Weight and film thickness

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In another thread buster said "Increasing the viscosity by one SAE weight, increases the film thickness over the previous weight by about 0.75 to 1.5 um." What's the basic film thickness of the various weights?
 

moribundman

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23,591
Ever since the issue of a correlation between an engine's clearances and the best weight oil has come up I've been trying to find out about my Audi engine (2.8 V6 12v, normally aspirated). I'll quote what someone who is familar with Audi engines has said regarding this: "I have found no clearances any tighter today on Audi engines than I encountered in the early 70's on Datsun engines. In fact for the most part the internal clearances/tolerances are eerily similar or even identical. When using Plasti-Gauge for assembling journal/bearing assemblies to the crank I get pretty much the same "squish" distortion on a corretly done assembly today as I was seeking back then. Pistons run the same skirt-to-wall clearances and the rings have pretty much the same ring-to-land clearances. Same for cam/journal, valve stem/guide, lifter/bore, piston/wrist pin, ad infinitum. I can find no area in these cars today that was manufactured or assembled to any significantly tighter spec than the Japanese cars of 30 years ago." I trust the guy who said this, and by the way, he runs straight 30 weight Redline in his Audi cars. A bunch of other Audi people seem to favor thicker oils, such as 15W-50 or 20W-50. There should be a correlation between clearances, and the perfect oil film thickness... [ July 17, 2003, 03:55 PM: Message edited by: moribundman ]
 
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