That's a hot plug. It will add to your detonation problem.
The last digit specifies where the plug is in its heat range. XR4 is a plug that cross references to a JY12 to JY14. This plug would be on the hot side for a 7.5:1 motor with a 2.76 gear in a 5000 lb Imperial that almost never sees more that 2000 rpm.
I would not put anything hotter than a -6, like a NGK r5670-6, and would consider a -7.
And I also agree with what folks are saying about your full advance timing (>50°) being too high, and possibly being too lean. Plugs is the easy first step.
A few questions..if i go to a 7 heat range, it wont start sooting up the plugs? It already stinks like hell at idle, but looking at the plugs, they look good. like correct mixture..Also, Looking at the NGK R5670-7, they are 7.00 each compared to the 3.00 each of the XR4, can you tell me of a cheaper alternative with the same heat range as the R5670-7 being this is experimental, dont want to spend that much if i dont have too..
crossing the R5670-7 that you recommended crosses to the champion J12Y, which you said also crosses to the XR4 i already have..? confusing
