Spark Plug
One gear at a time
Joined: Aug 2003
Posts: 5,203
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CI 8-9-10 Veteran
St. Jude Donor '07-'08-'09
Re: Spark Plug (Vetterodder)
My higher compression engines with HEI usually ran best gapped between .040 and .045". With higher com[pression (above 10:1) the spark may not be able to overcome the larger gap, especially at high RPMs.
Race Director
Joined: May 2000
Posts: 12,712
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Re: Spark Plug (wesmigletz)
The first year of the HEI - 1975 - GM speced all engine plug gaps at .060". In 1976 they respeced plug gaps to.045" for eight cylinder engines and .035" for fours and sixes.
Not sure why this was done, but there may have been leakage issues with the wide gap. The wider the gap, the higher voltage must rise before the gap ionizes. V8s with "integral" coils did not have as many potential leak paths as fours and sixes, all of which had remote coils.
In my experience plug gap doesn't make any noticeable difference in performance, but a wider gap will place higher stress on the secondary components such as plug wires. If you have the OE type wires that were not designed for the higher voltage generated by wider gaps, stick with .035". The HEI has double the per spark ignition energy as the point ignition and will light the fire no matter what.
I've seen old worn out plugs come out of HEI equipped engines where you literally had to measure the gap with a ruler and the engine ran fine.
Just to refresh your knowledge, the voltage generated in the secondary circuit is a function of the voltage necessary to ionize the plug gap, which is usually in the range of 5-10kV, so "50,000" volt coils are meaningless. Open circuit, a HEI coil will generate about 25-30 kV, but this voltage will never be generated in the secondary circuit unless a plug wire is disconnected or the ground electode breaks off the plug.
Duke
[Modified by SWCDuke, 11:42 AM 6/8/2004]
Not sure why this was done, but there may have been leakage issues with the wide gap. The wider the gap, the higher voltage must rise before the gap ionizes. V8s with "integral" coils did not have as many potential leak paths as fours and sixes, all of which had remote coils.
In my experience plug gap doesn't make any noticeable difference in performance, but a wider gap will place higher stress on the secondary components such as plug wires. If you have the OE type wires that were not designed for the higher voltage generated by wider gaps, stick with .035". The HEI has double the per spark ignition energy as the point ignition and will light the fire no matter what.
I've seen old worn out plugs come out of HEI equipped engines where you literally had to measure the gap with a ruler and the engine ran fine.
Just to refresh your knowledge, the voltage generated in the secondary circuit is a function of the voltage necessary to ionize the plug gap, which is usually in the range of 5-10kV, so "50,000" volt coils are meaningless. Open circuit, a HEI coil will generate about 25-30 kV, but this voltage will never be generated in the secondary circuit unless a plug wire is disconnected or the ground electode breaks off the plug.
Duke
[Modified by SWCDuke, 11:42 AM 6/8/2004]
