10.25:1 static cr
AFR heads

My AFR sheet says to use FR3LS which is now actually 41-629. Is this actually a heat range cooler than an R45TS?
Now if I find that I actually need a slightly cooler plug than the 41-629, what plug would that be?
Napa and the AC Delco dude he had on the phone weren't very helpful.

Found this at AC Delco:



Numbering
1st number denotes THREAD SIZE
2nd number denotes HEAT RANGE
In the early 1990s, an all-numeric, identification code was developed to ensure that the proper spark plug would be selected for replacement in specific engine applications. The numeric code does not in any way correspond to the heat range of the spark plug. Therefore, selection of a spark plug with a different code number is not recommended. The heat ranges may be drastically different between two different plugs, and engine damage or poor performance may result. See your local installer or retailer for details.
The prefix “41” stands for the manufacturer’s product line (e.g., 41 = spark plug)
Numbers after the prefix indicate the type of spark plug and also the specific application:

100 – 199 = Iridium
600 – 699 = Resistor
800 – 999 = Double Platinum

How are we supposed to change heat ranges now?

 

TTT...I need a little help here too. But I need to find a compatible resistor plug to my Autolite 3923.

 

check this out cross reference chart

 

even more useful still