Gapping Sparkplugs

-
Ive never heard anything bigger then .035 or maybe .040-.045 'magnum'& hemi for a stock chrysler app's

True that with hei-.045 is about right, with msd, they START at .045 and range from there to .060

I run .045 with my msd 6a 'capacitive discharge', and .035 with factory type ignition boxes 'none capacitive discharge'

but JMo
 
slantsixdan said:
I'm still callin' BS. I haven't seen every last plug gap spec for Mopars and GMs of the '80s and early '90s, but I've seen most of 'em, and I haven't seen any Mopars calling for 0.060" or GMs calling for 0.080". All I've seen is a couple people saying it -- just like we had a guy saying Chrysler switched from LH-thread to RH-thread wheel lugs in 1972 in response to a Federal mandate, none of which is true. Show us one make-model-year with a factory plug gap spec of 0.080" (or 0.070", for that matter), please.
That's not correct. Smaller gap = lower arcover voltage = lower-energy spark.
Click to expand...

I know you freely accepted the second opinion but I found these in my old repair manuals and checked online to validate.
Jeep 4cyl had .060 gap (RV15YC6) for a few years (GM 2.5L), later 77-79 GMs had gaps of .080 on some engines, predominantly 77-78 Pontiac V8s. I'm told these were reduced to .060 through a GM Technical Service Bulletin, but as I dont care much for GMs, I'm not looking that up. Lastly some of the aftermarket (I saw AC Delco's)platinum plugs for mid 80s GM's had a plug manufacturer's recommended gap of .080 although factory type plugs seem to have been .060. that's online, so take it or leave it. I had to look to find the info but I know I recalled seeing the parts on the shelf years ago and I have 10 years worth of jobber parts when those cars were all over the place.
On your lower arc-over deal... The capacitance spark is the "arc over" and a smaller distance does take less voltage to bridge. You're electrical theory-wise correct. I'm not an electrical engineer so you can talk down to me all you want about how wrong what I'm about to say...
It's my semi-educated opinion:
I'm taking more into account than just distance when I say the smaller gap allows more energy in a firing event. A spark plug firing event has two parts. The capacitance spark is the arc that bridges the gap. The inductance spark is what follows the initial arc and is a lower voltage, but longer duration event. If you have a coil that say has 30kV of power when saturated, and the capacitance spark only needs 20kV to jump, then that additional 10kV that is built into the coil becomes part of the inductance spark. The energy has to leave the coil during the firing event. That's where I'm coming from anyway. I believe the capacitance spark energy you are correct about... Less means less. But the residual energy has to be disipated and it's my opinon that the inductance spark is made hotter by the smaller gap.
I've never heard of that rotor... pretty cool. The cap stays the same?
 
moper said:
It's my semi-educated opinion:
I'm taking more into account than just distance when I say the smaller gap allows more energy in a firing event.
Click to expand...

No, your opinion still doesn't square with fact. The energy of the spark is contingent on the plug gap (within limits of what the coil can provide). The energy density of the spark, which it sounds like you might be trying to describe, is contingent on the cross-sectional area of the electrodes. Smaller electrodes = greater energy density at the electrodes themselves. Smaller electrodes also reduce arcover voltage, so for any given secondary voltage you can have a larger plug gap for greater mixture exposure to the spark, or for any given plug gap you reduce stress on the ignition system.
 
All I want to do is thank MOPER and Slantsixdan because you guys (and many others) are very helpful on this site. Thanks Much!! And God Bless America! Happy 4th!!!!!!
 
StrokerScamp said:
Some of the old GM products from the late 70s and early 80s ran as much as .080" plug gap. I know whenever I ran HEI on some of my old chebbies, I always carried a spare rotor because the ignition was so hot it would burn a hole right through the center of the rotor and ground to the distributor shaft. Nuthin wrong with the GM HEI. It's some hot ignition. Just carry a extra rotor.
Click to expand...

Rob, where have you been? I wanted to thank you for the battery tray, it fit perfectly. Brooks
 
You must log in or register to reply here.
-

Back
Top