SPARKING PLUGS.

Press, Volume LXVIII, Issue 20499, 18 March 1932, Page 6

SPARKING PLUGS.

THEIR INLUENCE ON ENGINE PERFORMANCE. For several years past, tlie trend of engine design has been towards higher speeds and higher compression. Theso changes have imposed a tremendous task upon the sparking plug manufacturer, to design and build plugs which will function under the enormously greater heat conditions that are set up. No motor can operate successfully beyond the capacity of its sparking plugs to stand up under the heat and strain imposed. Every plug has its limitations; that is why it" is essential to .make different models as each type has been designed to give service in a certain type of motor, and under conditions 'which are normal for that motor. 3 Plugs arq grouped into two classes, which are termed "hot" and "cold" (sometimes referred to by racing drivers as "soft" and "hard" respectively). (a) A "hot" plug is designed to oporate in a comparatively cool, slow-running, low-compression motor which has a tendency to foul sparking plugs. -(b) A "cold" plug is designed to function a high-compression, fast engine, having a shorter heat path (i.e., the distance from the tip of the insulator'to the lower copper gasket) allowing the heat to bo more rapidly dissipated into the cooling medium. The application of "hot" or "cold" plugs to. any engine depends both on the heat characteristics of the engine and the services required. If, for any reason, the engine gives unsatisfactory performance with the plugs fitted as standard equipment, the engine should be checked for the following items before concluding that different plugs are needed: (a) Compression, (b) Distributor points. (e) Spark timing. (d) Valves—action, timing, tappets. (e) Grade of petrol used., (f) Spark gap setting. If these items are checked and the plugs, after a hard run, show a tendency to continue firing after switching off, change to the next "colder" type of plug. If, on the other hand, the tendency is towards fouling, a "hotter" plug should bo used to meet tho particular conditions of service. Finally, a new set of sparking plugs fitted after 10,000 miles' service (an average year's driving) will be found to restore power and speed, give proper acceleration, and eventually pay for themselves in the saving of fuel, to say nothing 'of the satisfaction that coiues with smooth-operation.