A Detroit DD15 can run without oil pressure for somewhere between zero and about thirty seconds before measurable bearing damage begins, and a few minutes before catastrophic seizure. Engineering tolerances on a modern high-pressure-common-rail Class 8 diesel assume an oil film roughly 0.0008 inches thick on the rod and main bearings. Lose pressure, lose film. Lose film and the bearings start riding metal on metal at 1,800 RPM. By 30 to 60 seconds of sustained oil starvation, journal surfaces have worn enough to need at least an in-frame rebuild. By two to three minutes of running dry on the highway, the connecting rod is on its way through the side of the block. The question almost always comes from a driver staring at a dropped oil pressure warning. The right answer to that warning is: pull over now.
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