5. INTERPRETING THE PLUG

Why - because the plug is the nearest thing we have to looking in the cylinder and outside of very expensive data acquisition equipment, (which isn't always practical, legal, or even available), it is the best indicator of what is happening during the combustion process.

How - very important - with a light of proper magnification and brightness.  This seems so simple but many people misread a plug because the light is too dark, keeping them from seeing the plug as it really is and causing them to "read" shadows.  Also it is important to find a good magnification and stay with it.  (I prefer a magnification that allows me to see details, such as denotation and oil spots and still lets me see the whole firing end.)  Changes in magnification greatly change the "look" of a plug and if you are not familiar with it, it will only cause to confuse you.

Test Conditions:

1. Should be a new plug - some markings such as denotation and oiling will not burn off so it becomes difficult to see changes when the plug is cluttered with old information.

2. Need a good, clean power shut off - this does not mean at wide open throttle and 300 MPH flipping the ignition switch! It doesn't hurt the plug reading at all to push the clutch in while backing off the throttle, let the engine stabilize for a moment, and then turn the ignition off.  The important thing is to do this check at the highest temp., i.e., the end of the longest run, and not let it idle all the way back to the pits.

3. Be consistent - the engine should be up to operating temp., (water and oil), and be the same from test to test.  The duration of the test should be the same each time as well.  Anything that affects the engine will affect the reading on the plug, (RPM, air inlet temp. and press., coolant temp., etc.) so keep all the changes in mind while reading the plug.

Things to Look for in the Plug:

1. Air/fuel mixture or fuel ring - look for the color of the ring on the insulator.  As the engine gets richer the color ring will get darker and farther up in the insulator nose, (towards the ground wire), as the mixture becomes leaner the fuel ring will become more light brown in color and closer to the bottom of the insulator , sometimes disappearing completely. In the Champion racing plugs a ceramic coating is added to the insulator where it seats into the shell.  As we already stated as you tune the engine leaner and the fuel ring goes deeper down the insulator, finally disappearing altogether the next step is going to be the ceramic coating just mentioned, blistering and pulling in on the insulator so that you will see a thin, black, jagged ring just at the bottom of the insulator before it goes out into the shell.

2. Ignition timing - generally denotation is evidenced by the appearance of tiny, metallic looking balls on the insulator.  These specks can be either dark or silver, (indicating aluminum), but are always spherical in their shape.

3. Heat - this is a very critical reading because it it the result of three factors.   1-fuel   2-timing   3-heat range of the plug.  Excessive heat is recognizable by a glazing of the insulator and/or a discoloration of the ground wire and center wire.  Other indicators of the amount of heat in the plug are the location of the heat color line on the shell threads and the ground wire.  At the heat goes up. the color line will extend to more threads on the shell and move farther on the ground wire towards the shell.  For me personally the threads are a little difficult to read however the ground wire heat line is fairly visible and therefore easy to see changes in.  Once you see the heat in the plug, deciphering which of the three factors are causing it requires a knowledge of the engines performance characteristics and where reading the plugs while the engine is on the dyno (where you have the benefit of power readings) becomes so important.  For example, if there is a lot of heat in the plug at the point of best power for timing and fuel mixture, then you should try a cooler heat range plug.  If you feel confident about the plug heat range and at the track the plug shows heat then consider fuel and timing.  It all requires a great amount of base knowledge concerning your particular engine, and it's very difficult to look at a set of plugs for the first time and give the "best power" advice.  Another issue to consider is that different plugs show heat differently and you should be aware of these manufacturing differences while trying to read them.  A good example is the older Champion plugs had cement inbetween the center wire and the insulator, a lot of people looked for this cement to boil out the top as an indicator of heat.  This worked just fine but now we have removed the cement, which is going to pose a big problem if you keep leaning down waiting for the cement to boil out.  This change also causes the plug to run a little warmer, which causes the insulator color line to appear leaner.  Another difference could be the plating of the shell and ground wire.  Zinc chromates tend to discolor quicker then nickel coatings, which again would change your impression.  The best advice is to remain consistent or understand the changes so they don't fool you.

4. Similar conditions on all plugs - for optimum performance all of the plugs should look the same, meaning that all cylinders are operating equally and at top efficiency.  You must look at all of the plugs when tuning the engine to insure the fact you are seeing the worst case, but at the same time each plug should be treated as if it came from a one cylinder engine.  If the plugs vary in their reading it could be either from inconsistent cooling or inconsistent fuel distribution.  Fuel distribution problems can sometimes be solved by staggering jetting, (in carbureted engines this generally affects two cylinders per jet), or by manifold porting.  Varying plug conditions should not be ignored as they may be the first indicator of a problem which when ignored could develop into a more serious condition. Some people in an effort to make all the plugs look alike have staggered the heat range of the plugs in the engine.  While this may be appropriate to say, prevent denotation in a hot cylinder by installing a colder plug while leaving the other cooler cylinders with the warmer plugs, it should be used only as a preventative measure and not be confused with actually solving the causing condition or improving the performance of that cylinder.

5.  Strength of ignition - an indicator of the ignition strength is a "spark mark" on the center wire.  When a strong ignition is being used there will be a crescent mark on the center wire from the strength of the arc.  Sometimes the plugs will, in addition to losing this mark, take on a very cold, rich look when you feel the jetting and timing is appropriate, and the cylinder leakage check proves OK.  This can be from a weak ignition and solved simply by changing ignition parts.  Many times though this "weak" look comes from a poor electrical ground system. Do not rely on your metal motor mounts to ground the engine, a good strap from the engine to the frame has solved many a weak ignition.  The ideal ground system would be to run an 0 or -1 size copper cable from the negative side of the battery, up to a post welded to the frame and then run all your ground wires including the ground strap from the engine to this common post.

Other Problem Indicators:

Oiling - typically a dark shiny look that will not rub off in your palm. Sometimes when the rings haven't seated completely you will see small, flat, dark spots.  These spots can be distinguished from denotation by their difference in shape.  Sometimes a large single blotch will appear, this is usually from oil coming down a guide when the engine was off and landing on the plug.

Water in the cylinder - the early indications of a water leak into the cylinder is a lack of any fuel ring combined with a light gray look on the entire firing end.

Additives in the fuel - sometimes when you change fuel brands you will see a change in the look of the insulator.  Usually it is a color change indicating a different dye or additive used in the fuel.  It may have a yellow color tint or it may  have a crystallized appearance.  The yellow color typically is a sour crude  having a high sulphur content, (western crude oil is a sour crude having a high sulphur content), or it can be from a high amount of lead additive.  As for the crystallized appearance some tracks add a small percentage of alcohol to their fuel to absorb the water/condensation that gathers in their large, very seldom full, storage tanks.  This alcohol with it's water can give the insulator a crystallized appearance.

Thread inserts in aluminum heads - this past year we ran across an interesting situation.  A racer had stripped out the threads in one of the plug holes of his aluminum heads, so he fixed his problem with a steel thread insert.  The next time he ran the car the plug in this hole melted the tip off.  What happened is the heat dissipating ability of the head, and thus the plug, had been changed rendering that heat range much too hot.  In this case the steel insert certainly had an effect but just as importantly was the Loctite used to hold it in places as the bonding agent created a great thermal barrier.

Heat Range Selection - there are basically two theories on choosing a plug's heat range for an engine and the tuning that goes with it.

1. Choose as hot a plug as possible - this choice has been the approach for many years and is justified by the thought that you  eliminate any low RPM fouling and stumble, and that the hotter plug will light the flame faster at all RPM resulting in a increased burn rate. Advocates of this idea don't mind cooling the plug by adding more fuel, (richening the engine), and decreasing timing for the race. Most of the supporters of this theory are drag racers, where fuel economy isn't so important and a slight denotation can be caught before any damage is done.

2. Choose as cold a plug as possible - this is a fairly new idea but is gaining a lot of popularity among the oval track and road racers.  The approach here is to run a cold plug coupled with a lean mixture and sometimes an increase in timing.  This choice eliminates the possibility of the plug limiting the air/fuel ratio and ignition timing by becoming a pre-ignition point thereby allowing the tuner the ability to find "best power" in both of these cases.  Some reports are that fuel mixtures much leaner, and timing higher than previously thought possible have been run with success.  This can be a benefit where fuel economy is an issue.  Other benefits to the colder plug are that it is more sensitive to tuning changes as not so much of the fuel is burnt off by the insulator heat, also with the increase in compression ratios and subsequently cylinder pressures the colder plugs have provided some insurance against pre-ignition/denotation and are probably much more in-line with the proper heat range for the cylinder temps.  In my opinion the only concern with this approach would be in the low RPM situation where there could be the possibility of misfires.  However most of the racing done today is at a relatively high RPM and most racers could probably use a cooler plug with no ill effects and the possibility of some benefits, just be sure you have enough ignition.

Thoughts on Special Applications -  most of our talk has centered around the normally aspirated, gasoline fueled engines. The following are some thoughts concerning other racing applications.

1. Alcohol Fuel Engines - alcohol is difficult to read on the plug insulator as it burns so clean.  Basically you have to look for heat to indicate your rich/lean condition.  If the plug looks brand new and the ignition is working OK then you are too rich.  Some engine builders look at the top of the exhaust ports to check their fuel setting, with a light brown indicating a good air/fuel mixture.  Also keep in mind that alcohol fueled engines are not nearly as sensitive to being rich as gasoline engines are, (a 10% rich condition will show no power loss), so don't be concerned with the fuel mixture like you would be with gas.  If it shows a fair amount of heat and the engine is crisp its probably OK.

2. Nitro methane Fueled, Supercharged Engines - looking at these plugs is like looking down a gun barrel to see the bullet coming - if you see it it's too late!  You should look for heat in the center wire with a light blue being normal and the next step hotter being burnt and gone.  Concerning denotation, if you see it in the plug its too late.  You'd better start pulling it apart as the piston, rings and/or cylinder will surely be damaged.  In choosing a plug for these engines you must consider the tremendous rate at which the heat builds up in these cylinders.  It can build up so fast it will cause the center wire to  expand too quickly, cracking the insulator radially as well as vertically.  Because of this if the plugs are to be used again after one run they should be inspected under an inspection light.  Some engine tuners like to run an extended tip plug in order to help keep the fuel mixture from fouling the plug and then they read the percent of ground wire burned off as an indication of the rich/lean condition.

3. 2-Stroke Engines - basically these engines' plugs read very similarly to the 4-stroke engine as far as the insulator and electrodes are concerned.  The shell will always appear a but more shiny, as an indication of the higher cylinder temps. associated with 2-cycle engines.  As with the 4-cycle , rich conditions will darken colors where lean will be very reflective and have a glazed appearance.  Timing also is similar in its appearance but, even the slightest sign of denotation will destroy the piston and cylinder.  Choosing the heat range of the plug is a very critical factor in the 2-cycle engine.  You certainly need a plug hot enough to prevent low RPM fouling but if it is too warm the oil in the fuel mixture can become burned onto the insulator, which becomes hot spots when enough has accumulated, creating a pre-ignition situation.  The colder fine wire plugs work very well in these engines because of the low RPM anti-fouling characteristics of the fine wire design.  The thing to remember is that all of the conditions are more critical since the 2-cycle fires on every stroke, causing less time for cylinder and piston cooling.

CONCLUSION

As we said in the beginning, the spark plug performs a very basic function, it ignites the fuel mixture, nothing more nothing less.  This means that if the conditions are correct all the plug "tricks" are meaningless and account for all of the reports that say, "I tried that and it didn't do a thing."  However nothing is probably less understood, less documented, and certainly less repeatable than the combustion process and all the things that affect it.  Therefore, since the spark plug is the initiating factor in this mysterious combustion process it is imperative that we understand, 1. the basic design and materials so that we can make a proper selection  2. the factors affecting the plugs performance so we can most effectively use this plug  3. the plugs abilities so we don't expect it to do things it's not capable of and   4. we must understand and be able to interpret what this gauge, the window to the combustion process, is telling us. because there are literally no two identical engines concerning combustion, there are very few hard and fast rules as to application or interpretation and so experience is your greatest asset. The more you look the more you will understand and be able to more effectively tune your engine.