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DOHC Spark Plugs
Article by Guy Croft

From: Guy Croft
Email: croftengines@aol.com
Date: Fri Jun 16 05:24 PDT 2006

This message was in response to using expensive "designer" spark plugs in the DOHC engine. It is used with Guy's permission.

The spark plug must be selected to suit the combustion regime. The wrong temperature range will cause the engine to run badly. Let me expalin in bit more detail. The optimum temperature range for the electrode end of gasoline engine plugs with copper core (most plugs are these days, NGK thought of it first) is between 450 and 870 deg C. Below 450 deg C the plug will foul and above 870 it will overheat.

If the plug overheats it will cause pre-ignition by glowing and setting off combustion - before spark ignition takes place. And the exposed plug will blister and melt.

If the plug fouls - in other words - won't self clean, ie: it will soot/wet up and it is liable not to fire at all, due to leakage of the spark to earth across the nose, or else it will fire weakly/intermittenly and anyways the combustion energy will be reduced, less power, poor acceleration, high gasoline use.

Highly tuned engines are burning more fuel-air on each firing cycle and the in- cylinder temp is higher than a road car. They need plugs capable of withstanding high in-cylinder temperatures. In F1 engines, and high boost units, very high indeed.

A 'hot' plug - one which runs hot - is one with a long insulator nose - the high purity alumina (ceramic) that surrounds the center electrode. A long nose gives a big area to absorb heat and a long heat dipsersion path.

A cold plug has the opposite. If you put a hot plug in a race engine it will give preignition problems, and if you put a race plug in a road car it will foul up most of the time.

The worst case is to have the wrong plug AND say, wrong ignition timing, too lean mixture, inadequate cooling, very hot intake air, heavy combustion chamber deposits - these add up to a recipe for cylinder head and piston damage. Rule of thumb is a well running plug should have a brown or light grey insulator.

It IS important to have a clean plug thread and seat and make sure the plug is torqued up properly - because this joint is the only heat loss path from the plug to the coolant.