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Old 05-29-2003, 01:54 AM
Duke2.6 Duke2.6 is offline
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Join Date: Mar 2003
Location: Southern California
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Normal idle oil pressure is usually in the range of 1.0 to 1.5 bar. As the revs increase to the 1500 to 2000 range it should peg at 3 bar and stay there until the engine returns to idle speed. Idle oil pressure is a function of oil viscosity and oil temperature. As stated, the owner's manual says as little as 0.3 bar is acceptable, but I've never seen much below 1.5 at idle with 20W-50 oil in mild ambient temperatures, maybe 1.2 in hot weather.

Automotive oil pumps are known as "constant volume pumps" and if their output is dumped into a bucket the delivery volume would be approximately proportional to engine speed. Since the bearings and other metered oil flow inside the engine constitute a restiction, pressure will rise with delivery volume, so all oil pumps have pressure relieve valves that open at a predeterminied pressure, and as this point the oil flow into the main gallery remains fairly constant as it is driven by a constant pressure against a fairly fixed restriction. The extra delivery volume is usually routed back to the suction side of the pump.

If there was no pressure relief valve, the pressure would continue to rise until the oil pump became overstressed at which point something would break!

Actual hydrodynamic pressure in bearings peaks in the range of several thousand psi. The feed pressure is there to ensure that the bearings receive a flow of fresh oil when the hydro pressure at the oil feed is zero, which always occurs at some point in the rotational cycle.


P.S. One bar is approximately equal to 14.5 psi, not 14.7. The "bar" is not defined as standard sea level pressure.
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