Spark Plug Gap Experts only.
Thread Starter
Instructor
Joined: Mar 2014
Posts: 205
Likes: 0
From: Alpine TX
Spark Plug Gap Experts only.
On another thread someone stated that they recommend reducing the gap by .004" for every 50hp increase.
WHY!
From everything I have learned, the wider the gap, the more spark energy. The point of diminishing return is that the space between the terminals in the cap will start arcing or crossfiring. Ford increased their cap spacing with a cap-adapter to correct for this. Another factor is the coil gets overheated trying to keep up with the increase in spark energy, so that is another limiting factor. With our C5's, I don't see where HP comes into this. RPM yes! The coil has a recharge rate that I'm aware of, but with individual coils packs this is not a factor. For the life of me I don't see where the hp comes into the equation. I'm not saying it aint true. I'm saying I want to learn why.
Must have something to do with burn rate, but I thought that was mostly a compression ratio/timing and grade of fuel factor. IDK. Please educate me...
WHY!
From everything I have learned, the wider the gap, the more spark energy. The point of diminishing return is that the space between the terminals in the cap will start arcing or crossfiring. Ford increased their cap spacing with a cap-adapter to correct for this. Another factor is the coil gets overheated trying to keep up with the increase in spark energy, so that is another limiting factor. With our C5's, I don't see where HP comes into this. RPM yes! The coil has a recharge rate that I'm aware of, but with individual coils packs this is not a factor. For the life of me I don't see where the hp comes into the equation. I'm not saying it aint true. I'm saying I want to learn why.
Must have something to do with burn rate, but I thought that was mostly a compression ratio/timing and grade of fuel factor. IDK. Please educate me...
Last edited by Biotex; Aug 7, 2014 at 05:18 PM.
Thread Starter
Instructor
Joined: Mar 2014
Posts: 205
Likes: 0
From: Alpine TX
Le Mans Master
Joined: Apr 2012
Posts: 7,818
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From: Brighton MI
Said another way, for a given spark plug gap the greater the cylinder pressure the more voltage required to jump the gap. At some point, you start to exceed coil capacity...
Melting Slicks
Joined: Jun 2012
Posts: 3,163
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From: Oceanside California
Great answers here, I agree. Like FI and N2O, even big N/A power, all these increase cylinder pressure and require a smaller gap. Running a cooler plug helps too. That's why lots move to the TR6 for N/A and the BR7 for big nitrous hits.
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Melting Slicks
Joined: Jun 2012
Posts: 3,163
Likes: 6
From: Oceanside California
Well cylinder pressure is certainly higher than stock. If you aren't having misfires you should be fine. But a good way to tell is to judge your idle. If it isn't smooth, try closing the gap some. If it smooths out you'll know you just stopped some misfires.
Safety Car
Joined: Jul 2004
Posts: 3,904
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From: Vancouver Washington
The "only" was because it's a 408 cu in LS2 and I'm sure there are a lot of guys getting a lot more hp out of those. Thanks, I'll give that a try and see if I can tell the difference between the cam lope and misfires. I may need a professional ear. 
Le Mans Master
Joined: Apr 2001
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From: Was New Orleans but swam to Baton Rouge LA
Cruise-In IX Veteran
For my NA stuff I run a TR6 gapped around .040. I make just over 500 rwhp in a 383. With nitrous I change my plugs out to a 8 heat range and close the gap to around .030-.035.
Race Director
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From: oregon
Safety Car
Joined: Jul 2004
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From: Vancouver Washington
