I gotta say, that I had never seen a BRF31 spark plug, I have ran Motorcraft for years, BF42,32,22, and even BF12 (very cold). I always thought that , at least on the 18mm plugs for FEs, that a BF32 was one step colder than a BF42, a BF22 was one step colder than a BF 32, etc. Like Ken said, the BF22, (and the BF12 was even worse), would foul up quickly, at least with the old dual point ignition we were running years ago. Back then, I had a set of BF42 "Warm up" plugs that I would run the car on in the morning at the dragstrip, until it got up to temperature, and then put in the colder plugs for the drag strip runs. My 66 Fairlane 390 GTA had manual steering and brakes, with the single bowl master cylinder, so I could change all the plugs in about 15 minutes.