<VV>spark plug miss

[Jim Burkhard]

I know P knows what he is talking about here, but I think 
the explanation of what causes what might confuse some 
folks. I've seen this frequently when ignition systems are 
discussed.

Higher voltage itself doesn't "allow" a larger gap -- rather 
it is caused by one! (It is a higher voltage rated coil that 
*might* allow a larger gap ... more on this below). If the 
gap is increased, you raise the voltage level at which the 
gap breaks down and the sparkplug fires.  A little gap means 
little voltage is required to jump it. A big gap means big 
voltage is required to jump it.

Generally, increasing the gap improves idle quality because 
the higher voltage and larger spark area improve charge 
dilution tolerance. At idle, especially on an engine with a 
fair amount of cam overlap, you wind up with lots of burned 
gas remaining in the combustion chamber after the exhaust 
event. This is chiefly a result of overlap (both the intake 
and exhaust valves are open simultaneously near TDC). This 
allows burned gas to be retained in the chamber or even 
sucked back into the intake manifold because of the positive 
pressure differential between the exhaust and intake manifolds.

Problems can occur if you make the gap too large, however.
1. You might have some other place in the ignition system 
which will spark to ground at a lower voltage than the plugs 
themselves fire. There are certain limitaions in the 
distributor cap design itself, and you can also run into 
trouble using crappy or deteriorated wires. If the spark 
fires anywhere but at the plug, you will have a misfire.
2.  If you run an overly weak coil, it might simply not be 
capable of generating the increased required voltage. When 
saturated (fully charged) if the secondary voltage is below 
the breakdown voltage, no spark is fired anywhere. That's 
where changing the coil can help.

The actual spark breakdown voltage is determined by the plug 
gap, electrode geometry (pointy v. rounded), cylinder 
pressure (higher loads = higher voltage requirements ... 
ignition can be fine at low load but miss under high load, 
especially on turbos), and charge composition 
(lean/stoich/rich, burned gas dilution, etc.). The key thing 
to take home is that just changing the coil doesn't increase 
the spark voltage. It *may* permit a higher voltage (and 
more ignition energy overall), but everything else in the 
ignition system has to be in good shape or you run into the 
problems I mentioned earlier.

Jim Burkhard




Padgett wrote:
> Higher voltage also allows a larger gap and increased likelihood of 
> firing a less-than-perfect mixture. Is part of the reason for electronic 
> and "High energy" ignitions. (When GM first introduced the HEI, they 
> recommended a .060" gap and everything that could arc- did. They later 
> reduced the recommended gap to .045" and only went back to .060" with 
> the multicoil distributerless ignitions.
> 
> Part of reason for Pertronic Ignitor II was to allow a hotter than stock 
> coil if necessary, may be the next attempt.
> 
> I really like Rapidfire Platinums, have then in all my other cars. Too 
> bad they do not have one for a Corvair.
> 
> Padgett