Re: H Plugs! Hot or Cold?

Posted by Phil Auten on January 06, 2000 at 19:09:05 from (206.175.72.226):

In Reply to: H Plugs! Hot or Cold? posted by john on January 06, 2000 at 15:49:09:

John,
I'll try to explain this. A "cold" plug is slightly recessed into the threaded portion of the plug so it runs "cold" compared to a plug with a "hot" heat range which has a longer insulator and center electrode to extend it out into the combustion chamber. This causes the heat generated by the burning fuel to have to travel further through the center insulator before it reaches the head where it is carried away to the radiator by the coolant. The "hot" plugs ignite the fuel/air mixture more easily than the "cold" plugs. The difference in length between the center electrode of the coldest plug and the hottest plug for the same application is usually less than 1/4". There may be 4 heat ranges within that 1/4". In cars, one that gets all it's driving done on freeways might use a "cold" plug as it will usually be operating at optimum temperature. The same car, if driven mainly for short trips at low speeds might use a "hot" plug to make better use of it's ability to ignite the fuel mix that a "cold" plug might not be able to fully ignite, thus improving performance. Does this help any? The other thing that is often done is to substitute a "hotter" plug where the "normal" plug keeps getting fouled due to incomplete combustion forming those sooty carbon deposits. I've seen plugs foul in just a couple of minutes in this situation. Replacing Champion D-15 plugs with D-17's or D-21's can solve this problem. However, care must be taken to watch for signs of detonation caused by the spark plug's tip becoming too hot and firing the mixture too soon. This can lead to burned valves or pistons, which is NOT desirable! So there are many considerations to choosing the correct spark plug for your particular need.

Phil

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