post #1 of Old 02-08-2011, 03:46 PM Thread Starter
keepishop
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Learn me about hot and cold spark plugs!

I want to know about the effects of hotter and colder spark plugs. All I can find so far is that too cold will result in fouling, and too hot will cause detonation. What is the actual difference in purpose? Are they made for different climates? Compression ranges? Types of driving? Do some perform better cold and some better hot?

Any old spark plug gurus here?



Current: Black 89 limited, dakota bastard pack, zj coils, 1.5" lift, open cooling swap, bronco 2 carrier, rad fan switch
Rest in Pieces: Red 1990 cherokee dana30/8.25 - v8 zj coils & s10 bastard pack 2.5" lift - 31x10.5r15 tires - v8 zj tierod

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post #2 of Old 02-08-2011, 04:31 PM
5-90
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Look at the "business end" of the spark plug - see that porcelain bit inside the metal shell?

The difference between a "hot" and "cold" plug is the length of that porcelain nose. A "hot" plug has a longer nose to retain heat; while a "cold" plug has a shorter nose to drain it away.

A "hot" plug is usually used when you have a good deal of carbon fouling, slight oil fouling (be careful here!) or some fuel fouling - the additional retained heat helps to burn the working end of the plug clear.

(I note the care needed with oil fouling - oil getting into the cylinder lowers the overall octane rating of the air-fuel mix, which can lead rapidly to autoignition/detonation.)

A "cold" plug is generally used with forced induction - since compressing air heats it up. This lessens the effect of the retained heat in the spark plug working end, reducing the likelihood of autoignition/detonation due to retained heat.

Any changes in spark plug heat range should only be done one step at a time, and you want to run it for a couple of weeks of daily service to make sure it's not causing any other trouble (particularly if you go one step hotter - it's easy to wander into detonation problems.)

Monitor the colour of the porcelain on the working end of the plug - when everything is running properly, there should be very light deposits and a pale straw yellow colour to the insulator. Learning how to "read" spark plugs is a useful skill - I refer you to Chilton's manuals, which tend to have a very useful colour plate giving this information (it's the same in all manuals - so just find one and study it.)

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post #3 of Old 02-08-2011, 04:54 PM Thread Starter
keepishop
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Will plugs running too hot prevent good spark?


Current: Black 89 limited, dakota bastard pack, zj coils, 1.5" lift, open cooling swap, bronco 2 carrier, rad fan switch
Rest in Pieces: Red 1990 cherokee dana30/8.25 - v8 zj coils & s10 bastard pack 2.5" lift - 31x10.5r15 tires - v8 zj tierod

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post #4 of Old 02-09-2011, 09:10 PM
chris142
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Quote:
Originally Posted by keepishop View Post
Will plugs running too hot prevent good spark?
No. The term hot or cold in spark plug talk means how hot the end of the spark plug stays when the engine is running. Too cold and it will get fouled by soot. Too hot and it can cause a hole to be burned in the piston because the gas starts to burn before the plug sparks.

The temp of the plug has no bearing on the spark produced.
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