Since there are many things that can affect a piston, KB continues to put articles on its website that can assist you in managing your engine to its optimum performance. We found this article regarding spark plugs put out by AERA (January 2006 Shop Talk publication). AERA authorized KB to reprint this article for our website users.
What they can tell you?
Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused. Spark plugs are the "window" into your engine -- your only eyewitness to the combustion chamber -- and can be used as a valuable diagnostic tool. Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine's performance. The experienced tuner can analyze problems, or determine air/fuel ratios.
Each set of spark plugs removed from a racing engine should look alike in color and condition. Any difference in color or condition among a set of plugs is an indication that the combustion chamber temperatures or air/fuel ratios are not the same in every cylinder or that related engine components need attention.
If there are differences in the color or condition, they can generally be traced along with inspection of the respective systems to the following information:
Unequal cooling of the combustion chambers
Steam pockets, leaking headgaskets, cracked radiator header tank, water pump capitation, insufficient radiator area or restricted water passages.
Fuel induction system / unequal distribution
Unbalanced multi-carbs, throttle valves out of sync, vacuum leaks in the induction systems, disturbances in air flow, improper ram tube length, exhaust obstruction, dirty fuel, unequal float levels, fuel pressure out of spec, dirty injection nozzles, air cleaner restrictions of mismatched intake manifold runners to gaskets.
Weakness in the ignition system
Weak battery, inoperative alternator, misadjusted ignition dwell, points out of sync, bent distributor shaft, distributor cap arcing, defective rotor, cross fire, defective spark plug leads or resistors, corroded switches of wiring connections on both positive and negative connections.
Poor oil control
Rings not seated; broken rings; worn valve guides, valve stems or seals; excessive oil pressure or improper crankcase ventilation.
Unequal spark timing
Piston to deck heights, rod-crank-piston tolerances not within spec, leaking cylinders defected or unseated piston rings, unequal combustion chamber volume and improper valve lash.
How long does it take to color a spark plug?
This question cannot be answered by any rule of thumb. The coloring process depends upon the heat range of the plug, the position of the plug with the cylinder, type of fuel used, richness of the moisture and gap style of the plug. As you can see, a lot depends on the color of the plug so that it can be read.
Reading spark plugs in a racing application is different that reading automotive plugs. In racing we are concerned with the coloration (fuel ring) at or near the base of the insulator as it enters the shell, and the condition of the electrodes.
We do not regard plug core nose coloring as an actual means to interpret heat range. In racing, the time function, or mileage, may be insufficient to color a plug compared to that of a street application as mileage is adequate for the street car to achieve the full coloring process.
In some cases, brown plugs present a deception. If oil control is poor, the plugs will often develop a brown coloration. With practice and proper magnification tooling, usually 4-6 power, plug firing end conditions will allow the engine tuner to measure the conditions and performance of a race engine and pinpont any shortcomings that may exist.
When read properly, the spark plugs will help the tuner determine if the engine is operating at maximum potential and efficiency. They will also reveal if the ignition system, oil control, ignition timing and fuel delivery are correct.
Precise telltale reading require knowledge of which plug was removed from which cylinder, and which carburetor (Venturi) or injector feeds which cylinder. Keeping the spark plugs with the numbered cylinder will help to determine if there is a fuel delivery problem or any other problem in that cylinder. To get a good reading on a spark plug, some plug manufacturers state that the ignition must be cut off at full throttle and the clutch disengaged with sufficient momentum to coast into the pits or to a stop near the end of a drag strip. A car that is driven back to the trailer or pits can erase all meaningful plug readings.
Acceleration out of a corner or down the drag strip and general engine response in the corner should be relayed back to the crew from the driver and tied in with the spark plug analysis. (NOTE: If every spark plug is not the same general condition or color, the engine is not operating at peak efficiency).
Plug conditions are illustrated on the inset (above right). This should help in undertanding what the spark plug should look like either running on gasoline or alcohol.