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Old 01-25-2008, 01:51 PM
oldbogie oldbogie is offline
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Quote:
Originally Posted by Calderone
Is it no good to have a High volume oil pump?
i bought one,high volume,is it no good ?
some people say that the pan can get dry and cause the lack of lube.-
In fact they sell oil flow to the top restrictors
http://cgi.ebay.com/New-Chevy-V8-Ren...QQcmdZViewItem
Is it good to have a HV Oil pump ?
In the case of engine oil pumps pressure and volume are related.

The engine is a controlled oil leak, the pump cannot develop any pressure until the volume it's putting out exceeds the volume that can leak thru the clearances. To get more pressure, it has to deliver more volume. Ultimately pressure is controlled by the volume leak past clearances and the a "relief valve". The relief valve adds to the leakage past the clearances thus maintaining a certain pressure.

Pressure is variable to the viscosity (weight) of oil used, the temperature of the oil, and the speed of the pump. Starting backwards, at idle the pump is turning slowly and pressure will be less, so much so, that when the engine is hot the relief valve is probably closed and pressure is simply established by the leakage thru the clearances. Conversely, at high speed the pressure increases because the pump is pushing more oil (volume) into the engine. At some point the volume/pressure will exceed the relief valve's spring and it will be pushed open reducing system volume and pressure. This is also affected by the thickness of the oil. Cold oil of any given "weight" has more viscosity or thickness than when hot. The thickness of the oil determines how fast both the pump can pick it out of the pan and how fast it leaks past the clearances. Thick oil results in abnormally high pressure at the pump which causes the relief valve to open. As the oil heats, it becomes thinner which is easier for the pump to pick out of the pan and it leaks faster thru the clearances. This reduces pressure and it's highly likely that the pressure relief valve will close. The same thing happens for the weight of oil in regard to leakage past the clearances and the setting of the pressure relief valve. All these variables occur at the same time.

Note that the relationship of pressure and volume is different in an engine than in a closed hydraulic system, because an engine constantly leaks oil, a hydraulic system doesn't or at least isn't supposed to leak.

A high volume pump given say a relief valve setting of 60 psi, will provide a higher hot idle pressure because of the excess volume it moves at any given speed. This is important for a competition or other hard working engine as normal idle oil pressures tend to be low and don't provide enough protection when a high load is suddenly dumped on the engine till enough RPM is built high enough to supply relief valve pressure on the system. This is a case of being able to momentarily overrun the oil supply at the bearings. The down side of a high volume pump is at high speed where there is so much excess flow from the pump the relief valve is always open to vent off the excess volume and pressure. This uses a lot engine power for no useful purpose and heats the oil unnecessarily.

A high pressure pump assumes that the volume it can deliver is far above the engines clearance leakage rate, a stiffer spring is used in the relief valve. This may prove to be an ultimate pressure that can't be achieved till the engine, thus the pump, is spinning quite fast. Again there are several variables occurring at one. Not only is the oil's thickness a concern in the ultimate pump output but so is the pumps capacity which is variable by it's operating speed and its size. Faster delivers more oil than slower. Bigger delivers more oil than smaller.

In the end pressure comes down to how much more oil the pump can deliver into the oil galleys against how fast it leaks past the clearances. Given everything above this paragraph, larger clearances leak more oil than tight clearances. This and pump wear is why a high mileage car will have lower oil pressure than when it was new. This is why or at least one reason why, a competition engine with wide clearances needs a larger volume pump to maintain sufficient pressure against the greater leakage. The other reason for a competition engine is that pressure gap between idle or dropped throttle oil pressure and the sudden need under hard acceleration, this gets into gear selection among other things. Take a road racer that comes of the corner in to high a gear, the engine has too few revs and is suddenly exposed to a fully open throttle. It's very possible that the high lugging load will blow the oil out of the rods and that the pump will not yet be supplying enough volume to make up the loss. Next thing you know it spun a bearing or worse.

For me, I build engines with excess oil pump capacity and my customers live with the slight power loss and an oil cooler. I have found over the years that this results in a lot fewer crankshaft bearing failures. I like to see about 40-45 psi at a hot idle rather than the factory's 15 to 20. For a street engine I usually pop off the relief at 60-70. Beyond that subscription to 10-15 psi per 1000 RPM above 6000 is OK. I run blower and nitrous engines about 10 psi higher in all cases since these devices can really put a lot of load on the rods and mains very quickly.

Bogie
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