Reading a Spark Plug
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 these symptoms to track down the root cause of many problems, or to determine air/fuel ratios. 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 valvetrain 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.

Gap and Heat Range
Proper combustion depends on several factors, but most importantly upon the plug's ability to ignite the fuel/air mixture completely. The two components of a plug which have the largest impact upon this process are the gap and the heat range.

Gap: the distance between the center electrode and the ground electrode (the bent one). The spark travels across this distance to ignite the fuel/air mixture. The precise distance is determined by the engine manufacturer and is dependent upon the size of the cylinder and other factors. An improper gap setting will result in misfires and a general decrease in combustion efficiency. The correct gap setting can be found in the engine manual.

A spark plug's voltage requirement is directly proportionate to the gap size. The larger the gap, the more voltage is needed to bridge the gap. Most experienced tuners know that opening gaps up to present a larger spark to the air/fuel mixture maximizes burn efficiency. It is for this reason that most racers add high power ignition systems. The added power allows them to open the gap yet still provide a strong spark.

With this mind, many think the larger the gap the better. In fact, some aftermarket ignition systems boast that their systems can tolerate gaps that are extreme. Be wary of such claims. In most cases, the largest gap you can run may still be smaller than you think.