[Diy_efi] Spark

[Rick McLeod]

I seem to remember (been 35+ years now) in engine
training class that for a plug gapped to .025" in a
low-comp engine (~8.5:1) to simulate/stress/test the
ability of the ignition to light the gap in cylinder a
free-air gap of .166" would be required to be jumped.

I'm sure it changes under higher comp ratios or forced
induction, but this was hammered into my head in
diagnostics class that just because it will jump .025"
on a plug on top of the manifold/head doesn't mean it
will jump that distance under pressure.

I would suspect the free-air test gap might go to as
high as .250" or higher under a force-fed or high-comp
plant.

--- Bill Washington <bill.washington at nec.com.au>
wrote:

> Gents,
> The energy required to make the spark jump the plug
> gap is proportional to the
>   (probably absolute) pressure in the cylinder at
> the time and probably inversely
>   proportional to the degree of ionisation - along
> with other factors....
> 
> IE a 'weak' - (read damaged - shorted turns) coil
> may throw a spark across the 
> plug gap with the plug sitting on top of the engine
> - but screw it in and no 
> spark ..... It happened to me ...... and took quite
> a while (an hour or so) to 
> identify the problem ... I had fuel and "appeared"
> to have spark but the engine 
> would not start! new coil and 'hey presto' immediate
> action!
> 
> Regards
> Bill
> 
> > Message: 2
> > Date: Tue, 4 Oct 2005 12:10:42 -0400
> > From: "John Gross" <jogross3 at hotmail.com>
> > Subject: RE: [Diy_efi] The Hunt effect
> > To: <diy_efi at diy-efi.org>
> > Message-ID:
> <BAY103-DAV14BEF6317CBCCFCE428AD383830 at phx.gbl>
> > Content-Type: text/plain; charset="us-ascii"
> > 
> > It takes more energy to cause the fuel to "flash"
> without a specific
> > ignition point.  The spark from the plug on your
> car is more than enough to
> > start the burn on just about any unleaded fuel.  I
> say unleaded to try to
> > keep the discussion within reason in terms of
> octane ratings and appropriate
> > CRs for the engines.  It takes more spark energy
> to light off a 115 octane
> > fuel than it does an 87.  However, when talking
> about street-driven cars,
> > any delay in the formation of the kernel (the
> initial point of combustion
> > inside the spark plug gap) is so minute, that it
> is not worth considering.
> > Additionally, what makes it harder and harder to
> light the air-fuel mixture
> > isn't just the fuel, but the dynamic compression. 
> That's why blower motors
> > typically need a more robust and powerful ignition
> system..the turbulence
> > inside the cylinder in a blown motor (high dynamic
> compression) can actually
> > extinguish the kernel, thus stopping combustion.  
> > 
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