My 300J with the 390 hp 413 had J-9y's with a 9.7 comp ratio. When I rebuilt it, I dropped the ratio down to 9.0 and went with a J-8y as the 9's are no longer available. It runs fine at 32 degrees. It seems that every catalog shows pre-1970 Chryslers are supposed to use J-12Y's which were too hot, even at the 9.0 level.
In the 1966 Chrysler service manual, 9.2cr 383 2bbls were spec'd for Champion J-14Y plugs, with the 383-4bbls in the J-10Y range. Which was typical for Chrysler B/RB engines for ages.
In the '70 Dodge Polara/Monaco FSM: 383 2bbl 9.2cr J-14Y 383 4bbl 9.5cr J-11Y
440/350 9.7cr J-13Y 440/370 9.7cr Spec Cam J-11Y These uses had been typical for many years, at this time.
The 1965 Chrysler FSM indicates that the 383 2bbl (9.2cr) and other non-"Power Pack" 383 and 413 10.0cr) engines used J-14Y and the 413 Power Pack used J-10Y.
Our '72 Newport 400 2bbl 8.2cr spec'd J-13Y plugs. In all cases, the distributors usually had about 25 degrees (crank degrees) timing in them, when combined with the initial timing usually putting them in the 34-38 degrees total area.
My observations on the '70 Monaco 383N was that the J-11Y worked just fine in normal driving. Oil consumption was about 1qt/2500miles at 80K miles, fwiw. No clatter with premium fuel (the spec) at WOT.
On our '66 Newport 383 2bbl (40degrees total advance), the J-14Ys worked just fine. No evidence of them being too hot, ever. From new, it preferred premium fuel, which we fed it all the time, so it was happy. It did NOT like regular fuel of any brand, period, at factory specs.
One thing that was mentioned in prior times, was that the 4bbl HP motors usually had lower total timing settings and colder plugs so that the law enforcement operatives could use regular-grade fuel (From their normal "fleet" fuel supply) and it not compromise their WOT extended-use performance. Just cruising around, no need for premium gas, only during "chase" periods. So the colder plugs on engines which might get into those situations (i.e., the 4bbl motors). With something one notch colder if needed, from the normal J-10Y or J-11Y spec.
On the other hand, a normal "enthusiast" might tinker with their engines a bit more and be more inclined to go a bit colder with their normal plugs, if they drag raced, if necessary. Just as they might play with fuel curve calibrations and the distributor's spark advance rate/total timing.
Also note that this was back in the days of fully-leaded fuels to increase Research Octane ratings, rather than rely upon "aromatics" to do similar things. And, with a Sunn air/fuel meter hooked to the engine's exhaust system, once the engines free-running rpm got into the 2500rpm range (criuse no-load rpm), the meter's needle would head toward 14.7 "stoich" on the meter. Even with a non-CAP carb and 12.5 degrees initial in the '66 383 2bbl, it was possible to adjust to a 14.2 afr, but the idle was more shaky, so we went back to the "lean best idle" settings.
I don't know if I've seen any data on how E10 might affect spark plug heat range per se, just that stoich is more like 14.2 rather than 14.7 (due to the lower heat content of E10 fuel blends).
By observation there are several things which can affect engine "ping/clatter". Fuel octane is a main one, just as combustion chamber design is also in the mix, as David Vizard proved, without any electronic detonation limiter mechanisms. Extended-tip spark plug heat range can be one of the "fine tuning" items, I suspect.
Whatever works for you,
CBODY67
