prophet@umcp-cs.UUCP (Dennis Gibbs) (09/30/84)
<> > I know that the API service classes are tests of an oils actual ability > to protect, not just its viscosity. In that case, the obvious question > is: If a lighter weight oil can be made to pass the most severe > test that API has, why do we need heavy weight oils at all? > > -- Jack Hagerty, Zehntel Inc. There are several reasons for this. (By the way, that was a good question) One reason is that when an oil with a low viscosity rating is hot, it is very thin. In a sports car, or any car with a high-revving engine, it is difficult for the oil pump to pump such a thin oil. At high RPM's, the oil pump would tend to whip a thin oil into a froth, instead of actually pumping it along. The second reason, and probably the most likely answer, is that it is more dif- ficult to build an engine that handles and can retain a thin oil without leaking or burning it. There are many many older cars out there that were built with larger tolerances than the engines built today. If a thin oil were used in one of these engines, it would be burned or leaked at an incredible rate. A third reason is that some cars have very large oil return passages in the engine. In these types of engines, when the engine is shut down, a thin oil would drain very rapidly back into the pan, leaving a thinner film protecting parts upon start up. One way to tell if this is the case is to look at the size of the oil pan on your car. If it holds 3-1/2 quarts or less, it could be that the engine would have to have been designed with very large oil passages in order to get the oil back into the pan while the engine was running. This is really a guess, but my experience has supported this point so far. Of these three reasons, I think the second one is probably the most significant reason. Finally, I think it is very difficult to make a thin petroleum (not synthetic) oil that will not oxidize rapidly at high temperatures. Dennis -- Call-Me: Dennis Gibbs, Univ. of Md. Comp. Sci. Center. UUCP: {seismo,allegra,brl-bmd}!umcp-cs!prophet CSNet: prophet@umcp-cs BITNET: GIBBS@UMDB ARPA: prophet@Maryland