation, but I ask that you allow me to greatly simplify for those who may not understand anything about octane ratings.
Greatly simplified, octane is the resistance of a fuel to pre-ignite (knock). Pre-ignition is really, really bad. as an aside, just because you can't hear the knock from the cabin, with your middle aged ears, in the wind doesn't mean it isn't there. If you think it might be knocking, it probably is.
Again, greatly simplified-- greater heat-soak in the engine has the effect of increasing the tendency of the fuel to pre-ignite. Advancing spark can help an engine run better (to a point), but it also increases the tendency to pre-ignition. Increased dynamic compression ratio gives more power, but it also increased heat (and, of course, pressure), and can cause a fuel to pre-ignite.
Modern engines have knock sensors that retard the timing when "knock" is present, so fuel quality is something we've all become accustomed to take for granted. There is a great difference between 87 and 93 octane.
My thinking is: I want to run optimal timing, A/F mixture, etc.- but the limitation is always going to be heat and the tendency of the fuel to pre-ignite. I want to start with the things that are not changeable and begin tuning from that point. We have readily available 93 octane (with methanol as an oxygenator) everywhere in the midwest. I tuned one year for non-alcohol fuel, but it seemed stupid to me to NEED to run gas I could only buy at one station 15 mile from my front door.
Putting the best readily available fuel you can find in the tank costs about .25/gal more than 87 octane, which means a fill-up is going to cost about 3 bucks more. Even a midwestern tight-wad like me can live with that. I only drive the thing 3000 miles in a good year.
If I start with 93 octane, I can run optimal timing and optimal A/F ratios in my 10:1 engine up to about 90 deg or so. After that, I need to either back off the timing, or fatten up the mixture. Next tear-down, I'm going to dish the pistons to drop the C/R to about 9:1, which would allow me to do anything I wanted.
I'm running a DTM and nicasil cylinders, or I'd need to back that number off a full point.
If I remember correctly, you are running 8:1 or so in that engine. You should be able to do whatever you want with the timing and mixture (and shroud) if you run better fuel.
I'll bet you could get near 30 mpg out of it. I wouldn't let the nay-sayers pick apart what you are doing. I think you are going to love everything about your big, lightly stressed Type 4 just the way it sits.
