Classic Inlines
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Spark Plug Readings

Being able to "read" a spark plug can be a valuable tuning aid. By examining the insulator firing nose color, an experienced engine tuner can determine a great deal about the engine's overall operating condition.

In general, a light tan/gray color tells you that the spark plug is operating at optimum temperature and that the engine is in good condition. Dark coloring, such as heavy black wet or dry deposits can indicate an overly-rich condition, too cold a heat range spark plug, a possible vacuum leak, low compression, overly retarded timing or too large a plug gap.

If the deposits are wet, it can be an indication of a breached head gasket, poor oil control from ring or valve train problems or an extremely rich condition - depending on the nature of the liquid present at the firing tip.

Signs of fouling or excessive heat must be traced quickly to prevent further deterioration of performance and possible engine damage.

Here's two more great articles: Article 1 - Article 2

Heat Range Reference

  • NGK - 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 - The larger the last digit in the part 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 - Typically Champion numbers in the same series the larger the number the hotter the plug.

Normal Life
A worn spark plug not only wastes fuel but also strains the whole ignition system because the expanded gap (due to erosion) requires higher voltages. Normal rates of gap growth are as follows: (0.000126~0.00252 inches/1,000 miles).

Normal Condition
An engine's condition can be judged by the appearance of the spark plug's firing end. If the firing end of a spark plug is brown or light gray, the condition can be judged to be good and the spark plug is functioning optimally.

Dry and Wet Fouling
Although there are many different cases, if the insulation resistance between the center electrode and the shell is over 10 ohms, the engine can be started normally. If the insulation resistance drops to 0 ohms, the firing end is fouled by either wet or dry carbon.

Overheating
When a spark plug overheats, deposits that have accumulated on the insulator tip melt and give the insulator tip a glazed or glossy appearance

Deposits
The accumulation of deposits on the firing end is influenced by oil leakage, fuel quality and the engine's operating duration.

Lead Fouling
Lead fouling usually appears as yellowish brown deposits on the insulator nose. This can not be detected by a resistance tester at room temperature. Lead compounds combine at different temperatures. Those formed at 370-470°C (700-790°F) having the greatest influence on lead resistance.

Breakage
Breakage is usually caused by thermal expansion and thermal shock due to sudden heating or cooling.

Abnormal Erosion

Abnormal electrode erosion is caused by the effects of corrosion, oxidation and reaction with lead - all resulting in abnormal gap growth.

Erosion, Corrosion and Oxidation

The material of the electrodes has oxidized, and when the oxidation is heavy it will be green on the surface. The surface of the electrodes are also fretted and rough.

Lead Erosion

Lead erosion is caused by lead compounds in the gasoline which react chemically with the material of the electrodes (nickel alloy) as high temperatures; crystal of nickel alloy fall off because of the lead compounds permeating and separating the grain boundary of the nickel alloy. Typical lead erosion causes the surface of the ground electrode to become thinner, and the tip of the electrode looks as if it has been chipped.

Oil Fouled
Oil Fouled
Carbon Fouled
Too Cold
Too Cold
Cold or Rich
But OK
Cold or Rich
But OK
Cold or Rich
But OK
OK
OK
OK
OK
Better
Best
Better
Good
Good
Good
OK
OK
Hot
But OK
Hot or Lean
Questionable
Hot or Lean
Questionable
Hot or Lean
Questionable
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
Too Hot or Lean
Pre-Ignition Range
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Other related articles:
Understanding Lobe Centers
- Static vrs Dynamic Compression
Cam Installation Tips - Adjusting Your Valves






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