text: Obviously, spark ignition timing is an impor- 
  tant factor controlling the initial 
  rise of pres- 
  sure in the combustion chamber. The ignition 
  of the fuel mixture must begin at the proper 
  time to allow flame front propagation and the 
  release of heat to build up peak pressure for the 
  power stroke . 
  
  The speed of flame front propagation is a 
  major factor affecting the power output of the 
  reciprocating engine since this factor controls 
  the rate of heat release and rate of pressure rise 
  in the combustion chamber. For this reason, 
  dual ignition 
  is necessary for powerplants of 
  high specific power output. Obviously, nor- 
  mal combustion can be accomplished more 
  rapidly with 
  flame 
  fronts rather than one. The two sources of 
  ignition are able to accomplish the combus- 
  tion heat release and pressure rise in a shorter 
  period of time. Fuel-air ratio is another factor 
  affecting the flame propagation speed in the 
  combustion chamber. The maximum 
  flame 
  propagation speed occurs near a fuel-air ratio 
  of 0.08 and, thus, maximum power output for 
  a given airflow will tend to occur at this value 
  rather than the stoichiometric value. 