Tech Tip: Spark plug selectionvdl
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.