Ab Jenkins and 20 weight oil

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The discussion on oil viscosity I’m afraid has triggered another ‘Cliff Claven” moment.

One of the interesting things in the movie “Flight of the Arrow” is the corny endorsement of Pennzoil 20 weight oil. The record breaking 24 hour run in the heat of the Bonneville Salt flats at sustained speeds of over 120 mph using straight 20 weight seems daring at first. Pierce recommended nothing thicker than 30 weight in summer, 20 was for cooler temperatures. The thin oil films that keep the bearings and pistons from scraping get thinner as the oil viscosity drops from increasing temperature. On the other hand as long as the oil film is thick enough to keep the parts from scraping the friction and heat is reduced. The other main factors are the loads and the speeds.

The conditions with the thinnest oil films are actually at low rpm and high load – lugging the engine at low RPM with a lot of throttle such as accelerating at low speed in high gear. Ironically high RPM provides the thickest oil film and the greatest margin from breaking through the oil film. Ab Jenkins run was done with a souped up V-12 which presumably had high overlap cams that required a higher than normal idle speed to keep it running during pit stops. This would have kept things from failing at low RPM with a thin, hot oil but the thinner oil would have had less friction at high RPM giving a bit more hp and speed.

Pierce bragged about having the largest con rod bearing area. That is a trade to have more load capability at low speed at the expense of more friction and heat at high RPM. This leads to using thinner oils reducing low RPM load capability. It is a trade and a bit of a dog chasing its tail.

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