Thread: Spark Plugs

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Hey guys, I have a new 383 Small block chevy, 10 to 1 compression, aluminum high flow heads, roller cam shaft with .528 Intake .536 Exhaust lift, 221 Intake / 226 Exhaust duration @.050 and a 110 degree lobe separation; running a Holley 750 cfm carburator and a HEI Distributor. It has long tube headers with 2.5" dual exhaust, with Flowmaster Super 10s on it. The engine dynoed at 426 HP at 5700 rpms and 452 ft lbs of torque at 4300 rpms. The issue I'm having is that I'm running the cold racing spark plugs that it was dynoed with (Champion 792), gapped at .045", getting spark through a new HEI dizzy and sometimes during cold start I have a spark plug that will have an intermittent miss that goes away completely as soon as the engine starts to warm up.

This only happens after a few short trips in the car, where the engine doesn't get a chance to warm up all the way and doesn't get driven at cruise speeds. I'm wondering if I should step up to a little hotter spark plug, or maybe even extend the spark plug gap out a little to help with the issue.

I have a motorcycle that's the same way and you simply don't start it unless you are going to take it out long enough to warm it up completely and cruise it for a while at highway speeds. I'm just hoping that I can make the engine a little more street friendly with a little hotter plug; any suggestions or advice on the issue will be appreciated.

I should add that the only thing I reused on this new engine is the spark plug wires, they are around a year and a half old and since they've been running behind headers for that long it's possible that I've cooked one or two of them by now enough to cause issues during cold start and high humidity situations. So I just went and ordered me a new set of 8MM wires from Select, hopefully that'll take care of the intermittent miss on cold starts.

That said, I'm still wondering if I can or should run a hotter spark plug for the street, especially when colder weather returns?

UMmmmmm... YES!


Have you a loupe? Are you experienced enough to read plugs? Changing your heat range should be no big deal. Same with the gap. Both easy tuning tools provided you can read the plugs. On my stroker, I found dropping the gap to .040" really smoothed out the cold starts. No other changes, just dropped the gap by .005" smaller made a lot of difference.

Heat range is read by where the color change is on the ground strap. If the color changes out near the tip of the strap, then the plug is too cold. if the color changes back by where the strap welds to the plug body, the plug is too hot. If the color changes at about the halfway point, the heat range is correct for your tune.
Wallace Racing - Spark Plug Reading

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Thanks for the advise guys, I am a retired USAF aircraft mechanic with 30 years of wrenching on just about anything with wings or wheels, but while this engine isn't radical by any standard, it's probably the the most radical I've daily driven. I think my plan of attack on this will be to first swap out the wires with a new set, if I still get some misfire, I'll look at the plugs and try setting the gap to .040". Like you mentioned reading the plugs should give me a hint of where I'm at on the heat range.

I've evolved to NGK plugs.

I'm running E3's in both the Vette and the Dakota and so far, so good.

Ok guys, this is what I did and what I found. I bought some new wires and was going to change them out this morning when the engine was stone cold, but I got bored last night after dark and wondered if I could see any arcing under the hood after dark. I didn't see any arcing, but what I heard under the hood sounded like I had a header gasket just starting to blow out on the passenger bank of the engine, so I shut it down made a mental note to chase down the header leak in the morning and called it a night.

Got up this morning and over my cup of coffee I wondered if the leak I heard could be from a loose spark plug, so that's where I started this morning and I found the #8 plug was loose and leaking compression. It wasn't loose enough to wiggle around, but it was almost a turn from seating on the washer. I inspected all the wires and they all look good and since I saw no arcing last night I put the new set on the shelf in the man cave for later on sometime. I also checked all the header bolts and intake bolts and they were all good, so I'll drive it a few days and hopefully the miss is gone for good.

Good catch. Sometimes - heck, quite often, really, educated curiosity, and just sitting down with a cuppajoe and thinking for a while is good. Hope this did it for you.

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Yep, always keep a chair in the garage, so you can just sit and think about this or that.

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Originally Posted by Msgt Luttrell

Ok guys, this is what I did and what I found. I bought some new wires and was going to change them out this morning when the engine was stone cold, but I got bored last night after dark and wondered if I could see any arcing under the hood after dark. I didn't see any arcing, but what I heard under the hood sounded like I had a header gasket just starting to blow out on the passenger bank of the engine, so I shut it down made a mental note to chase down the header leak in the morning and called it a night.

Got up this morning and over my cup of coffee I wondered if the leak I heard could be from a loose spark plug, so that's where I started this morning and I found the #8 plug was loose and leaking compression. It wasn't loose enough to wiggle around, but it was almost a turn from seating on the washer. I inspected all the wires and they all look good and since I saw no arcing last night I put the new set on the shelf in the man cave for later on sometime. I also checked all the header bolts and intake bolts and they were all good, so I'll drive it a few days and hopefully the miss is gone for good.

Nice catch and thanks for your service too!

Thanks Ryan, for the reminder....
Master Sergeant Luttrell, thank you for your service to our country and may God love you and keep you safe and well. Without fellows like you, we'd all be peasants and slaves under some pissy little dictator.

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Thank you guys for the sentiment, but it was truly my pleasure to serve, I wouldn't trade the experience for the world and would do it all again in a heartbeat.

Ok guys, time for a little update, since the car is just a weekend play perty and doesn't get driven that much, I just got around to noticing that I still had a plug or two that were acting up on cold start. So I bought a new set of champion 792s and gapped them at .035" down from .045 I had gapped the other set at, installed them along with the new set of good wires. I believe with the camshaft and 750 Holley I'm running, combined with the higher compression was keeping the spark from igniting the air charge during cold starts. The cold start this morning was much smoother and the throttle response seemed better as well. I just did the 500 mile break in oil change on the new engine last week, I'll get her lined out soon enough.

As an (old?) aircraft mech, if you're ever having trouble getting a threaded fastener to "take" that first thread, do you ever close your eyes so that you can "feel" the threads better? I still remember some activities where we had to work "blind", reaching back into a void to set a washer, start a nut, tighten and then torque the nut that could only be seen with a light and mirror.... I often closed my eyes to kick the other senses up a notch.