Spark Plug Cross Reference Chart
The following are the most popular spark plugs for use in Briggs flathead racing engines today. There are many other brands and specifications available that meet our application; please be sure to consult with your engine builder on what his experience has lent him to prefer for use in your engine.
Heat Range: The heat range of a spark plug determines your engine's cylinder head temperature to a point. A hotter spark plug will increase cylinder head temperatures, while a cooler range plug will yield cooler cylinder head temps. This is accomplished through the induction and transfer of heat through the porcelain around the center electrode and ceramic built into the plug itself. As a general rule, each plug heat range is worth 10-15* of cylinder head temp. That is, if you go one step cooler on the heat range of spark plug you are using, you will likely see a drop in cylinder head temperatures by ten to fifteen degrees.
Reading a plug: Much to do is made about "reading" a spark plug. What this is referring to is examining closely the color and burn pattern on the plug itself as it relates to the combustion in your engine. You'll typically hear someone talk about a preferred "light tan" color for proper engine tuning. While this is somewhat true, is is also a bit misleading. More importantly, when we are examining a spark plug from an alcohol fueled engine, we are concerned with the heat transfer on the metal ground strap and the bottom of the threads of the plug, or the base of the plug itself. While the light tan color is easily read on a gasoline fueled engine, (ie Nascar), on an alcohol engine, the rich fuel mixture and cleansing of alcohol often times washes any color from the white porcelain surrounding the center electrode of the plug. Keep in mind that when the guys at Nascar do a plug check, they will typically push in the clutch near peak torque in the rpm band (about halfway down the straightaway), and kill the ignition and coast back to the garage area to get a "clean burn" reading on the plug. If you think about it, this makes a lot of sense, since at an idle, engines will run rich and give a false reading or sense of color on the plug.
While this practice is somewhat impractical with a kart racing engine at the track, we do occasionally do plug checks in the dyno cell here at the shop. Again, don't try to read too much into the plug check. Look for pre-detonation concerns or other issues that may lead to catastrophic failure. Also, don't be overly concerned with monitoring cylinder head temperature...at best it is an insurance against an engine meltdown. A true idea of what is going on inside the combustion chamber can be found by monitoring exhaust temperature, (or egt).
If you have any questions, please feel free to contact us directly.
Carlson Racing Engines / Vector Mfg.
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