Tech Tip #15

(Spark Plugs)

The Spark plug in an internal combustion gas engine is far more than most people realize. Even some mechanics think that a spark plug is a spark plug and there is no need for all the different types. 

One of the first things a race engine mechanic learns is how very important the correct plug is to an engine. It does not matter what kind of engine we are talking about. A lawn mower, a rail dragster or an Indy car engine. They each must have the kind of plug needed for the design of the engine, it's condition and it's use. Even the lawn mower engine needs a different type of plug when it gets old and worn vs. when it was new. Some race engines get so critical that they need one kind of plug to start and warm up with and another to race with. Lets look at some basics first.

The spark plug has an insolated post in the center and one or more posts (or place) for the spark to jump to. An ignition coil (transformer) supplies the voltage to jump the gap between the insulated center electrode and the post or area around it. To understand which plug should be used in an engine, you should know a little about what kind of voltage is available from the coil. You don't need to know exactly what kilovolt (thousand volt) amount is available from the coil but you do need to know approximately the range of voltage. Meaning, high voltage, standard voltage or low voltage. The higher the performance of an engine the more closely you must know the  voltage available from the coil. A garage mechanic needs to know if a coil has the capability of producing 35K (35,000 volts) or only 25k. on a particular car he is trying to diagnose. He has a "Scope" (Oscilloscope) to tell him this. If you have a "Scope", you will not need to read this stuff as you will already know this. For the rest of you, read on.

The "Old time" mechanic needed to know this when most didn't have scopes. How did he do that? He would pull a plug wire off the plug and hold it close to the engine and say "Hit er once" (meaning he is telling someone to turn the starter on or he used a remote starter switch) He watched the spark jump to the head and would vary the gap to see how far the spark would jump and the color of the spark. If he seen a wide blue spark that would jump 3/8 or a half inch, he would say, "Notten wrong with the spark". If he seen a thin yellow spark that would quit at a 1/4 or 3/8 inch. He would say, "This thing aint got enough far".  Keep in mind that this was on a car that only produced about 35K. One time I watched an old timer check a running car by placing his fingers on the plug ends. He then said, "Number 2's not firing". Just the thought of that put shivers down my spine. He couldn't do that on high voltage cars of today. Lets use for example a coil that normally produces 35K. with a .035 in plug gap on a plug that is clean but well used. Here are some conditions that will effect the spark on this engine. Increase the combustion chamber pressure and the spark could stop jumping the gap. lean the mixture out and the spark may stop jumping too. (a lean mixture can also stop firing even though a spark is still there) The voltage that jumps the plug gap is what voltage it takes to jump that gap, meaning if you open the gap the voltage goes up and if you close the gap the voltage comes down. If you file clean the electrodes of a used plug the voltage comes down. A rich mixture will make the voltage come down. Newer cars have high energy ignition systems with wide gaps mainly because they are running lean and there are not enough fuel molecules in the gap to start a fire. High performance engines use high energy ignition systems not because they are lean but because they experience high combustion chamber pressures and extreme conditions in the combustion chamber.

It is very important to keep deposits from forming on the insulation between the center electrode and the case or grounding electrode. The design of this insulator and location of this insulator controls the amount of heat build-up allowed. The heat that the insulator accumulates is what burns off and prevents deposit build-up, which shorts out the plug. It does this by dissipating it's heat out to the case of the plug to the cylinder head.  Generally, a long thin insulator will not be able to transfer it's heat well to the case so it will get hot easily. This is called a "HOT" plug and is used in slow, low performance engines or an engine that is old and burning oil. If you try to use a Hot plug in a high performance engine and run it hard, the plug will get so hot that the center electrode and the insulator will start to glow and ignite the fuel mixture before the spark can which starts the fire out of time and causes "Preignition" and "Detonation" which in turn will destroy the engine. (see tech6) A hot plug can be used in a high performance engine as long as it is use for just light loads. Such as warm up or just to get the engine running.   

The other end of the scale is the "COLD" plug. This plug has a short thick insulator that can easily transfer it's heat to the case and on to the cylinder head. Thus it runs cooler and can be subjected to much higher combustion chamber temperatures. This plug can cause a problem also when a high performance engine is cold and just started or like a car or motorcycle that is used on a dirt track where a wide range of throttle and RPM is used. The fuel mixture is generally set richer. (up to 40% richer than stoichimetric (ideal mixture for a given fuel) produces the most horse power) The richer mixture is also cooler running especially when there is light or no load on the engine, thus deposits build very quickly on the plug insulator shorting it out (fouling a plug). Plug manufactures make a wide range of plugs in-between the "HOT" and the "COLD" plug. This is usually represented buy the number in the part number of the plug. UNFORTUNATELY, the manufactures never got together on a standard for this numbering system. A Champion plug has the number go down to indicate a colder plug. An NGK has the numbers go up to indicate a colder plug an there is no correlation between numbers to different brand plugs. On most brands of plugs, the part number tells all the features of that plug. However, you need a menu of what each part of the number means for each brand of plug. For example, an NGK  "BP6ES" the "B" means 14mm threads, "P" is a projector core, "6" is the heat range. (2 being hot and 11 being cold), "E" is 3/4 in reach threads and "S" means standard size center electrode. The "BP6ES" is about the equivalent of a Champion "N9Y"  In Champion the "N" stands for a 3/4 in 14mm thread, the "9" is their heat range (higher number meaning hotter and lower meaning colder) Bosch also runs high numbers for hot and lower numbers for colder. Keep in mind that heat is accumulative so if you have a correct heat range for an engine and the plug is not tight, it will act like a hotter plug because it will not be able to transfer it's heat to the cylinder head. 

All of this may be confusing if you have not been working with different types of plugs. There are some basic rules that you can follow that will keep you out of trouble. 

1. Any time you have a stock engine with no modifications and it is used for normal driving ALWAYS use the factory recommended plug or equivalent in another brand or go one range colder.

2. Any time you use a stock engine for hard use, go one to two ranges colder.

3. Any time you modify an engine, use several ranges colder and if it continually fouls plugs move one range at a time toward hotter until it quits fouling. If you get down to the factory plug and it is still fouling, fix your engine, it's not the plug heat range causing the problem.

4. Any time you use a hotter plug than recommended for what ever reason, never apply heavy loads or high RPM to that engine.

5. You can remove most of a plugs extra features like "Projector Core", "Resister", "Booster Gap" etc. but never change the "Reach" (length  of threads). You better do a lot of research if you want to add one of these features. The only feature that I can recommend to any engine is the multi electrode. Generally you should see some increase in performance and longer plug life.

"Reading the plugs" as it was called, was a skill many mechanics had when fuel was leaded. That skill is somewhat lost with unleaded fuel . Not completely lost but lessened. A hard full throttle run and kill the engine, pull a few plugs and you could tell a lot about the engine. A correct fuel mixture would leave a light brown or tan color on the insulator around the center electrode. As the mixture was richer, the color got darker brown on to flat black. An incorrect heat range plug would alter these readings. A "Blued" or discolored negative electrode indicated incorrect ignition timing. Gloss black was either oil burning or a fowled plug. Unleaded fuel leaves this insulator clean unless it is very rich which makes it show flat black. You can get a little reading from the edge of the plug. A lean mixture will leave that clean but as you progress richer you will start to see a flat black dusting on to complete flat black and some flat black dusting of the insulator on to both being flat black. Keep in mind that engine temperature, engine condition (oil burning), ignition timing and even altitude will alter mixture needed thus effect plug heat range needed. Example, you have a modified engine and running it a little on the rich mixture side for quick throttle response and power. You have the coldest plug that will not gas foul. Now you run up a mountain road, and as you go higher in altitude your mixture that was already rich, is now too rich and the plug that was just on the edge of OK is now not hot enough to keep the plug from gas fouling. Another example, same car, you have been driving all winter with no problems and one spring day it gets nice and warm. Think I will go for a drive. Now the mixture that was OK in cold air is now too rich again and fouls your cold plugs. I can see the gears turning in your head now! OH! I see! I should use a range hotter plug?   WRONG!!! You should correct the fuel mixture for the conditions. Newer fuel injected cars usually have temp and altitude compensation built in, but hoped up cars with carburetors usually don't. Cars with SUs or Stromburgs w/adjustable needles are easily adjusted, but Webber or Solex etc need a jet change. Hopped up or high performance engines are more critical to temp and altitude changes than a standard low performance engine. Many race car & motorcycle racers have to alter mixture at each track they go to, checking the local temp and barometric pressure.

Most sports car owners who only run an occasional autocross will probably only need to set a compromise. Mixture set close to stoichimetric and a plug only one or two colder range than standard and only adjusting mixture between winter and summer and only switching to a colder plug when auto-crossing.

If you have any questions or comments, contact me at, 

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