Brand new BBF527 ford with 13-1 compression with C12 fuel. AR3934 kept fouling out at idle. Changed to NGK iridium 6764 . Started back up and the sound of the motor changed. No more fouled out plugs after 5 minutes of running. Both plugs are the same heat range. I use iridium plugs in every car I drive and in every tune up I perform. I tired them in race motors in 2012 and found them to be the way to go. NGK gap was .031 vs AR3934 was .025 max. More testing will be performed on race day between the two plugs for better results .