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How to read spark plugs

Moparts Member
Posts: 2970
Registered: Mar 2001
posted 06-27-2002 10:54 PM

How to read spark plugs ;
*** Mostly valid only for full-load dyno tests ,
or if engine is quickly turned-off after
finish line, and then towed back to pits,
and spark plugs removed for readings there !

Plugs, best viewed with a 10x power illuminated mag. glass

Heat Range = Ground Strap , the ground strap indicates the
heat-range of the spark plug. If the "color" of
the ground strap "changes" too-close to the ground-strap's
end, (which is above the center electrode), then the
heat-range is "too-cold" , meaning that the strap is
loosing heat too-quickly to the base-ring, and is not
able to burn-off deposits until near its end.
If the "color" of the strap changes near where it is
welded/attached to the base-ring(last thread ring),
then it means that the plug heat-range is "too-hot",
because heat is not being tranferred/cooled from the
strap to the base-ring quickly enough !!!! The strap
might begin to act like a "glow-plug", eventually
causing preignition/and/or detonation later on.
Proper heat-range is when the "color" is at the
half-way point on the strap, neither too-cold or

(Color= meaning the evidence of heat/or lack of heat
by the appearance dark vs lightened color of metal)

Jetting = the air/fuel mixture ratio shows up on
the base-ring (the last thread ring, it has the strap
welded to it).
You want 1 full-turn of light-soot color on the base
ring!!! If you want to tune for max. power, then you
want 3/4 to 7/8ths of a full-turn of light soot color
to show-up on the base-ring, but this is on ragged-edge
of being too-lean, but will make the most HP on most
engines. To be safe, leave it at a full turn of light
soot color.If the base-ring has a full-turn of color,
but there are "spots" of heavy build-up of "dry-soot"
on top of color, then jetting is too-rich .

NOTE> If the base-ring has a full turn of color with
some spots of heavy dry-soot, then jetting is too-rich,
REGARDLESS,if porcelin maybe "BONE-WHITE", jetting is
still TOO-RICH !!!
NOTE> Do not look at the porcelin to read jetting !!!

Porcelin = the porcelin shows up preignition/detonation,
it will not accurately reveal jetting/air/fuel ratios.
To look for the first/beginning signs of detonation,
search the white porcelin for tiny black specks or
shiny specks of aluminum that have fused to the porcelin.
When detonation occurs, part of the air/fuel mixture
explodes instead of burning, the explosion is heard as a
"metallic-knock", this audible knock is the result of a
sound shock-wave, this shock wave travels back and forth
across the clearance volume "disrupting" the cooler
boundary layer gases that cover the entire clearance volume
area,this disruption allows "more" heat to be transferred
into parts, especially, domes/piston tops,...along with the
very rapid rise in pressure like a hammer like blow, pistons
can get torched/melted sides/holes !!! With the early signs
of detonation, the shock-wave will also rattle rings causing
the tiny amounts of oil that now gets by rings, to be fused
to the white porcelin as tiny black specks, also fused as specks
are soot that was clinging to clearance volume surfaces
in the relatively "still-air" of the boundary layer.

One step beyond the black specks,will be tiny specks/balls
of alumimum coming off-pistons that will be fused to white
porcelin,....the next step to be reached is occasional pieces
of the porcelin being broken-off as detonation gets worse,ETC.
Soon after that, are holes, blown head gaskets, broken
connecting rods, ETC.

NOTE> additional signs of beginning detonation are piston
rings. By comparing ring's "free-diameter" to "out-of-box"
free-diameters of new unused rings will reveals beginnings
of detonation before much harm is caused !!! This is a result
of the above previous explanation about heat being transferred
more readily because the of the relatively insulating/cooler
boundary layer being disrupted by the shock-wave.
A piston ring is a simple spring,
when a spring is over-heated it will loose tension.

Center Electrode = the very tiny sharply defined porcelin -"ditch"
that encircles the center electrode, will also showup early
signs of preignition/detonation/wrong heat-range. Look for signs
of the ditch beginning to be filled up with melted porcelin,
you will need a 5x or 10x mag. glass.

The "Ground-Strap" = Heat Range
The "Plug's Base-Ring" = Jetting
The "Porcelain" = signs of preignition/detonation

Moparts Member
Posts: 4164
From: West Palm Bch. Fl.
Registered: Jun 2000
posted 06-28-2002 12:34 AM

where did you run across that? it's good info and i agree with 99 % of it.

Moparts Member
Posts: 1333
From: Trinity, AL. USA
Registered: Oct 2000
posted 06-28-2002 10:06 AM

I like DRAM, agree with most of that. What I don't agree with is reading the strap for heat range. The stripe on the strap is usually more a result of timing than heat range. I know you are posting this because of your recent addittion of oxide. In a nitrous motor you need to pay very close attention to the strap. Nitrous motors already have a fairly cold plug, so you read the strap for timing adjustments. You want the heat line on the strap, right in the bend, if is is closer to the electrode, add a degree or two of timing, if the line is down the strap toward the base ring, take some out. If you have no stripe and the strap shows a blueish tint, you have a good bit too much timing and the gap will likely be closed a little, this shows a great deal of heat on the strap. Conversely, if the strap looks like new with no heat mark, add some timing.
As far as reading the mixture, I pay way more attention to the porcelain than the base ring. You need to read the porcelain at the base(way up in the plug) and you want just a very faint ring of color right at the base of the porcelain. The color on the base ring of the plug is usually due to idle and midrange circuits and may soot the plug here while driving to the lanes. Even if the base ring was sooty, if the porcelain shows no color, I doubt you would be rich.

Monte Smith

New Member
Posts: 82
From: Napa, CA. USA
Registered: Mar 2002
posted 06-28-2002 11:33 AM

So I have been looking at them wrong this whole time? i was checking the porcalien for a tan color which has gotten only slightly tan and I have gone from 72/74 to 78/78 jetting in the 750 Holley VS. Now I,m not sure which way to go. Would a very mild 340 need this much jetting? I realize headers will lean a motor out but??

Moparts Member
Posts: 4261
From: Stinkerdener (Pasadena) TX
Registered: Feb 2000
posted 06-28-2002 01:06 PM

But doesn't todays hot aftermarket ignition systems make the 'art' of plug reading more difficult!? Learning to read plugs is difficult at best. Killing the engine 'clean' at the very end of a run is usually not possible for automatic cars with a reverse shift pattern either. The standard shift guys has us there. An engine that is running rich is a little hard to tell with the MSD being so hot that the plugs stay pretty clean. Ask TOD about that. But it's easier to tell if you are too lean imo. The car picked up better ET when it was jetted down some and the plugs were even whiter. Imo, reading plugs was a lot easier in the day of point systems and much harder now with the great ignition systems we have now.

Moparts Member
Posts: 2126
From: Vacaville,CA
Registered: Nov 1999
posted 06-28-2002 02:09 PM

I think there is some good info in CD's post. However I also do not agree with the heat range and ground strap portion.
I tend to agree with Monte. The ground strap is an indication of timing in the motor. You are looking for the movement of the "thin blue line" on the strap. It is an indication of timing not heat range of the plug. As stated before you want that thin blue line to be at the bend in the strap.

As for the color of the plug all the info here is excellent in both CD and Montes posts.

Zippy is correct. Unleaded pump gas makes plug reading very difficult. They tend to color plugs very little making reading them some what more difficult. On the other hand if you ad octane boosters to your fuel they will leave orange to red color on the plugs.

Keep in mind this info only pertains to dyno pulls or passes at the track. The info is not very usefull if the car is being driven,especially a car with a Holley carb as they are notoriously rich under part throttle and idle.

Another indication for those unsure is MPH at the track. Generally the best MPH will come from the optimum A/F ratio for the engine. If you jet changes result in more MPH you are most likely going the right direction. If however you make a change and the car slows down you may be going the wrong direction. This is a generality but usually true. You still need to check your plugs.

The best time is just after a pass. Under ideal conditions make a pass and shut the car down and pull the plugs on the return road. I know this is not always possible but it is the best way to get an accurate A/F reading on the plugs.

Remember lean is mean but fat is safe

Moparts Member
Posts: 1333
From: Trinity, AL. USA
Registered: Oct 2000
posted 06-28-2002 02:18 PM

Some plugs are much easier to read than others. Autolites hardly ever take a set pattern and are extremely hard to read. Champion race plugs are fairly easy to read, but the best plug by far for your racecar(as far as reading goes) is an NGK. It takes some practice, but you get used to what to look for. As far as neutraling an auto car in the lights, that is tough, but you need to shut the car down as soon as possible, tow back or pull them at the end of the track, replace with another set for the trip back to the trailer. You will need a good plug light, Powerhouse, Childs & Albert and many others make good lights. You can read plugs with unleaded gas, just a little tougher. This is the ONLY true way to tune your engine to the right mixture. EGTs and data loggers can be misleading, as I have seen motors burn up at 1200* and other motors run fine at 1600*. So EGT readings can really only be used as a reference point once you get the motor right. Spark lead can also alter your EGT readings drastically, so as stated, plug reading is the BEST tuning trick you can learn to get the most from your combo.
Monte Smith

Posts: 6973
From: Ohio
Registered: Nov 1999
posted 06-28-2002 03:40 PM

Alot of good info here!!
Heres the mag lite I use from tavia, and theres others also.


Moparts Member
Posts: 2126
From: Vacaville,CA
Registered: Nov 1999
posted 06-28-2002 04:25 PM

IMO you need to pull ALL the plugs. Believe me I know how big a PITA that can be after a run. However it is the only way to get a good handle on how the engine is doing. I am sure you have a cylinder or two that tend to be lean on occasion. I also agree that the NGK is the easiest plug to read..The black coating on the Autolites make reading the timing on the strap difficult at best.
As for best fuel that is tough as they change from area to area. I have seen some fuels that put color on the plugs that make it difficult to read. I assume it is an additive in the fuel. I have seen some bright yellow colors with certain fuels. My opinion is choose the fuel you like and stick with it. I recently learned the hard way on the race car how critical switching fuels can be. We switched from VP to Sunoco(at a higher octane rating) and melted 3 pistons on the same tune. The specific gravities of the fuels can be significantly different from brand to brand. Something I knew but thought we would be ok..Of course we were trying to spray a 400 shot but on the other hand we had no problems with the VP and the same tune up. Just a word of caution.

Moparts Member
Posts: 1944
From: Buffalo, MN
Registered: Apr 2001
posted 06-29-2002 10:21 AM

So would it be best to just make one pass on a set of plugs? There are some tracks around here I can push the car all the way to the burnout box, fire it up make the run and change them at the end of the track(plenty of room)

Moparts Member
Posts: 2126
From: Vacaville,CA
Registered: Nov 1999

Jerico-IMO checking the plugs after every pass is the BEST way to know what is going on with the engine. You will get a feel for when to replace them. We replace the plugs after every pass on the race car but I do not in the Belvedere. I will run a set for a at the track and leave them in for the street driving and then put in new ones when I go back to the track. Still have buddies I cannot get to put in new plugs to go to the track. Then they ask me to look at thier plugs and all I can say is they look like plugs to me ...You need a fresh set to determine timing and AF ratios.

info provided by members listed above.


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