Spark plug reading
Reading spark plugs is another window into the engine, what happens in there. It's an important methodology because it can tell you so much, but it's also a difficult methodology, it can easily be misinterpreted. To make it easy to read the plugs we'll discuss 3 steps:
1. Getting the proper heat range spark plug
2. Reading spark plugs for problems
3. Reading spark plugs to help tune the engine
The biggest problem is that if you read a plug and it looks rich, there are 2 possible causes. One is that the plug itself is too cold while the jetting of the carb is OK, the second is that the plug is the correct heat range but the carb is jetted too rich. These 2 causes require very different remedies, so it's critical to find out which it is.
On 1. Getting the proper heat range plug
If your engine is stock the manuals will tell you which plugs too use. If they read rich or lean it's because the carb jetting needs tuning. Be especially careful if you change brands, make sure the translation from one brand's heat range to another is accurate. Don't trust the guy in the local shop selling it to you, it could be that he sells you what he has on the shelf. If your engine is a performance engine, having improper heat range plugs can kill the engine. If in doubt, throw them out, buy new ones, plugs are cheap.
If your engine is stock plus a nitrous system you have to go 2 heat ranges colder, the same for a blower engine. Remember that on American spark plugs, the higher the number, the hotter the plug while on European and Japanese plugs it's the other way around.
It's better to have too cold a plug than too hot. So if in doubt, go for the colder one. When it then constantly fouls and you know your engine is in good tune, buy the next hotter range until it doesn't foul anymore. So what's bad about a hot plug, it sounds great, being hot is generally a good word and it sounds like it could aid combustion. But if it's too hot the electrodes burn away too fast and even worse it can glow red hot and cause detonation which can kill your engine. Having too cold a plug doesn't include such risks, the only risk is that the plugs foul.
So here's how to find the right plugs if your engine is not stock. Drive a while on the freeway (remember that new plugs may take 500 miles to get their colour). If the engine from time to time pings or detonates, try colder plugs until it goes away. If not, then stop somewhere safe and pull the plugs. Are they soothed black try hotter plugs until that condition goes away. When you get to the right heat range plug, there will be one tell tale that can't be confused with carb jetting. When the heat range is right there will be a narrow bluish ring around the electrode, approximately 1mm from the tip. If the heat range is right and the carb mixture is right you'll be able to see it right away, but if the heat range is right and the carb mixture is too rich you have to clean the plug first.
Always buy copper plugs their effective heat range is wider, so they're more forgiving if you don't have the exact right heat range, and more important they can better handle the different chores of keeping the plugs clean when idling and not getting too hot when accelerating full throttle.
On 2. Reading plugs for problems
Yes, we know, it's not fun looking for problems. But if there are problems, they have to be remedied before we can tune the engine. Let's try to look at the serious symptoms one at a time:
• Electrodes are melted: Over-hot condition, way too lean. Fix immediately or expect to kill engine on next run down the track if it's not already dead
• Electrodes are broken/bent: Mechanical interference of the worst kind. Did you just put high dome piston in it? Or longer thread plugs. If you find that the gap between the electrodes tend to decrease over time, they could be just kissing the pistons at high rpm
• The plug looks wet: If it's gasoline, this can normally be fixed by tuning as we'll do later (smell it), but if it's oil, forget about tuning, you need to rebuild your engine first
• Small bubbles on center electrode and shiny look on side electrode and porcelain: Too hot plug, see previous paragraph
• Small black or silver deposits on porcelain: This is bad or very bad, it's sign of detonation. If it's black, it is the carbon deposits that are in every combustion chamber that's been rattled loose and is melted onto the porcelain. Remember that detonation is like hitting the piston with a hammer. If it's silver-like it's even worse, then it's part of your aluminum piston being thrown all over the combustion chamber, maybe it's time for a rebuild...
• Plugs are not the same: At the start you have to pull all plugs, not just one. Later when it comes to fine tuning and you know you don't have fuel distribution problems one will be enough. If you pull all plugs and remember which goes where, and see differences this has to be addressed. It could be jetting that needs to be changed on one side. If some plugs look much leaner than the rest there could be a vacuum leak near them. We had a boat with a blower and a single Holley Dominator. Cylinder 1 and 8 were always leaner than the rest. We tried turning the Dominator 90 degrees, we modified the intake manifold, we tried all sort of jet combinations in the Dominator, none helped. In the end we bit the bullet, bought 2 smaller Holley blower carbs and an adapter for them to sit on top of the blower, did some jetting and only then were all 8 cylinders the same.
On 3. Reading plugs to help tune the engine
So with the proper heat range plug and no serious problems, we can start reading the plugs to help us tune the engine. The ideal plug will look something like this: The porcelain is chocolate colour (leaded fuel) or very light brown, almost white (unleaded fuel), the tips of the electrodes (center electrode and side electrode) are grey, the rest of the side electrode is straw colour. There will be some deposits on the plug, but only dry and low gloss, no shiny and no wet deposits. And of course you can see the narrow bluish ring around the electrode, approximately 1mm from the tip.
If the mixture is too rich the plug will be sooty brown/black. If the tips of the electrodes are grey, then the plug is still firing, so it's a mixture issue, but if the electrode tips are also sooty brown/black, the plug doesn't fire, and that has to be fixed before the plug can be read, the sooty brown/black stuff will maybe disappear when the plugs starts firing again. If it's only one or two plugs looking like this, it's very likely it's an ignition issue that has to be solved first.
If the mixture is too lean and you use leaded gas the porcelain will be all white and the center electrode tip can be white as well. If you use unleaded gas, things are more difficult because an engine in perfect tune will almost not colour the porcelain. Look for white center electrode, signs of overheating like bubbles and burnt electrodes to help find a too lean condition.
Since we're trying to build a performance car more than an economy car, we recommend again to go on the rich side. And since it's difficult to see the difference between OK and lean, that's another reason to go a too rich, for peace of mind. So our recommendation is to be a little too rich, have plugs that are somewhat sooty brown, at least not white. Then you know you're alright and has margin of error on your side.