Post by Chuck on Feb 18, 2012 1:22:38 GMT -5
Sparkplug Heat Ranges
I have been hearing some talk at the track about adjusting heat ranges of plugs as the temperature warms up this spring/summer. Any thoughts or insights about which way they are going, hotter plug with warmer temperature or cooler plug with warmer temperature?
First, I think its important to understand what the heat range specification for a spark plug really means.
When a plug is rated as cold, this rating is NOT a indication of the intensity of the spark. This rating is a indication of how well the plug can transfer heat from the electrode itself to the head. A colder plug transfers more heat from the electrode tip to the head, therefore the business end of the plug,(electrode), is at a colder temperature PRIOR to the time the ignition attempts bridge a spark between the gap. If this colder plug is sooooo cold, (its relative), that the fuel mixture can't be vaporized from the surface of the electrode and insulator, (from the heat generated from the previous strokes fire), PRIOR to the application of the spark, the potential that is supposed to jump across the gap will bleed off by conduction thru the wet surface across the electrode and ceramic to the head. The result is a fouled plug. And the converse, a plug that is too hot will retain too much combustion heat and tend to pre-ignite the fuel mixture before the ignition fires. This is pre-ignition, and causes detonation.(very, very bad). Just a few seconds at the rpm we run will cause destructive failure of the engine.
The very best way to look for this is via the use of a magnifying flashlight. Look for aluminum deposites on the ceramic insulator, especially down around where the ceramic meets the metal. If there are deposits of aluminum, this is a indication of a lean mix. These deposites are actually piston/head material being broken off during mild detonation. I say mild because anything more and you wouldn't need to check the plug. Since we can't alter the amount of air,(O2), injected into our engines, the only thing that will affect the leaness of a fuel mixture is the air-density.
The adjustments we make with our jets to obtain that same great performance we had last week are just compensating for the changes is air-density.(a temperature compensated measurement of oxygen contained in a given volume of air). The greater the O2, the more fuel needed to get the best performance. The more of both,(at correct ratios), we can get, the more power we will make.
This air-density measurement also plays a part in plug selection.(hot or cold). On a day that is colder, the fuel/air mixture is much colder and can then cool the plug to such a extent that the wet mixture can't be vaporized before the spark arrives. The plug fouls and your out in the grass watching instead of racing.(been there)
It is the opinion of racers everywhere, in all forms of racing that you want to run the coldest plug possible that won't foul while operating at lower cylinder temps. This way the chances of going into pre-ignition/detonation are less.
There is also a (SMALL) side benefit of colder plugs. A colder plug will take up more combustion chamber area because of the greater amount of ceramic around the electrode. (Take a look at a W-27 and a W-31 sometime). This is a plus for spec classes since the cc's are checked with the plug removed.
I have been hearing some talk at the track about adjusting heat ranges of plugs as the temperature warms up this spring/summer. Any thoughts or insights about which way they are going, hotter plug with warmer temperature or cooler plug with warmer temperature?
First, I think its important to understand what the heat range specification for a spark plug really means.
When a plug is rated as cold, this rating is NOT a indication of the intensity of the spark. This rating is a indication of how well the plug can transfer heat from the electrode itself to the head. A colder plug transfers more heat from the electrode tip to the head, therefore the business end of the plug,(electrode), is at a colder temperature PRIOR to the time the ignition attempts bridge a spark between the gap. If this colder plug is sooooo cold, (its relative), that the fuel mixture can't be vaporized from the surface of the electrode and insulator, (from the heat generated from the previous strokes fire), PRIOR to the application of the spark, the potential that is supposed to jump across the gap will bleed off by conduction thru the wet surface across the electrode and ceramic to the head. The result is a fouled plug. And the converse, a plug that is too hot will retain too much combustion heat and tend to pre-ignite the fuel mixture before the ignition fires. This is pre-ignition, and causes detonation.(very, very bad). Just a few seconds at the rpm we run will cause destructive failure of the engine.
The very best way to look for this is via the use of a magnifying flashlight. Look for aluminum deposites on the ceramic insulator, especially down around where the ceramic meets the metal. If there are deposits of aluminum, this is a indication of a lean mix. These deposites are actually piston/head material being broken off during mild detonation. I say mild because anything more and you wouldn't need to check the plug. Since we can't alter the amount of air,(O2), injected into our engines, the only thing that will affect the leaness of a fuel mixture is the air-density.
The adjustments we make with our jets to obtain that same great performance we had last week are just compensating for the changes is air-density.(a temperature compensated measurement of oxygen contained in a given volume of air). The greater the O2, the more fuel needed to get the best performance. The more of both,(at correct ratios), we can get, the more power we will make.
This air-density measurement also plays a part in plug selection.(hot or cold). On a day that is colder, the fuel/air mixture is much colder and can then cool the plug to such a extent that the wet mixture can't be vaporized before the spark arrives. The plug fouls and your out in the grass watching instead of racing.(been there)
It is the opinion of racers everywhere, in all forms of racing that you want to run the coldest plug possible that won't foul while operating at lower cylinder temps. This way the chances of going into pre-ignition/detonation are less.
There is also a (SMALL) side benefit of colder plugs. A colder plug will take up more combustion chamber area because of the greater amount of ceramic around the electrode. (Take a look at a W-27 and a W-31 sometime). This is a plus for spec classes since the cc's are checked with the plug removed.