Gapping Sparkplugs

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Drache

Drache

1971 Dodge Dart Swinger
Joined
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Is there a formula for knowing how much to gap sparkplugs?

I have a 318 in my car but it's not exactly stock anymore.
 
What kind of ignition system are you running?
 
Drache said:
Do you mean the distributor? Im currently on the original mechnical type but my mechanic will be switching my over to a HEI type first thing tomorrow morning.
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stock points or electronic? stock coil and ecu??


.035 even with HEI you'll probably keep it at .035-.040
 
You'll need to experiment, but let's talk about you choice of ignition, HEI? Really? Did your Chevy mechanic talk you into this?
 
Guitar Jones said:
You'll need to experiment, but let's talk about you choice of ignition, HEI? Really? Did your Chevy mechanic talk you into this?
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No actually my cousin owns the shop. My original distributor was really worn. He happened to have one of these in his lunch room:

http://www.topstreetperformance.com...ibutors/ready-to-run-chrysler-318-sb-red.html

He gave it to me for free, I just had to pay for the install.

Yes it's far from the best but I also am on a strict budget right now due to work.
 
Electronic set it at .045. No sense in over engineering plug gap. If you had a MSD with a 60,000 volt coil and they reccomended .060 with platnum plugs that would be a different story. .045 should work fine. toolman
 
I've never gone past .045. I don't fuss a lot with plugs. So factory type ignitions get .035. NO2, gets .035. MSD, or an upgraded ignition/coil package gets .045.
 
moper said:
I've never gone past .045. I don't fuss a lot with plugs. So factory type ignitions get .035. NO2, gets .035. MSD, or an upgraded ignition/coil package gets .045.
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Moper, does MSD recommend .045? I didn't look it up.
 
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.
 
I'm 100% certain I've seen the spec on radiator support emmision stickers. Seems like it was on the big Buick motors. I know it was the bigger GM cars though.
 
I think MSD says start at .045" and work larger by testing at the track. Factory specs go as large as .080 on GMs and .060 on Mopars. That was for emissions in the 80s and early 90s and the old plug part numbers that had the suffix "060" or "080" were that way. I always figured it's more energy in a smaller area (meaning hotter spark) when the energy is sent over a smaller gap. It's also less resistance which helps stay away from energy being leaked out connections and misfires. Especially with NO2 or rich fuel mixes with larger cams and bad overlap. But that's just me.
 
I know a couple guys on here run that dizzy. I think one company really makes them and a few companies market it. I would get a good coil and run it at .045.
 
moper said:
Factory specs go as large as .080 on GMs and .060 on Mopars. That was for emissions in the 80s and early 90s
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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.

I always figured it's more energy in a smaller area (meaning hotter spark) when the energy is sent over a smaller gap.
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That's not correct. Smaller gap = lower arcover voltage = lower-energy spark.
 
I know a few Oldsmobiles called for .080. Early HEI 455's and 350's around the mid 70's. I looked at an old 84 AC plug book and it showed the Toronado 5.0 calls for .080. Wouldn't run it that high but thats what it says. I'll try to get a pic later.
 
No pic needed -- if you found one, that's good enough for me. Wonder if it was one of the specs that was revised downward (way downward, like to 0.055") in the slew of TSBs GM issued on ignition issues in the early '80s. Rapid deterioration of caps, rotors, and plug wires, just like StrokerScamp says.

Steering the topic back to Mopars: If you use the longer-nose rotor (Echlin MO-3000), it reduces the rotor-to-cap gap so you can open your plug gap wider to get greater spark exposure of the fuel/air mix in the combustion chamber, without increasing the overall gap and therefore without increasing secondary voltage, thus without increasing wear and tear on the secondary ignition components. The MO-3000 rotor, which usually must be specially ordered at NAPA, has a contact 0.060" longer than the regular rotor, so check for cap compatibility. If your cap is ground off-center so some of the individual cylinder contacts are bigger than others, the longer rotor tip can snag.
 
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