SparkPlug Gap MSD
Race Director
Joined: Apr 2000
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
I run .035" on my race plugs, that is what they recommend, I tired .045" just for fun the other day and it would not start, I put them all back to .035" and it started right up. I don't understand this but it happened, maybe a fluke or something or maybe 11:1CR coupled with a few other things on the motor make it like .035" better 
Thread Starter
Pro
Joined: Sep 2004
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From: Philadelphia PA
Thanks. I am at .025 Autolite race plugs (sidegaps). I think I'll bump up to .035 to start.
Originally Posted by MotorHead
I run .035" on my race plugs, that is what they recommend, I tired .045" just for fun the other day and it would not start, I put them all back to .035" and it started right up. I don't understand this but it happened, maybe a fluke or something or maybe 11:1CR coupled with a few other things on the motor make it like .035" better
Last edited by Highhat; Nov 14, 2005 at 11:14 PM.
Racer
Joined: Dec 2003
Posts: 304
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From: Pasadena Maryland
I've always gapped my plugs at .035 and go from there. I run it for a while and check the plugs. Then open them a litlle and run them. When I get to the point where it rdoesn't run so good I close them a little. I've found out from experience that .045 to .050 seems to be the range my systems like and runs the best at. Race Plugs almost always recommend a gap of .035.
Pro
Joined: Mar 2003
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From: Valparaiso IN
Originally Posted by Highhat
Thanks. I am at .025 Autolite race plugs (sidegaps). I think I'll bump up to .035 to start.
Thread Starter
Pro
Joined: Sep 2004
Posts: 500
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From: Philadelphia PA
I didnt see 0.25. The guy who installed the angine after rebuild by another put them in that way. I happen to take one out to check, and then another, and they measured at .025. It turns out that is about where they are gapped out of the box. But, the MSD site recommends .050-.060 for my compression --which I thought was rather large.
Originally Posted by OldSchool
WOW .025 !!!??? No wonder my Autolite AR133 sidegaps wouldn't spark. Mine were .045. HIGHHAT, Where did you see the recomended gap as .025? Or were you just freelancing?
Race Director
Joined: Nov 1999
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From: Foxfield CO 1970 Convertible
Originally Posted by Highhat
Running 388ci, 10.25 compression, MSD 6A ignition.
Any recommendations on optimum spark plug gap? (MSD recommends .050-.060, but that seems rather large)
Any recommendations on optimum spark plug gap? (MSD recommends .050-.060, but that seems rather large)
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Tech Contributor
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From: At my Bar drinking and wrenching in Lafayette Colorado
Be careful with going to an excessive gap on a high compression and/or high performance engine. As cylinder pressure increases, the resistance across the plug gap increases. Eventually, resistance across the gap gets so high that the plug does not fire. If you have an ignition system with a lot of power (like an MSD), the spark will find the least resistant path to ground once the plug gap resistance gets too high. This will result in the ignition system crossfiring inside the cap, across plug wires, or anywhere other than at the plug gap. It has nothing to do with the MSD system: You are limited by the relative resistance across the plug versus resistance other places in the system. If you have lots of system power, the spark will arc someplace other than the plug. For this reason, high performance/high compression engines will usually run .035" gaps, and seldom more than .040". The stock HEI systems on our Vettes used very big gaps becasue they were very low compression engines. If you bump compression up, keep the plug gap modest. 
Team Owner
Joined: Sep 2003
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From: eville in
Originally Posted by lars
Be careful with going to an excessive gap on a high compression and/or high performance engine. As cylinder pressure increases, the resistance across the plug gap increases. Eventually, resistance across the gap gets so high that the plug does not fire. If you have an ignition system with a lot of power (like an MSD), the spark will find the least resistant path to ground once the plug gap resistance gets too high. This will result in the ignition system crossfiring inside the cap, across plug wires, or anywhere other than at the plug gap. It has nothing to do with the MSD system: You are limited by the relative resistance across the plug versus resistance other places in the system. If you have lots of system power, the spark will arc someplace other than the plug. For this reason, high performance/high compression engines will usually run .035" gaps, and seldom more than .040". The stock HEI systems on our Vettes used very big gaps becasue they were very low compression engines. If you bump compression up, keep the plug gap modest.
Pro
Joined: Mar 2003
Posts: 553
Likes: 1
From: Valparaiso IN
Originally Posted by lars
Be careful with going to an excessive gap on a high compression and/or high performance engine. As cylinder pressure increases, the resistance across the plug gap increases. Eventually, resistance across the gap gets so high that the plug does not fire. If you have an ignition system with a lot of power (like an MSD), the spark will find the least resistant path to ground once the plug gap resistance gets too high. This will result in the ignition system crossfiring inside the cap, across plug wires, or anywhere other than at the plug gap. It has nothing to do with the MSD system: You are limited by the relative resistance across the plug versus resistance other places in the system. If you have lots of system power, the spark will arc someplace other than the plug. For this reason, high performance/high compression engines will usually run .035" gaps, and seldom more than .040". The stock HEI systems on our Vettes used very big gaps becasue they were very low compression engines. If you bump compression up, keep the plug gap modest.
Steve
Safety Car
Joined: Oct 2004
Posts: 3,921
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From: Youngstown Ohio
Originally Posted by lars
Be careful with going to an excessive gap on a high compression and/or high performance engine. As cylinder pressure increases, the resistance across the plug gap increases. Eventually, resistance across the gap gets so high that the plug does not fire. If you have an ignition system with a lot of power (like an MSD), the spark will find the least resistant path to ground once the plug gap resistance gets too high. This will result in the ignition system crossfiring inside the cap, across plug wires, or anywhere other than at the plug gap. It has nothing to do with the MSD system: You are limited by the relative resistance across the plug versus resistance other places in the system. If you have lots of system power, the spark will arc someplace other than the plug. For this reason, high performance/high compression engines will usually run .035" gaps, and seldom more than .040". The stock HEI systems on our Vettes used very big gaps becasue they were very low compression engines. If you bump compression up, keep the plug gap modest.
Excellent advice......I've always used the closer (.030-.035) gaps, as they have provided slightly better top end performance at the strip.
Race Director
Joined: Apr 2000
Posts: 17,672
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From: Who says "Nothing is impossible" ? I've been doing nothing for years.
Originally Posted by lars
Be careful with going to an excessive gap on a high compression and/or high performance engine. As cylinder pressure increases, the resistance across the plug gap increases. Eventually, resistance across the gap gets so high that the plug does not fire. If you have an ignition system with a lot of power (like an MSD), the spark will find the least resistant path to ground once the plug gap resistance gets too high. This will result in the ignition system crossfiring inside the cap, across plug wires, or anywhere other than at the plug gap. It has nothing to do with the MSD system: You are limited by the relative resistance across the plug versus resistance other places in the system. If you have lots of system power, the spark will arc someplace other than the plug. For this reason, high performance/high compression engines will usually run .035" gaps, and seldom more than .040". The stock HEI systems on our Vettes used very big gaps becasue they were very low compression engines. If you bump compression up, keep the plug gap modest.
Thanks Lars backs up my thinking as to why mine would not start at .045"
