Spark Plug Reading


Reading Spark Plugs

Thanks to Larry Meaux of Meaux Racing for providing this information and pictures.

Plugs are best viewed with a 10x power illuminated magnifying glass.

Heat Range:

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 too hot.

(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 a 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 the porcelain is "BONE-WHITE", jetting is still TOO RICH !!!
NOTE: Do not look at the porcelain to read jetting !!!


Porcelain = the porcelain 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 porcelain for tiny black specks or shiny specks of aluminum that have fused to the porcelain. 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 blow, pistons can get torched with melted sides and 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 porcelain 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 aluminum coming off the pistons that will be fused to white porcelin,....the next step to be reached is occasional pieces of the porcelain 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 porcelain - "ditch" that encircles the center electrode, will also show up early signs of preignition/detonation and the wrong heat-range. Look for signs of the ditch beginning to be filled up with melted porcelain, you will need a 5x or 10x magnifying glass.

Reading Alcohol Spark Plugs

full-turn = richer , safer, almost all air is used

full-turn thats darker= richer , plenty safe or maybe too rich
darker means not enough heat to burn deposits off
and/or the soot of rich combustion is leaving its color deposited onto base-ring.
Thicker layer of darker soot all the way around the base-ring

3/4 turn= leaner than full-turn, leanest you should ever be,
thinner soot deposits

1/2 turn= leanest ... edge of preignition / detonation, almost all fuel is burned
excessive heat is burning off thinner soot deposits

courtesy of
Meaux Racing Heads
9827  LA  Hwy.  343
Abbeville,  Louisiana  70510

Racing Spark Plug Cross Reference Chart



Autolite Racing PN
NGK Stock Number
AR12 RV9YC R5674-7 3535 
AR13 RV9YC R5674-6 4449 
AR23 RV9YC BPR6FS 2623 
AR24 RV12YC BPR6FS 2623 
AR25 RV15YC4 UR4 6630 
AR32 RF9YC AP8FS 2227 
AR33 RF9YC AP7FS 2127 
AR50 N59YD R5672A-10 3395 
AR51 N61YD/N6YC R5672A-9 7405 
AR52 N63YD/N7YC R5672A-8 6564 
AR53 N9YC BP6ES 7333 
AR72 J12YC R5670-7 2891 
AR73 J12YC R5670-6 6659 
AR74 J12YC R5670-6 6659 
AR92 S57YC R5724-10 7993 
AR93 S59YC R5724-9 6434 
AR94 S61YC R5724-8 4175 
AR103 RS9YC BPR6EFS/TR6 3623 
RF10C AR6FS 3323 
AR131 V55C R5673-10 4050 
AR132 V57C R5673-9 3442 
AR133 V59C R5673-8 3249 
AR134 V63C R5673-8 3249 
AR135 V63C R5673-7 4367 
RV8C R5673-6 4256 
N3C R5671A-7 4091 
AR472 S57C B9EFS 1085 
AR473 S59C B9EFS 1085 
AR474 S61C B8EFS 1049 
AR764 RS12YC TR5 2238 
AR2592 C59 R5671A-10 5820 
AR2593 C61 R5671A-9 5238 
AR2594 N2C B9ES 2611 
AR3910 C59YC R5672A-9 7405 
AR3911 C61YC R5672A-8 7173 
AR3923 RC9YC BKR6E 6962 
AR3924 RC12YC FR5 7373 
AR3931 C55C R5671A-11 6596 
AR3932 C57CX R5671A-10 5820 
AR3933 C59CX R5671A-9 5238 
AR3934 C61CX R5671A-8 4554 
AR3935 C63C R5671A-7 4091 
L87YC BP6HS 7331 

AC Delco to NGK Cross Reference

NGK Stock
40TS R5674-7 5034
41XL R5671A-8 4554
42S R5670-6 2746
42TS R5674-6 4449
436T R5673-9 3442
436TS R5674-9 6468
436XLS R5672A-9 7405
438T R5673-8 3249
438TS R5674-8 5657
438XLS R5672A-8 7173
43S R5670-6 2746
43TS R5674-6 4449
440T R5673-7 2817
440XLS R5672A-8 7173
44S R5670-5 2298
45S R5670-5 2298
46S R5670-5 2298
C44S R5670-5 2298
M43S R5670-6 2746
M90 R5670-9 3913
MR41T R5673-8 3249
R42S R5670-6 2746
R43S R5670-6 2746
S40XL R5671A-10 5820
S41XL R5671A-9 5238


Heat Range Reference

  • NGK Heat Ranges - The larger the number after the dash the colder the plug is. Example a R5671A-7 (a good street/strip plug) is a cold plug where a R5671A-10 is a very cold plug used for large 400hp and up nitrous and other power adder applications)
  • Autolite Heat Ranges - The larger the number the hotter the plug is. Example a AR3935 (good street/strip plug) is hotter than a AR3932 plug which is very cold used for lots of nitrous.
  • Champion Heat Ranges - Typically Champion numbers in the same series the larger the number the hotter the plug.