Why not run a wider spark plug gap?
 

OK, so I installed a Mallory Hyfire 6A ignition box, new wiring, new Ford Duraspark dizzy, a Mallory Promaster 29440 coil, and MSD spiral wound 8mm spark plug wires.

I am running a stock 360 FE engine so this is totally overkill, I know...

What I'm wondering is, why not open up the spark plug gaps...a lot? I think they are at .40 or so right now. Why not open them up, I am sure this "racing" ignition I put on can handle it. Why did I put on a racing type of ignition, you ask? For fun mostly, and I guess I thought it would help mileage or emissions or something. Plus, my original ignition system was really screwed up - I had a bad Duraspark module, a bad stator and reluctor on my dizzy, and the P.O. had a wire fire that melted some of the ignition related wires so it was nice to re-wire it all. I also like having the Mallory coil mounted on the sheet metal instead of on the engine.

Anyways, enlighten me, I am pretty ignorant when it comes to spark plugs, if I run a bigger gap wouldn't that get me smoother idle, better emissions, better combustion in general? Is there a downside to running a bigger gap? I always heard the downside was your ignition might misfire, but I really don't think that will happen, I have a pretty nice ignition setup here.

Also I don't care if the plugs wear out faster, they are cheapo plugs from Autozone, they are definitely expendable.

You could open it up to about .045". The wider the plug gap the more voltage is required to fire the plug. The higher the voltage developed the easier it is for the spark to find some alternate path to ground besides the plug gap. That means it will find the weakest point in the ignition, -which may be a wire near a metal part, or across the rotor, or thru the distributor housing, or to another cylinder where the cylinder pressure is not as high. Any of these things can cause a misfire. If the spark jumps across or thru some insulator it leaves a carbon track which destroys that component since it is just that much easier to jump the path again and again and again...

With your ignition system, you could open up the gap, especially if you change the plugs out often like you said. You could experiment and go even wider, than .045, but you will have to find a happy medium that works for your engine, and is not to aggravating as far as having to change the plugs all the time. Your 360 has fairly low compression, so that will make the spark easier to jump the gap also.

Hey guys, I bought a nice feeler gauge and checked the gap on my spark plugs. It was only about .032! I think the computer said .035 but it only was .032 on my guage.

So, I opened the gap up... I figure, I have a racing type of ignition, why not try a bigger gap! So I put it at .05 for now :) We'll see if it makes a difference!

Even on a stock Duraspark ignition it was supposed to be .044 right? I'm pretty sure the specs that came with the truck say .044 - I wonder why Autozone computers said .035 was the right gap?

I gapped my plugs at .54 in my 94 460 with a Jacobs Pro Street ignition and it worked fine.

WOW! And the piston didn't hit them?

It's not that close in there. I have heard of people going all the way up to .080 gap. Chevy actually ran a .060 gap from the factory for a few years.

The sparkplug gaps are like everything else. The factory sets a compromise spec based on a lot of testing. The gap they give for the stock system gives alright power and long service. You can widen the gap, especially if you put a high energy system on, but you my have to change the plugs more often. The spark likes to jump from sharp edges. Once the sparkplug wears down a little bit, it gets harder for the spark to jump a large gap.

I was kidding him. He typed " .54 " but I knew what he meant. The devil is in the decimals.

Oops! I just noticed that. That would be a little fat wouldn't it?

Hey guys, something strange happened!

I was running the cheaper Bosch plus, about $1.19 each, gapped at .05, and it worked at first but then something happened...

We were draining the radiator and flushing out the water jacked and everything before putting in new water. The reason was because there was soot in the coolant from when they hot tanked my intake manifold. The water passages in the intake manifold had a bunch of soot in them and turned the coolant black. I wonder if that affected the cooling ability of the coolant?

Anyway, we had a hose in the radiator washing things out and the engine running, and some water got on the spark plugs and it started missing. The next day, everything was totally dry but the engine was STILL missing.

I started unplugging wires to see which cylinder was not firing and it was #3. Fearing the worst, I quickly checked compression - it was fine on that cylinder. I tried hooking up the spark plug outside the engine and the plug was definitely sparking. However, when I screw in the plug and run the engine, that cylinder misses. So, I swapped that spark plug with cylinder 1 and then cylinder 1 would miss. So the problem moved around with the spark plug, so I knew the plug was bad.

I don't really like those plugs anyway, they came gapped at about .03 and getting them to .05 was difficult, I did not touch the center electrode, I used needle nose pliers to carefully adjust the gap, but maybe I damaged something somehow. So, I just bought a set of Motorcraft plugs, and guess what? They came gapped at .044 already, so I just kept that gap for now. I did test them all to make sure their gap was all the same and it was, all of them .044 so I put them in. Engine runs smooth again!

I wonder what was wrong with that plug? There is no visible damage, I can't see anything wrong, and the plug sparks fine when you test it outside the engine. It's a nice brown color that doesn't look like it was getting too hot or anything. Crazy...

Like I said, the spark will find the weakest point to jump. Ethylene glycol and water or water alone is a conductor. Once the spark has jumped over an insulator it leaves a carbon track if there are any organics present. Even a slight film of oil like "road film" can provide the carbon. The carbon track is permanent, fused into the part and ruins it.

Outside of the engine at normal atmospheric pressure it may be easier to jump the gap than to arc over.

Remember the wider gap will cause more failures elsewhere in the system and will require more frequent maintenance and $$$. Also, always use silicone dielectric compound on all of your plug wire connections. It is also good practice to never touch the insulator on a plug either. Your finger prints leave salts and oils on the surface that make it just that much easier to jump over the surface.

The proper wire type plug gap gage comes with a tool that has a couple notches in it to adjust the side electrode:
http://www.toolsource.com/OstImages/...ages/67279.jpg
Professional tool stores have these tools or you can get them at places like:
http://www.toolsource.com

Those round metal gages with the varying thickness edge are NOT the right gauge to use like:
http://www.aircraftspruce.com/catalo...s/12-21375.jpg
-totally bogus.

Flat feeler gages are not correct either:
http://www.autoblog.com/media/2006/03/Plugs-1.JPG

Flat gages are semi-OK for brand new plugs but are worthless on a used plug.

Seem to me that I read or heard somewhere that cylinder pressure can cause a weak plug to mis-fire. So while it may look good it's not necessarily so.

Supposing your ignition coil is up to the job, you can run an .080" gap. But in the high pressure, turbulent environment of the combustion chamber, the voltage required to arc across that gap is astronomical. As the coil will put out the necessary voltage to release it's energy, it will be challenging the insulating qualities of every componant in the HV side of the ignition. So for a daily driver, extreme plug gaps are going to sacrifice reliability.

That's true, because more air packed in the cylinder makes it harder for the spark to jump the gap. Turbocharged and supercharged engines have this problem more than others. The spark needs to be hot, and the gap carefully(and conservatively) set for these engines.

I have a old champion plug tester which is fired by a Model T buzz coil (came that way from Champion). You install the plug, start the spark buzz and look into the glass window as you turn up the regulator to increase air from a compressor. I'm set to 140 psi maximum, you'll see the spark size decrease to less than half under pressure like inside a running motor. Any plug that idles ok but misfires under heavy load will fail in the tester. A plug with a cracked porcelain tip will show up in the tester. A neat tool that must be 60 years old.

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