Primer Cups on Old Automobile Engines: Their Use

Below was taken from an AACA discussion post. Yes, these cups inject raw gas into a cylinder. The cup opening is the process that primes the engine. But I heard let night another function of doing this. Especially in the winter, when engine oil is very thick, it was very difficult to turn over an engine. The priming process was really to use the gasoline to wash down the cylinder walls of oil, thus making it easier to turn over the engine! 

You can see the priming cups on this 1918 Cadillac we sold a few years ago. They're the little brass fittings next to the spark plugs, and as described, you put a few drops of fuel in them, turn the valve, and the fuel drips into the cylinder. Close the valve or else you'll be blowing combustion out through the cup (note that the priming cup nearest the firewall has been replaced with an exhaust whistle). I've never used them, as most of the cars start pretty readily today and aren't used in the winter, but I can imagine some of those massive 800 cubic inch road monsters needing it simply because the distance between the valves and spark plug is massive.