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, Wed, 19 Apr 2006 12:12:37
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Engine bearings (especially main bearings, but also rod bearings in a good design) are actually a non-contact application at steady state. What I mean by this, the that the crankshaft actually rides on a small cushion of oil (a hydrodynamic wedge), not directly on the journal bearing. When you speed up RPM in an engine, the crankshaft actually climbs in the bore (it starts at the bottom and climbs in the direction of rotation toward the top of the bore, usually). This wedge of oil is what protects your bearings from wear. But if you bog an engine down (high load, rpm too low to form the hydrodynamic wedge) then you get wear (accepting S. Goldbergers point that less oil pressure is also likely to be available at low rpm). Main bearings are usually well fed, so they can tolerate this best, but rod bearings typically do not see quite as much lube and have smaller surface areas. Either way, 1300 rpm is to close to idle for my taste. Bogging down an engine constantly is probably no less harmful than revving one constantly, just a different mechanism of wear.
FWIW, this is related to the rationale of why it is sometimes suggested that one not cruise for long periods of time at constant speed during break in. During break-in the wedge is not as effective in preventing wear because the bore is tight on the crank (and asperities protrude from the mating surfaces of both the crank and bearings), and the thought is that you will wear one spot on the bearing preferentially to others because of particular height in the bore that the crankshaft climbs at the particular rpm you are cruising at.
I don't know how much this adds to the discussion, but I hope it is food for thought anyway.
posted by 136.237.60...
, Wed, 19 Apr 2006 12:12:37, Fri, 21 Apr 2006 07:08:31
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