Early ’49 Cadillacs used AC 48 spark plugs, a very “hot” plug. The difference between hot and cold spark plugs is how quickly the plug body dissipates heat. Hot spark plugs dissipate heat slowly, keeping its temperature higher, which allows the plug to burn off harmful carbon deposits quickly. They are used in cars that are usually driven slowly and in stop and go traffic, whose motors frequently don’t get fully warmed up. When designating the spark plug for the new ‘49, Cadillac engineers thought most of them would be used under such conditions, but apparently this turned out not to be the case as the following 1949 Cadillac factory “Serviceman’s Supplement” explains:
So, to be absolutely authentic either AC 48 or 46-5 spark plugs can be used in early motors, but only 46-5s should be used in motors from number 31,613. Nonetheless, NOS AC 48 plugs seem to be more easily found today. Also, your motor may need a hotter plug anyway if you don’t drive much at high speeds, so you may want to stick with them. And, between us ‘49ers, how many cars show judges would know this obscure factoid anyway?