I'm probably going to run NGK spark plugs in my 400 rebuild, and saw on their web site they recommend a heat range of '4' - the second hottest plug they make! Why would you want to use such a hot plug in an engine known for pinging? I guess Ford specified that way back, but knowing they are prone to pinging, especially with the LOW octane fuels of today, would a colder plug be better?

I'm for sure going to use a 6 or maybe even 7 in my 400.

Thoughts?

 

. Are higher numbers cooler in NGK? Champions are opposite...


. I'd just run cheap Motorcraft 25's or Champion RV15YC's (assuming your engine in good shape and not burning oil) (I think Champion has switched back from Mexico to USA manufacture and they seem to work better now) (Hotter RV19YC in oil burners)

 

BuzzLOL,

Yep, NGK is odd that way, their hottest plug is a 2, then 4, 5,6,7....

It just strikes me as odd that the recommended plug is a 4. If I had detonation problems I'd for sure try a 5 or 6. Since my 400 is pretty performance oriented, I'll try a 6 first. I think with a nice tight engine and the MSD ignition it should be able to keep them from fouling.

 

We pretty much use NGK exclusively.

OE head we would use a R5673-6 or -7
as you have stated, 7 is colder.

Start on the cold side to be safe.

NGK threads have a coating on them,
the first 3 threads will be burnt with the proper heat range.

 

Thanks Tim! A wealth of knowledge as always!

Sounds like I'll start with a 7 then. My son recently purchased a 78 F-150 with a 400, 4bbl. and headers, maybe we'll try a 5 in his.

 

I'm running autolite 25 plugs in my cleveland. About 15* initial timing on 87 octane gas. Runs good.