Reading spark plugs

[TZ350]

This is some useful info I scraped of the net.

http://www.carcraft.com/howto/116_07...eat_range.html

Choosing The Right Spark Plugs For Your Custom Engine
Unleaded fuels do not color plugs as clearly as leaded fuels, so you have to look closely for the A/F ratio ring at the bottom of the center porcelain. The big things to look for are tiny flecks of aluminum that have transferred to the ceramic from the piston or perhaps the combustion chamber during an oh-too-lean condition that has caused aluminum to melt.

Ignition timing plays a huge part in terms of spark-plug temperature. The best place to read this is on the spark plug's ground strap.

The tip of the ground strap will tend to color first, and then, as temperature increases, this coloring (called annealing) will travel down the length of the strap toward the shell. An ideal ignition-timing temperature range for a given spark plug will create a light blue ring around the strap about halfway down its length.

If the ground strap shows no coloration, the engine could respond to more timing. If the ground electrode begins to color in blues and greens or the tip begins to melt, the timing is over-advanced. Of course, if you are tuning at the dragstrip, these conditions will have raised red flags in terms of lost e.t. or trap speed as well.


The Demands of Superchargers, Turbos, and Nitrous Power-adders do a fantastic job of increasing cylinder pressure and heat. This also places much greater demands on the spark plugs. Nitrous is perhaps the most abusive because of its reputation for extremely fast cylinder-pressure rise time. Some rules of thumb include running a colder heat range and using nonprojected-nose plugs.

The general consensus among power-adder tuners is that it's best not to use platinum plugs, since this material acts as a kind of heat sink that will cause pre-ignition problems.

Other tips include running a tighter gap, since high cylinder pressures place much greater demands on the ignition system. The best plug gaps are the largest ones that will survive; start with 0.030 inch.

For nitrous, a safe recommendation is one step colder for every 75 to 100hp increase in power. So, for a 300hp engine adding 200hp of nitrous, this would mean using plugs that are two steps colder. This is also a good starting point for similar power increases either normally aspirated or with a supercharger or turbo.

Specialty Plugs
This is the age of designer spark plugs. SplitFire rode the first wave of specialty plugs with its V'd ground strap, and now it seems that every spark-plug manufacturer has its own version of what's hot. While the debate will continue to rage over whether diamond-shaped or split-V ground straps are worth more power/mileage/emissions, we do know that plugs using more exotic metals enhance durability. Platinum arrived a few years ago, with Bosch leading the charge with three different versions of its platinum-tipped spark plugs. Then NGK upped the ante with its Iridium IX series of plugs. The plan with virtually all these designs revolves around reducing tip or ground-strap erosion that increases the gap between the center electrode and the ground strap. Some plugs, like the Bosch Platinum +4, use four ground straps to also minimize the overall gap to ground.


The ground strap is a great heat indicator. Over-advanced ignition timing can cause the ground strap to discolor and in extreme cases can cause the strap to melt.

Learning to read spark plugs can start with looking at the three tiers of engine operation as indicated on the plugs' porcelain insulators.

Mixture A/F Ratio

A light tan ring near the bottom of the porcelain (arrow) is an indication of A/F ratio with unleaded fuel. This is a very subtle trend, so you should also use other measuring devices, like a wide-band oxygen sensor and/or quarter-mile trap speed numbers as reinforcing evidence.

Ignition Timing

Ignition timing plays a huge part in terms of spark-plug temperature. The best place to read this is on the spark plug's ground strap. The tip of the ground strap will tend to colour first, and then, as temperature increases, this colouring (called annealing) will travel down the length of the strap toward the shell.

An ideal ignition-timing temperature range for a given spark plug will create a light blue ring around the strap about halfway down its length. If the ground strap shows no coloration, the engine could respond to more timing.

If the ground electrode begins to colour in blues and greens or the tip begins to melt, the timing is over-advanced. Of course, if you are tuning at the dragstrip, these conditions will have raised red flags in terms of lost e.t. or trap speed as well.

Too much ignition timing can cause detonation. This is most easily read on a spark plug's ceramic insulator as black speckles.

Pre-Ignition

Here's what happens when you run a too-hot plug in an E85-fueled engine at 12.5:1 compression. E85 is a very pre-ignition-sensitive fuel, and this plug literally melted. On E85, Champion recommends plugs three steps colder than a comparable gasoline heat-range.

The rest of the story is here! http://www.carcraft.com/howto/116_07...eat_range.html

[Skunk]

Good info TZ350.
I have a two stroke plug reading chart on my website. May help some too.

[TZ350]

Skunk! your chart makes very interesting reading. It's good to see all the plugs laid out like that.

[gammaguy]

if you are really serious about 2T tuning,you also need to read pistons

[speedpro]

I've found that if it goes real quiet about half way down the straight that the engine is either a bit lean or a bit advanced, or both. Reading the piston as suggested has always supplied clues as to what went wrong. Ash white with a hole in the middle, coating the inside of the exhaust, and/or coating the plug are all clues.

[bucketracer]

.
Yes....... But clues to what????......

[gammaguy]

clues to the mixture.its the most important thing in a 2T.there should always be a small amount of unburnt carbon on the piston crown,a cleanly burned top denotes too lean a mixture or ignition issues.if its burning clean,its close to detonation

[CM2005]

I've found that if it goes real quiet about half way down the straight that the engine is either a bit lean or a bit advanced, or both.

or you've run out of gears!!

[Buckets4Me]

have you seen his avatar
it went quiet then
but nobody could find the piston to read it

[TZ350]

if you are really serious about 2T tuning,you also need to read pistons

Yes your right. Easy to do on a two stroke. Can read things like rich/lean end gas detonation, transfer port flow.

But if you rely want to know if your getting close to the max combustion temperature for your engine look under the piston.

The colour of the oil baking onto the underside of the piston crown tells you a story. But once the oil starts to chare you have gone to far.

A plug can tell you about ignition advance too.