If I recall correctly. Champion's old number system worked like this:
In the middle, the number denoted heat range. The higher the number, the hotter.
Following the number were letters denoting design characteristics: "Y" means extended nose, which usually acts like a little hotter plug. The extended plug nose hangs out there in the burning gases and gets deposits burned off a little better. There were other letters, but I don't know most of them, except a "J" suffix meant a second gap inside the plug (J6J outboard plugs) and for a while many plugs carried a "c" for copper core. Platinum plugs carried a "P", except the old Homelight H0-8A.
The prefix immediately preceding the number usually means thread size and reach: J means 3/8" reach, and 14mm thread. A "K" here means 18mm thread such as was used in the old one-lunger snomobile engines of the 1960's
Before that is sometimes another letter. A "C" means a shorty plug. A "K" in this spot or ahead of the "C" means resistor, pretty much the same as "R" does, except that the "K" plugs used a NiChrome element and most of the "R" plugs used a carbon pellet.
The NiChrome element fired at a little less peak voltage, but sustained the spark longer than carbon. Its possible that Pioneers spec'ed it for this reason. I imagine it doesn't matter oiutside the test lab, but if you're concerned, use the NGK "R" equivalnet because I think NGK still uses a metal resistance element. I think that number is BPMR7
30 years ago i did a stint as the parts and service mgr for an OPE wholesale distributor. I took this down as notes back in 1980; I'm proud I saved them and even prouder to be able to find tthem after all this time!
