Street Gasoline & Low Compression Ratios
Tips To Reduce The Octane

• Build the engine with about 8.5:1 CR (Compression Ratio).  More CR needs more octane.
• Use a four-inch or smaller bore. (Bigger bores require more octane)
• Use aluminum cylinder heads if possible. (Fewer hot spots with better heat transfer)
• Use a 180 degree thermostat, a good water pump, efficient radiator and effective fan system (High temperatures need more octane)
• Use an aluminum intake manifold. (These improve cold start driveability)
• Be sure air / fuel mixture is correct. (Lean mixtures require more octane)
• Cool inlet air is a benefit. Use a fresh air system if possible. (A 25 degree increase in inlet air temperature requires 1 more octane number)
• Live at a higher elevation. This may not be practical, but for every 1000 feet above sea level, your engine needs 1 less octane number, but makes less power.
• Live in a humid region. Again, this may not be practical, but if you live in Florida and the temperature is 85 degrees, your car will be satisfied with a gasoline that is 2, or 3 octane numbers lower than it would if you were in Phoenix at the same 85 degrees temperature. This is why water injection works in the southwest U.S.
• Reducing spark timing by 2 degrees will reduced the engine octane appetitie by 1 octane number. 4 degree is the maximum for this "fix".