[Mgs] spark plug gap

[PaulHunt73]

Ignition systems have for decades been more than capable of generating a spark that will jump gaps much bigger than the plugs we use, up to 1/4" in my tests with the system as used in the MGB and cars of that era.  However that generates very high voltages which can break down ignition components like cap, rotor, lead insulation and even the coil.  It's the size of the plug gap plus that between rotor and cap contacts which determines the peak voltage in the ignition system, and it's the requirement for that peak voltage not to exceed a certain amount that leads to the manufacturer specifications for plug gap.

When an ignition coil generates a spark it generates a certain amount of energy which is dependant upon the design of the system, and modern electronic ignition systems generate a lot more energy than ours.  If a lead is removed from a plug with the engine running this will generate voltages much higher than ours - so much so that they come with health warnings, but the plug gap has typically only increased by a few thousandths of an inch.  

When an ignition system fires the voltages rises over time - a very short time.  When it reaches the breakdown voltage of all the air-gaps in the system i.e. plug and cap to rotor (where applicable) the plug fires and current starts to flow.  At that point the voltage doesn't rise any further, the energy starts to dissipate as current, and the higher the energy produced by the system the longer the current will flow.  It's the duration of the spark that is more relevant to modern high-energy systems than the length of the spark i.e. plug gap.

PaulH.
  ----- Original Message ----- 


  I always thought that the larger gap gave a bigger spark and was better, as long as the system generated a spark capable of jumping that far.  The gap was a compromise, big enough, but not so big that you end up with no spark.  Modern cars are more capable of reliably handling the larger gap.
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