Re: timing? 230? plug gap? electronic ign.

Posted by Bob M on January 02, 2007 at 10:44:16 from (151.190.254.108):

In Reply to: Re: timing? 230? plug gap? electronic ign. posted by John T on January 02, 2007 at 08:15:14:

Hey John T - In case you haven't already figured, I kinda enjoy "tricity" discussions too...(!)

Anyway here’s how the effect of spark gap length was explained to me by an AC Delco engineer back when I worked at GM - back when HEI (electronic) ignition were just beginning to appear on cars:

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First, the longer the spark the more of the fuel-air mixture the spark travels through. Also the arc is hottest at the middle of the arc, since quenching from the (relatively) cold plug electrode surfaces reduce the arc temperature near the electrodes. Thus a longer spark results in more consistent ignition of the mixture and therefore improved combustion.

Note: We demonstrated this on a Chrysler 225 slant six (with a stock mechanical ignition) on a dyno in college. We discovered increasing the plug gap from the spec’d 0.030 to 0.045 would cause a slight but very definite (several %) decrease in specific fuel consumption. Then reducing the plug gap back to 0.030 made fuel consumption go back up.

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Second, an electronic ignition coil might be considered essentially a “constant current” source. And the longer the plug gap, the higher the voltage required to initiate the spark arc. And the energy (heat) dissipated by the arc = current x voltage. Therefore within the design limits of the coil system, spark energy must increase with increasing arc (plug gap) distance. And the greater the spark energy, the more reliably the fuel-air mixture will be ignited.

(I’m sure the above is an oversimplification – and it may be all bunk. But that’s how it was explained way back when….)

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This being said there’s tradeoffs involved. Large plug gaps/higher spark voltage results in an increased sensitivity to problems from moisture, bad plug wire insulation, dust/dirt accumulating on spark plug porcelains, etc. Also higher voltages cause corona problems, can generate massive RFI, etc.

If you are inclined feel free to email me offline - so we don't continue to bore everyone else over here!

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