coil plug gap relationship?
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,351
Likes: 654
From: Tampa, Florida
coil plug gap relationship?
Whats the relationship between them? I came across a post that said plug gap should be opened up for high output coils. I have an Accel 45,000 volt coil. What should I gap the plugs at? I realize there's no performance gain with that coil but its all the local parts store had in stock.
'70 350..mostly stock.
Thanks,
Jim
'70 350..mostly stock.
Thanks,
Jim
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
The larger the gap, the more 'spark' to fire off the cylinder. The bigger the coil, the larger gap the spark can jump. So, you want as large a gap as you can get....but you have to make sure the coil can continuously supply that amount of energy AND the it will ALWAYS do so (no misses).
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
Whats the relationship between them? I came across a post that said plug gap should be opened up for high output coils. I have an Accel 45,000 volt coil. What should I gap the plugs at? I realize there's no performance gain with that coil but its all the local parts store had in stock.
'70 350..mostly stock.
Thanks,
Jim
'70 350..mostly stock.
Thanks,
Jim
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
A plug will fire as soon as enough potential energy has built up in the coil to 'jump the gap', even if that's a lowly 5-10Kv. The spark does not wait until the coil has built up it's maximum potential of 45kv or whatever it's rated at.
Opening the gap may increase the actual discharge voltage slightly, but so what- makes no difference in performance.
Opening the gap may increase the actual discharge voltage slightly, but so what- makes no difference in performance.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
It's simple physics that it takes a given level of energy [voltage] to jump a specific gap (assuming other environmental factors are equal). The larger the gap, the larger the voltage required to jump it. If there was no benefit to be gained by increasing the plug gap along with higher potential coil(s), why would every manufacturer today choose to do so? {Maybe....they just want to put costlier components in your car for no good reason?} 
Drifting
Joined: Feb 2005
Posts: 1,738
Likes: 2
From: San diego ca
It's simple physics that it takes a given level of energy [voltage] to jump a specific gap (assuming other environmental factors are equal). The larger the gap, the larger the voltage required to jump it. If there was no benefit to be gained by increasing the plug gap along with higher potential coil(s), why would every manufacturer today choose to do so? {Maybe....they just want to put costlier components in your car for no good reason?}
- increased resistance to fouling
- increased resistance to deposit build up and thus bridging
also I'm not sure but I do believe the Hei spark is of slightly longer duration than a points coil.. as such it would need a more powerful coil i would imagine.. and a longer duration spark is of some advantage IIRC.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,118
From: Crossville TN
I was referring to the "...makes no difference in performance." comment from Brother Ward. A 'hotter' spark [or hotter spark "potential"] will provide more assurance that a cylinder will fire and that it will not foul. 'Performance' isn't only measured in HP/torque; sometimes reliability of the cylinders firing properly is a 'performance' asset.
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
I didnt see anyone mention there was no benefit....there may be a negligable performance benefit but other benefits that increased gaps and the resultant increased energy of the spark may include but not be limited to...
- increased resistance to fouling
- increased resistance to deposit build up and thus bridging
Modern cars use 'hotter' coils to help fire extremely lean mixtures, something that didn't exist in the days of early C3s.
The two main reasons GM came out with HEI was to deal with the above and to stay 'in tune' for a much longer period as mandated by federal regulations.
Putting a hotter coil on an engine that runs OK with a standard coil will gain nothing, same as with electronic ignition kits.
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Race Director
Joined: Jul 2001
Posts: 15,892
Likes: 42
You've got have an initial problem for a 'fix' to be applicable.
Le Mans Master
Joined: Dec 2003
Posts: 5,727
Likes: 38
From: Fremont CA
Large plug gaps KILL spark plug wires. The best figure to run is around 0.040" or 1mm of plug gap to save your plug wires. GM had a bad expirience with the HEI in the 1970's running a 0.060" plug gap and later backed down the figure to 0.040-0.045" on all its manuals.
Thread Starter
Race Director
Joined: Apr 2003
Posts: 19,351
Likes: 654
From: Tampa, Florida
I started a thread in regards to reading my plugs and the consensus was I was running rich but they weren't fouled. The remaining 5 plugs looked the same if not better than the 3 I pulled.
I installed AC R45's which are similar in heat range to the Champion RJ12YC's I pulled out and gapped them .040 based on this thread. I also turned in the idle mixture screws 1/4 of a turn.
Does everything sound reasonable?
I installed AC R45's which are similar in heat range to the Champion RJ12YC's I pulled out and gapped them .040 based on this thread. I also turned in the idle mixture screws 1/4 of a turn.
Does everything sound reasonable?
Tech Contributor
Joined: Jun 2004
Posts: 20,905
Likes: 962
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
It's simple physics that it takes a given level of energy [voltage] to jump a specific gap (assuming other environmental factors are equal). The larger the gap, the larger the voltage required to jump it. If there was no benefit to be gained by increasing the plug gap along with higher potential coil(s), why would every manufacturer today choose to do so? {Maybe....they just want to put costlier components in your car for no good reason?}
Burning Brakes
Joined: Dec 2007
Posts: 952
Likes: 0
That's crap.
Instructor
Joined: Aug 2007
Posts: 122
Likes: 0
From: Enterprise Ut
I replaced a factory setup in a 75 jeep with a straight six with an HEI conversion. New wires for the HEI and set the plug gap to .045. I noticed more power through the entire RPM range. I dont want to argue with anyone, maybee the old coil was weak? But that is my experience.