Fw: RE:What is proper spark plug gap with electronic ignition?

["John Fielding" <johnf@domain.elided>]

----- Original Message -----
From: John Fielding <johnf@domain.elided>
To: Kevin Trent <trentkc@domain.elided>
Sent: Monday, August 12, 2002 9:22 AM
Subject: RE:What is proper spark plug gap with electronic ignition?


> Hi Kevin,
>
 The use of a larger plug gap when using a high energy ignition system is quite common.
To allow a greater spark front the gap can be opened up.  The idea is that with a more
potent  spark the higher voltage available can usefully be used to start the flame front
burning
over a larger area, hence the recent introduction of 3 and 4 point ground electrodes
(Bosch and NGK being two manufacturers using this technique).
>
> A trick which a lot of racers use is to grind away the ground electrode so that it is a
> knife-edge situated above the centre of the fixed electrode.  This provides a more
central
> spark and helps ignite the mixture.  From electrostatics, the use of a sharp point or
edge
> makes the electrons crowd into a smaller area and causes the brakdown voltage to be
lower.
> If you observe any high voltage apparatus you will see that in order to prevent arcing
the
> surfaces are curved or spherical and highly polished.  This discourages the presence of
> high spots and stops the elctron crowding, so providing a high breakdown potential.
> Grinding the plug ground electrode reverses this effect and causes the easier
> establishment of an arc.
>
> How much the spark gap can be widened depends to a great extent on how much voltage the
> coil can develop.  This is often limited to what breaks down first, it could be the
leads
> or other parts of the high voltage circuit limit the potential voltage.  An excessive
plug
> gap can cause the idle or high rpm performance to suffer if the ignition system is
running
> near its limit.  I normally start with 40 thou (1mm) and work back from there.
>
> Running a high CR or boost or very lean or rich mixture can push the required voltage up
> more than the ignition system can safely develop. In my CDI systems this isn't a problem
> as I can adjust the outpout power during manufacture so as to develop a spark energy 5
> times more than a conventional IDI such as the Marelliplex or similar and this holds up
to
> 8000 rpm with a 4 cylinder motor, dropping off gradually from this point as the rpm
> increases.  I have also built even higher energy systems for V8 motors, here the energy
is
> about 15 times a Marelliplex IDI system and can support a speed of 7000 rpm @ 8
cylinders,
> but the spark plug gap wear starts to become excessive with so much arc current flowing.
>
> Hope this helps.
>
> John
> Durban
> South Africa
> Alfeta 1.8L turbo
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