340 and 318 Spark Plugs

[challengermaniac]

Looking for recommendations for mfg and size of spark plugs for my 340 and 318 motors and whether the same or interchangeable? 

The 340 has a fresh rebuild with Autolite 65's and yet to have it's first start.  Keep or change these plugs?

The 318 has an old set of Champion RN12YC.  Would the Autolite 65's work for this motor?

PS: and the 318 has a miss, although all of the Champion plugs appear to look the same. 

[70 Top Banana]

I bought a 70 T/A 340 6 pack car not long ago. I did a tune up and I pulled out Champion RN 12 YC plugs. I went to 4 different websites just to check and all called for Champion  RN 12 YC. The car ran a lot better once I changed the plugs. I can't help you on the 318 as I can't remember what the car had in it and I had that car for quite a while.

[ply64post]

Sparkplugs as I remember 68-70 340 in champion were N9Y and 318 would have taken a N12Y.

[benguin]

Agree with the above- the 340 typically has a bit of a colder plug than the 318.   

Many years back, I switched to using NGK BPR5ES plugs in my 340 after some fouling with the Champions, but that was pre-ethanol.   I just fitted a new set of NGKs in mine, but haven't fired it up you. (I think I was swayed during the '80 Winternationals and seeing that Paul Rossi used NGKs in his 440-6 Challenger.  I still have one of his used plugs.  )

[70 Top Banana]

I agree the original plugs were Champion N9Y for a 340. But I could not find them and all the places I looked were newer plug numbers.

[71383bee]

I use autolite 64 in my 318 in my convertible. It runs a rev n ator setup with a firecore distributor with timing set at 18 initial and total at 34.  So far she runs real clean. I've been meaning to pull one to look at the plugs and check how it's burning. From seat of the pants feel it runs nice.

Been running autolites for years now and have been very happy with them.


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[HP2]

318, 340, 360, 5.2 and 5.9 spark plugs are all interchangeable in physical dimensions. The only difference is the heat range recommended for them

All the numbers designate all aspects of the plug for all manufacturers, but they all use their own schema.  For Champion, RN12YC translates as R=resistor, prevents radio buzz and electronic interference, N=shell thread size and reach, in this case 14mm x 1.25mm, .750", gasket/joint, 13/16 hex, 12 is the heat range, for Champion higher numbers are hotter, Y=non-projecting electrode of  .060"-.090" C= copper core electrode.

So a factory N9Y would directly interchange with an RN9YC. Honestly, I'm not even sure if you can find an N9Y any longer as nearly all plugs made for general consumer use are resistor now days.  It also used to be a rule of thumb that the higher performance the engine, the cooler plug you wanted to use in them to provide better sustained high RPM firing. Since most of our cars are cruisers now days, a hotter plug may not be a bad idea to allow it to clean any deposits at lower rpm ranges. Even the occasional strip blast isn't bad with a hotter plug, but if you are doing it regularly, then step down heat range.

[MoparLeo]

Chrysler products have always come with Champion as standard equipment.
https://www.championautoparts.com/Products/Spark-Plugs.html