Spark plug Q's

Any one tell me what the difference is in Autolite racing plugs AR3910 and AR3924? Heat range maybe? I've been running a Champion RC12YC. The engine is a 440, 13.5:1, Stealth heads, MP .590 solid cam and VP 110 fuel, race only.

Thanks, Brian

here is a autolite spark pkug heat range chart i found on line on day.mopar65

ok let me try it again.mopar65

here is the spark plug conversion chart.Mopar65