SPARK PLUG DESIGN Every year the range of NGK spark plugs grows to accommodate the ever increasing demands of modern engines. Spark plug design must take into consideration many features of an engine including physical dimensions; combustion chamber shape; cooling abilities; fuel and ignition systems. Spark plugs play a vital role in producing the maximum power from an engine whilst keeping fuel consumption and emissions to a minimum. Choosing the correct spark plug type will help a vehicle manufacturer meet legislated emission targets and assists the motorist in getting the best from their engine. Increases in the size and the requirement to improve the cooling of the inlet and exhaust valves have meant that the space available for the spark plug is severely restricted on some cylinder heads. A change in spark plug design; possibly the adoption of a taper seat and extended reach (threaded portion) or even the use of a smaller diameter is often the answer. Some engines require the use of two spark plugs per cylinder and again due to space restrictions these might be of different sizes. Changes in fuelling systems and the fuel itself have meant some special features being adopted at the firing end of the spark plug. Extra projected types push the spark position into the heart of the combustion chamber to promote better combustion of the fuel/air mixture; which is weaker than ever in an effort to improve economy. Modern engines manufacturers often require increased spark gaps to allow a longer spark duration; which again aids more efficient combustion. To combat the cold fouling effect that can occur in some engines using unleaded fuels specially arranged ground electrodes are sometimes used. These can force the spark to discharge across the insulator and thus burn away any built up carbon that could cause poor starting or misfires. It is not uncommon to find two; three or four ground electrode arrangements in new vehicles. The use of precious metals on the tips of