The RCJ6Y is colder.

The RCJ8Y is hotter.

I use the RCJ8Y at high altitude with 91 octane gas thats got 10% ethanol in it. I would like to find "real" gas, but I don't think it exists in Colorado. The hotter plug and higher octane gives me a couple hundred RPM. I'm not saying that will work for you. It works for me, flying at 6000' altitude.