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Old 12-15-2008, 05:21 PM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by y2k600f4
I have a 383 stroker (less 1K on build) with a 7qt oil pan (total system capacity with oil filter almost 8 qts); running 10W40. I just installed a sunpro electric oil pressure gauge and at cold idle (600-1K RPM) I am seeing ~75PSI and ~85PSI at RPM. When the engine is warmed up to operating temp (160 F) I am seeing a drop in oil pressure at both idle and higher RPMS of ~10PSI to ~65PSI (Idle) and ~75PSI at higher RPMs (numbers approximate). I was wondering if anybody had a thought if the engine builder possibly put a high volume/high pressure oil pump in ? Thanks.
This looks to be a high volume pump, the idle pressure with these will be high with a narrow increase, if any, to high RPM pressure. The change from cold to hot pressure is normal when oil looses some viscosity as it heats up; multi-grades less so than single weights but they all do it to some extent.

Basically the difference between a high volume pump and a standard pump is the size of the gears, high volume having a longer set of gears in a deeper cavity, or more teeth in a wider cavity. These move more oil which will show itself as a higher pressure at lower RPMs than a standard capacity pump. This assumes back to back testing on the same engine with the same bearing clearances as pressure in an engine is the difference between the volume the pump puts in against what leaks out the bearing clearance and/or is popped off by the relief valve.

Pressure, or more exactly maximum pressure, is established by the spring force controlling the relief valve. This is independent of pump size, it's just that a larger volume pump gets to the relief pressure at a lower RPM than does a standard capacity pump.

Often people shim the spring or replace it with a stiffer one on standard volume pumps to increase their maximum pressure. These will show more pressure than a standard pump once the RPMs reach a point where the relief valve opens. Below that point the pressure will look like a regular production pump.

A high volume pump may also be configured to provide more maximum pressure, again, with a shimmed or a stiffer replacement relief valve spring. These will show more pressure everywhere when compared to a high volume pump with a standard pressure relief spring.

There are a number of us hanging out in these parts that feel the SBC does not have enough at and off idle oil pressure for hard working engines, we, therefore, are proponents of high volume pumps to boost lower RPM pressure. There's just as many folks hanging around these parts that feel the standard pump is just fine. Where you live probably has a lot to do with this; here in the mountainous and deserty west it always struck me that the warranty replacement of failed SBC bottom ends justified more pump and heavier oils than these energy savings types in engines that have to put out hard work from point A to B. Especially in these hotter running, higher geared EFI engines, I think it's easy to get these to lugging when hot and the oil film in this low pressure 0W-nothing swill gives up on a long mountain grade at 120 degrees of outside air temp and 220-250 degrees of coolant temp.

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