Spark plug reading by Mike Canter
 

This great article should be made into a sticky in both street and the race sections.

Here's the text from the article on Dragstuff.com
 

Spark Plug Reading 101
by Mike Canter, Last Updated Dec 19, 2014

Other than installing thermo-coupler sensors in the combustion chambers the only real way that you know what is happening inside your engine is to read the spark plugs. Both the fuel mixture and the ignition timing result in coloring of the spark plug's porcelain and ground strap. The trick is to how to get the correct coloring without going into detonation and destroying the engine or by going too rich and raising the ring lands.

Spark plugs can only be correctly read if the car has been shut down immediately at the end of a run without driving it back to the pits. Get off the track and coast to a place you are safely out of the way and either read the plug there or change one or two with some you have in your pocket so you can read these uncontaminated plugs when back to the pits. Or you should tow the car back to the pits. It may take a couple of runs to get see the sparkplug color.

Reading For Air Fuel Mixture

The porcelain around the plug’s center electrode can be divided into three areas for reading. The area that is closest to the tip is affected by the idle and transition circuits carburetor circuits and is of no real concern to a racer. If this area is gray then you drove the car back to the pits and you cannot correctly read the plugs. The middle area is only colored when you drive down the road at around a steady 30-40 mph and is normally affected by the primary circuit jetting with the power valve closed and this is really of no concern to the racer. The area you are interested in is that third that is all the way up inside the plug where the sun don't shine. This area is colored when all is wide open under full power because the combustion chamber heat totally cleans off the other two areas. It will take a special plug reading flashlight with the magnifying glass to view it correctly. Plugs cannot be correctly read by just quickly looking at them with the naked eye. You see people doing it all the time because they do not know how to read plugs.

Normally aspirated cars should have a light gray or tan hydrocarbon ring or as some call it a "fuel ring" all the way up inside around the third area closest to the point where the porcelain is attached to the metal jacket of the plug. The actual color may depend on type of fuel you use. This fuel ring should appear like a light shadow. Most VP C-15, C-16 or C23+ fuels will show as a light gray when correct. This fuel ring starts to color on the porcelain side that is below the ground strap and works its way around either side of the center electrode until it completely joins. Sometimes it may take two or three runs to see a good coloring. Note: New engines or engines that pump a little oil may show a thin oily line way down inside on the porcelain where the porcelain meets the metal wall of the plug. This oil line has nothing to do with the air/fuel mixture but may be confused with the fuel ring you are looking for.If you are having a hard time figuring out if what you are reading is correct or because you are not sure if the plug heat range is correct then tow the car back to the pits and drop the headers and look inside the pipes. If they are black then you are too rich, if they are light gray or white then you are too lean. The pipes should be a medium to dark gray or tan color.

Normally the white area of the porcelain has a chalky appearance. If you see the porcelain take on a shine then it is time to change the plugs because the glass that is in the porcelain has been melted and has glazed the surface. If the car has been running rich (due to lots of idling or incorrect fuel mixture) then it is possible to glaze the plugs and short them out during a run because of the sudden heating of the plug with the soot on the porcelain. This glazing appears to be a glossy coating on the porcelain with a splotches of color of greenish yellow or brown. These two different glazings will cause the plug to short out and misfire and raise ring lands or make a popping through the exhaust when going down the track.
Reading For Ignition Timing

Ignition timing is directly responsible for the heat in the combustion chamber and therefore the color of the plug’s ground strap and the color of the first few threads on the outside of the plug. The ignition timing can be checked by looking at the color of the plug’s ground strap and the position of the "blue line" on the strap. The blue line really indicates the point at which the strap has reached annealing temperature of the metal. To help to understand this think of a bar of steel (ground strap) on a table that is being super heated with a acetylene torch at one of the tip ends. As the end heats up and the heat starts moving down the bar you will see a blue line across the bar at some point down the bar away from tip with the torch. This blue line reflects the temperature that is the annealing point of the metal. As the temperature increases the blue line moves further down the bar away from the torch. Similarly, the blue line moves down the spark plug ground strap as you put more heat in the engine.

If you are using a gold colored ground strap like with an NGK spark plug then not enough timing will show the ground strap as still gold or going light gray maybe with a few bubbles on it after a run. As you advance the ignition and put heat in the engine the plug ground strap will turn darker gray as well as the metal at the end of the threaded area. As the metal turns medium to dark gray you should start looking for the blue line (band) around the ground strap. Ideally, you want this blue line to be just below where the ground strap makes the sharp bend and above the weld. If you advance the ignition too far the blue will disappear off the strap and the strap will pick up rainbow colors (blues and greens). The next step beyond that is to start melting the strap from the tip end and detonation. When you are close to the correct timing then only change the timing by one degree at a time. If you ignition system has the capability of adjusting the timing of each cylinder independently (ICT) then you can use that feature to have the blue line in the same position on all the plugs. First, adjust the basic timing to get as many of the plugs to have the blue line just at the sharp bend in the strap. Now adjust the ICT to move the blue line to the same point on the remaining plugs. Once all the plugs read the same you can advance the ignition a little at a time to put the blue line just above the weld on the strap or whatever point gives you the best performance.
Other Things To Look For

The round flat circular area of the plug at the end the threads should be dark gray or flat black and should not be sooty. If it is sooty then it can mean that your plug has not been tightened enough and you are sucking and blowing fuel and air past the threads of the plug.

Detonation shows up on the plugs as spotting on the porcelain. There are two different types of spotting seen. One type appears as just black spots and the other appears as little bright spots like diamonds. The black spots (look like pepper sprinkled on the plug) indicate a little too much heat on the plug which causes detonation by having the heated plug fire off the mixture prior to the spark firing. This creates two flame fronts that collide and can cause great amounts of damage. If you see black spots on the porcelain and you know the tune-up is correct then you may need a colder plug. If you are not sure then increase the carburetor jet size slightly, take out some timing, or go to a colder plug. If you hold the plug in the sun and you see what appears to be small diamonds on the porcelain then your detonation is severe enough to be blowing off the aluminum from your piston and you need to add fuel and/or take out timing now.

Spark Plug Heat Range

If you keep on adding timing until your finish MPH falls off but you still have no color on the plug’s ground strap but the porcelain has good color then your plug is too cold.

If you have lots of color on the ground strap but the porcelain is clean and white then the plug heat range is too hot. The heat from the plug is cleaning of the fuel ring from the porcelain.

(This article reprinted with permission from Mike Canter ©Copyright Mike Canter 2003 all rights reserved)
Additional Spark Plug Related Resources

Blown Alcohol Spark Plug Reading 101
Detailed Plug Pictures page
Plug Reading Discussion going on in our message board.
Another excellent "How to read Spark Plugs" article
Racing Spark Plug Cross Reference for Autolite, NGK and Champion
How you can read spark plugs and select them - by Gordon Jennings (Note: Geared towards Motorcycles but very good info here. Make sure and read the part about how detonation makes the plug look cleaner.)

Yours should be made into a sticky .

It's been decided here, today's fuel doesn't allow you to read spark plugs 🙄

The problem with "reading spark plugs" is that this new fuel takes a LONG time to color them up. Combine that with combo's that are tuned to be really efficient it's going to take quite a few miles to determine whether your plugs are the right color of white, tan, brown, black or something in between all that. With that said exactly what are you looking for, white, almost white, very light tan, tan, light brown, brown, dark brown, dark brown with black specs, light gray, gray, lightly blackened, deep black, sooted all to chit, etc, etc?

I tune for a living and consider spark plug "reading" pretty much a waste of time. This assumes that you know how to tune, and or have enough experience to look at the engine combo in front of you, what's it's using for CID, compression, camshaft events, intake, exhaust carb and distributor and dial it in pretty close right off the bench and well before any "fine tuning" is done.

In any and ALL cases the engine will tell you where it's at and what it wants. I've built tens of thousands of carburetors, and a LOT of distributors, and had scores of troubled vehicles brought to the shop as for custom tuning. NEVER in one single instance have I "read" a spark plug to guide my efforts. Even if I did, what if it's looking really light tan to almost white? Is the tune too lean, just right, or am I only looking at the primary side of the carb, or what the A/F is at full throttle?

I think at this point we are WAY past doing a quick jaunt down the street driving like a little old lady, then turning off the key and coasting over to the curb to pull and read a plug.

Same with track testing and tuning off the key a the end of the pass and coasting down to the return road, pulling over and reading a plug.....complete waste of time as I use ET and MPH from my time slips to tell me where I'm at and where I need to go......and with ALL that said I personally prefer to er just a tad "fat" at WOT vs going lean. EGT's are way down, and you are staying further away from possible engine damaging detonation, and contrary to popular belief you aren't leaving very much if anything at all on the table for vehicle performance.

I've done countless test & tune sessions and private track rentals testing full throttle A/F for ET and MPH. I'd tune from so rich the pipes were bellowing out a little black smoke to so lean the engine started to "surge" slightly.....and guess what? The entire spread of ET and MPH was seldom my over .03-.05 seconds and 1 mph or so. NEVER once with all that sort of testing have I found the "magic bullet" and picked up a half second or so in and 5-10 MPH in trap speeds.......FWIW.......

Modern day version of reading plugs is guys staring at a AFR gauge thinking they need a specific number. Instead of using it as a tool to see what the engine needs or likes.

Link to article in DragStuff.com
https://www.dragstuff.com/techarticl...ark-plugs.html

Blown Alcohol plug readings
https://www.dragstuff.com/techarticl...ark-plugs.html

I was just going to ask that, do how do blown Ally guys read there plugs?

Simply by heat range or if the electrode is even still there after a pass?

Thanks for the link I will read it this evening.

Aside from the above I don’t see how only todays fuel cut by 10% and having no lead changes plug readings massively.

Ive said that a lot Steve, referring to your last sentence. Unleaded fuels, especially those with ethanol, are not that good at coloring plugs as we used to see them with good ole leaded fuels. So it really starts to become a pointless exercise

Like cliff said, I find it better and easier to just give the engine what it wants. Drivability is more important and it doesn't always agree with what someone thinks it should be.

I like using a dyno for wot to get it in the ball park. Mainly just to make sure it's safe, but like cliff said you won't find a ton of power with big swings in afr. Anything from say 12.2 to 13.0 on an N/A engine won't really make a big swing in hp. Now if you're way out in left field then sure, that's a different story. At the track D/A conditions might make you take a big swing at it. But drivability tuning just comes from experience. Give the engine what it likes. Plug reading with unleaded fuels can be a bit misleading when it comes to fuel mixture.

Timing marks and studying the porcelain for specks can be useful however.

To really read a plug properly you either have to cut the plug apart anyway, or you need a lit magnify eye piece to really look down in there. I doubt anyone here even goes that far. Just pulling a plug out and looking at it is barely half the story anyway.

It was a rare occurrence when I removed a spark plug from any of my engines. The last 455 developed an intake manifold gasket leak and "miss" at idle speed so I pulled the plugs out of it for cleaning and inspection. They were very light tan to almost white and by appearance anyone looking at them would cry "its WAY too lean", when in reality the tune was dead nuts on the money from dyno/track testings and a LOT of street miles.

In any and all cases when it comes to tuning, whether it be dyno, track, or street by the "seat of your pants", ALWAYS tune for best results in all areas no matter what spark plug "reading" or A/F meters tell you is going on. Matter of fact when folks call here for "tech" help and they have installed an A/F meter (they are ALWAYS frustrated from "chasing" the gauge all over the place) the FIRST thing I tell them is to put a piece of black electrical tape over the gauge. Then tune the carb, and distributor for best results in all areas, starting with initial timing and idle mixture screw settings. When you are done tuning to the best of your abilities remove the tape from the gauge and observe the readings. These will be your "base-line" numbers for further tuning to try to make improvements.........FWIW......

That's a big issue with AFR gauges. Mainly because people have these set numbers in theirs heads.
"Oh it has to be 14.7 for cruise" or "it has to be 14.7 for idle" and on and on.

Well....no, that doesn't mean that's where the engine is going to be happiest. Every single engine I tune requires something different. Especially since ethanol fuels change the stoich that everyone wants to chase anyway.

Most cases they will idle and cruise at richer ratios, and many times I find that making them leaner does nothing for gas mileage anyway.

A lot of stuff I have is happier idling in mid 13's AFR and a lot of them don't like cruising any leaner than 14.0. leaner and you can feel the engine lay over. Some cases leaner creates a situation where mileage suffers.

It's more about what the engine wants, not what someone wants to see on an afr gauge, but that point is impossible to get access to people.

I have a feeling that the 14.7 is a mathematical equasion based on 2 materials ( fuel and ambient air) in a labratory environment assuming perfect fuel atomization perfectly mixed with ambient air a theoretical relationship

In a internal combustion engine you have not so perfect fuel atomization combined with not so perfect mix with incoming air in a confined space ( a port, plenum) with a chance of fuel drop out here and there

So 14.7 is perhaps in real life not where you want to be but it is a useable guidepost

Based on the molecular weight of the fuel and air.
Different fuel has different weights, plus alcohol weighs lot different. (Nitro a whole lot different.) The specific gravity is also affected by temperature.

The Lambda is what should be used, basically what EFI uses, I think.

EDIT:

Try this calculator to see what the lambda and AFR are.

Air/Fuel Lambda

:)

So if I'm understanding correctly, the "butt meter" works accurately for fine tuning metering rods, jets and vacuum advance but fails miserably when picking camshafts, cylinder heads and intake manifolds. 🤔

We initially tried tuning the Grocery Getter with the goal of ignition timing putting the blue line at the bend of the ground strap. It was an oft-repeated guideline. After eating too many pistons, we started retarding the timing. Each time we'd take a half degree out, the engine durability went up AND ET went down. The engine was happy and fast with no blue on the ground strap at all. We had individual cylinder timing. The most advanced was 10 degrees, the least advanced cylinders were 7-8 degrees BTDC.

As an old engine machinist told us "sometimes you have to ignore the experts and let the engine tell you what it wants".

Eric

This plug reading is for naturally aspirated gas engines. (the other link for alcohol)

NO2 is a little different 'rules', there is a txt on it somewhere.


:)

"So if I'm understanding correctly, the "butt meter" works accurately for fine tuning metering rods, jets and vacuum advance but fails miserably when picking camshafts, cylinder heads and intake manifolds."

I prefer to use "Dairy Queen" tuning for camshafts, cylinder heads and intake manifolds. I measure how far I can do a burnout in a hard right hand turn leaving the DQ parking lot at WOT. I back that tuning method up with how far it "chirps" the tires on the 1-2 up-shift.

My last tuning method for LOW compression engines with big camshafts on "tight" LSA's is to see if they will spin the tires when I drop the rear of the car off a floor jack at WOT........

Stoichometric for e10 is 14 to 14.1, not 14.7. But you're never sure of what the actual mix you get is unless you test it.

With e10, high output ignitions, and SCRs above 10.0:1, it's pretty worthless reading plugs these days. (exception = specs/particles)

My efi tunes started very conservative, and I am creeping up on idle/cruise/WOT AFR numbers. Currently, my idle is 14.4, my higher vacuum cruise too, mid-to-high vacuum cruise is 14.20-13.20, WOT is 12.5. But I know it's still conservative and needs work.

Have to also change timing as you change AFRs, so it's not as easy as just doing one or the other in entirety and move to the next.

Dyno number give you a 'baseline', but in-use is much different. I see so many setting up strictly based on dyno numbers and get in trouble. Can't do that if you want it to live.

Most dyno shops tune for best peak numbers, variables such as gas quality, altitude, and load are not taken into account.


.

I find the dyno extremely helpful.

My biggest benefit is finding wot timing and where the engine likes it.
From there I can fine tune the curve once in the car. I've never really found a need to change the total because I dyno them at sea level, which is where the track is anyway, but I drive them most of the time at higher elevation where engines typically want even more timing, I just prefer not to add any more.

Fuel is a different deal as that changes quite a bit with elevation. On the dyno I really don't see a ton of change anyway going from 12.2 to 13.0. maybe 3-4 HP and I usually don't chase that

If you don't run anything more than max WOT timing it's like running without vacuum advance. You're missing a lot of benefits.

.