Engine Parameter              Volumetric Efficiency    Combustion Time                Ignition Timing Requirement
                              (% filling of cyl with   (result of flame speed)
                              fuel/air from carb)

Engine RPM                    highest at same RPM      Reduced at torque peak RPM     Less advance after 45% of peak RPM
                              as torque peak                                          (due to increased turbulence and fuel
                                                                                      atomization, higher head/piston temps,
                                                                                      temps, higher compression when "on the

Higher compression ratio      Minimal effect           Reduced                        Less advance

Increased fuel octane         No direct effect         Increased.                     More advance
                                                       less likely to reach
                                                       knock limit

Air/fuel ratio                No direct effect         Less near ideal                More advance required for rich mixtures
                                                       fuel/air ratio              

Improved fuel atomization     Minimal effect           Reduced                        Less advance

Increased air temperature     Lower                    Reduced; may reach knock       Less advance
                                                       limit where end gases ignite

Increased humidity            Slight reduction         Increased                      More advance
                              as water displaces air

Increased cyl head temp       Minimal effect           Reduced; may reach knock       Less advance
                                                       limit where end gases ignite

Combustion chamber design     Minimal effect           Reduced                        Less advance for efficient combustion
w/high squish and swirl                                                               chamber designs