Tech Tip: Spark plug selection

One of the most misunderstood tuning habits is reading spark plugs.  For the in-depth discussion of this topic, refer to Dan’s paper presented to the Automotive Engine Technology Conference (found on the tech page).  Here’s the short version:  a rich engine can look lean when the plug is too hot.  Conversely, a low compression engine can look rich on the plug when it actually has the correct air/fuel mixture if the plug is too cold.  So, here’s a quick guide to spark plug heat range selection: 

9:1 compression = 59 to 61 heat range 

10:1 compression = 57 to 59 heat range

11:1 compression = 57 heat range

12:1 compression = 57 heat range

13:1 compression and up = 55 heat range

Please note that these heat ranges are Champion’s numbers and can be easily converted to the matching plug for any major brand.

Extended tip plugs (indicated by the Y in Champion numbers) are never harmful as long as you have piston clearance.  There is really no magic here unless you are covering up for a poor chamber design or bad plug location.