38racing wrote:so confusion is understandable when my supplier list for BPR6ES has Denso W20ETR-S yet for BP6ES it has W20EX-U , the latter being consistent with what you have in the R version. And what about the P versus non P for those engines?
"P" is extended tip - the insulator nose (and ground electrode) extend out from the threaded portion of the plug, commonly used where the combustion chamber is "dome" shaped - it sticks your spark a little further out than a non-P plug will, it can have a noticeable impact on engine performance and fuel efficiency, so it pays to use the correct plug.
The thing about supplier and spark plug cross references is - not all plugs are the same. For example, NGK makes FR4, BCPR5ES and BKR5E - all three of those plugs, according to Champion, cross reference to RC12YC.. However, if you reverse that and look up RC12YC in NGK book, it cross references only to BKR5E or FR4
Any of the 3 plugs can work in applications where you want a RC12YC (Common on Briggs & Kohler OHV) and depending on your needs, and experimentation of the engine's use and loading, you can move up or down in heat range a notch or two - gives you some "wiggle room" if you really understand the interaction of spark plug on engine performance and want to tweak the most possible power and performance from your engine for a specific application. Important thing is getting the heat range "close" to what it should be, and using the right thread, reach, and extended nose (or not) plug so you get the spark in the right place within the combustion chamber.
How poor are they who have not patience. What wound did ever heal, but by degrees? - Iago (Othello Act II, Scene 3)