Fuel mix for my 69 Camaro L78

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When I ran my car at SCR7 its was running speeds of 98 and 99 mph all day.I was using 93 octane unleaded pump gas My tank was down to about 1/8 of a tank and Greg who had the orange clone copo Chevelle there had some aviation fuel, I added 5 gals and made another run and I lost 31/2 mph. The burn rate must have been way to slow so I guess more octane isnt always the case.
This brings up a point I have been wondering about I have a L78 Chevelle a L78 Camar and a L72 Camaro I have a hard time to get any one of them to spark knock set at 8 degrees with the stoick distributors. They also seem to run better at 12-14 degrees. does this new fuel even burn slower than the old leaded fuel? I even went out and bought a new timing light thinking my old one was bad. Anybody else notice this.