How can I tell a hotter spark plug from a regular one?

[mrtleavitt]

Cylinder #4 on my 390 is the only plug that seems to be wet when I checked it today. It has a fairly steady miss from that cylinder at idle, but isn't very noticeable when driving. I haven't diagnosed it too much other than new plugs, wires, cap, and rotor. I figure the problem is either a piston ring or perhaps a valve. Anyways, I don't want to tear my engine down but I figure if I ran a hotter plug it may help a little bit. The motorcraft spark plug part # is sp-420. What would be a hotter plug for this (perferably a motorcraft)?

[MIL1ION]

I forget the formula but .

IIRC, the shorter porcelin plug is hotter plug

a longer porcelin plug is cooler plug

[binder56jd]

this article has 2 strikes against it 1 being mopar 2 being bosch

http://www.moparmusclemagazine.com/tech ... index.html

the info seems good


another

http://www.summitracing.com/tech/how2/s ... k_plug.htm

and

http://en.wikipedia.org/wiki/Spark_plug

[jcwillis]

The older Motorcraft/Autolite number system had BF-22, 32, 42, 52, with 22 being cold (longest insulator). I do not know the new number system but it looks like you are using a 42 (a guess because the same numbers are within 420).
I would GUESS sp-520, going by the old number system and applying it to the new number system. However, when you go to pick up the plug, ask the parts counter man and they will steer you in the right direction.
Hope that helps.
Jim

Cylinder #4 on my 390 is the only plug that seems to be wet when I checked it today. It has a fairly steady miss from that cylinder at idle, but isn't very noticeable when driving. I haven't diagnosed it too much other than new plugs, wires, cap, and rotor. I figure the problem is either a piston ring or perhaps a valve. Anyways, I don't want to tear my engine down but I figure if I ran a hotter plug it may help a little bit. The motorcraft spark plug part # is sp-420. What would be a hotter plug for this (perferably a motorcraft)?

[mrtleavitt]

Thanks everyone, sounds like you're on the right track jcwillis. I'll go for the 520!