So, back in the day our big blocks ran Champion J11Y plugs...I have just acquired a set of J11 plugs...what is the difference between these two if any ? For my 1969 383...

wasn't the Y series a projected nose design ?
just going off [from what's left of] my memory.........

Champion did use the Y suffix to identify the projected tip spark plugs
OP, these are a non resistor plug also and may not be legal in your state
OP, if your motor has stock heads and pistons I would look at using the old Champion part # RJ12YC
if you have aluminum heads you will need a 3/4 reach plug which will have a N or RN prefix or RC prefix. The RN uses a 13/16 inch body socket size and the RC uses a 5/8 inch body, both use gaskets and are resistor plugs

there was an oj11y resistor plug but is gone like the j11y. projected tip plugs should used.

thanks for the replys...have stock pistons and heads..will go with the recommended RJ12YC

J11 is a lawn mower plug. Probably also works in a small block chevy

Personally, I prefer NGK XR5 for a stock compression iron head BBM.

don't know how much if any resistance your looking for in plugs but from my experience the rj12y is rather dismal performer. i just tested an rj12y and t had 17,000+ ohms of resistance. an autolite 85 will have 5000-6000 ohms. ngk has a resistor and none resistor plug for the application, not in love with either. the ngk resistor is advertised at 5000 ohms. i see the real issue as the j11y and j13y were designed for big block mopar iron heads. those two plugs had a longer projected tip vs what's currently available. now all we have is small block chevy plugs. no real good choices.

Why would they be illegal? I have never heard about such a thing,

The federal government outlawed non resistor spark plugs along with solid core spark plug wires to eliminate EMF, electromagnetic interference caused by them.
Back in the day, around 1962 I think, before them being outlawed you see and hear that electrical interference on the T.V. and hear it on AM radios
In todays digital world they probably would be not causing the same issues

I started working in a gas station back in 1972. I personally installed a lot of the J11Y & J12Y plugs into big block Mopars back then. We didn't start using the resister plugs until the electronic ignition was pretty standard among our customer's cars, probably in the mid 70s (or maybe we couldn't get the non-resistor plugs by then).

I have hundreds of original Made In USA 8 packs of the Mopar Champion J12Y spark plugs if anyone is interested. The original cartons have the sticker that garages used to stick on the B pillar for when the service was done. The packaging states Mopar/Champion
The original part number on the box is 4106121.
Made by Champion Spark Plug Co. Toledo, Ohio
Sorry this is not in the "for sale" section. I really never thought about selling any until someone mention them.
Pics upon request