Marinette Boat Forum

I am going over and getting running the 1978 28 express I just got. I looked up the plugs in a book and it said Champion RN12yc were the right plugs, I pulled the old plugs yesterday to put oil in the cylinders and the engine had RN14yc plugs in it.... Which is correct?

I'm running RN9YC plugs. Pretty sure the manual calls for RN9YC, at least for the mid eighties 360 Chrysler's.
According to Champion the 9,12&14 are the same spec except for the heat range. Looks like the 14 is at the top of the hot range, then 12 and 9 in the middle range.
"heat" refers to dissipation at the tip of the plug.

Plug info
http://matchlessclueless.com/mechanical ... mperature/

Can also google for the Champion plug heat range pdf document.

IF you do a lot of cruising on plane, a slightly cooler plug would be a good idea. That's what we do, and I've seen the plugs run a bit too hot for my tastes.

But if you only plod along off plane, a bit hotter plug MIGHT make 'em run better.

Jeff

You can find spark plug information at www.sparkplugs.com. I run AC Delco R44XLS which is essentially an Autolite 65. The next cooler plug is an R42XLS (Autolite 63). Since the plugs are readily accessible, I run less expensive copper core plugs.

Checking the articles referred to in the post just above and on another thread, the conclusion is very important:

It is also important to remember that it is better to err on the side of a spark plug which is too cold, rather than one which is too hot. At worst, a plug which is too cold will result in fouling which can easily be remedied with a quick change, but a plug which is too hot could cause severe engine damage

1985 twin 318s. I usually go slow around our lake. I am planning on using the hotter Champion RN11s in the aft 4 cylinders because everyone says those cylinders foul plugs faster due to running richer than front four cylinders. I will use the oem RN9s in the forward cylinders.Seems logical.....

WHat you are thinking about sounds interesting. But I wonder why Chrysler didn't recommend that practice in their engine manuals.

Got this idea from FastJeff (I think). Maybe Chrysler tested engines in a level configuration. The slant in our engines cause the rear cylinders to run richer than the front. I am grasping at straws, but that is all I can come up with. I will try it out. 1 step hotter than oem plugs. Not even sure that will make a difference.

I am running a Pertronix flame thrower coil and wires. I have been running them for 3 seasons now with no issues. I will be changing plugs in a few weeks. It will be interesting to see how the soft Champion plugs held up to the higher voltage coil. I will report my findings. Jeff chime in if you have tested the hotter plug application.

RN12YC is what Champion recommends for today's fuels in the small blocks.