I posted this question in the middle of another thread and i guess i should have asked the question in my own thread so here goes. I'm a mechanic by trade and have been for about 15 years now. I don't know it all nor do I claim to even though i'm pretty good at what i do. When i see someone who does claim to I have serious reservations about the individual! I know there are high volume and high pressure pumps out there. I want to know the difference between the two. I've always thought that oil pressure was determined by the clearances in the engine (main, rod, cam bearing and lifter bore). I figure that only so much oil can be passed through these clearances at one time and the excess trying to be pushed through creates pressure. So how does one pump create a higher volume and one create higher pressure without having both? Everytime i ask i always get this "Oh one has higher volume and one has higher pressure" Yea but I'd like to know how and why. This may sound like a dumb question to some but i bet that there are a lot of people on this forum that really don't know and would like to. Thanks in advance for any reasonable explanation.

The volume of an oil pump is controlled by the size of the pump. The bigger the pump the more oil it will push. Pressure is controlled by the relief valve in the pump. Like you said the biggest factor is the engine itself.

I don't use high volumn or high pressure pumps, stock is good enough for street and even mild racing applications and uses less HP.

Melling has a pretty good explaination -- but he really likes high volumn and wants to sell you one.....
http://www.melling.com/techbul1.html

Ditto, If you check out the high volume pump, it's gears and gear housing will be about a 1/4" longer then stock for more volume. Now the high pressure pump has had the by-pass spring either changed, shimmed or stretched to increase the pumps pressure.

But as you mentioned, ultimately it is the bearing clearances that determine the oil pressure. pdq67

A high pressure pump has a higher rate relief spring than normal.

A high volume pump has a larger rotor pack than normal.

Bearing clearances do determine operating pressure. If too much clearance exists then pressure cannot build. The spring just regulates the maximum pressure.

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The Red Beast http://www.geocities.com/casanoc

Bedanobub,

I think I see what you are getting at. If, for example we have a test engine and it has a stock oil pump on it. It holds 35 psi at 2000 rpm and the relief spring opens at 55 psi. If you installed a high volume pump on this engine that had the same pressure relief as stock, you would see an increase in pressure as well as volume because you are trying to force more oil through the same opening. If this same engine with the stock oil pump held 55 psi at 2000 rpm with the 55 psi relief, the high volume oil pump would increase flow, but not pressure because of the 55 psi pressure relief. The increased flow would go out the pressure relief, not through the rest of the engine. In a case like this, a high volume oil pump would do nothing but waste energy. The only way to increase flow through the engine in a case like this would be to install a higher pressure relief spring in the pump. This would force that extra oil to go through the engine, but at a higher pressure because, again, you are forcing more oil though the same opening.

Like Carl C said, in general, a high volume oil pump has the same pressure relief as stock, but flows more oil. It also will usually increase pressure over a stock pump because most stock oil pumps don't open the pressure relief springs during normal operation. A high volume pump doesn't usually open the pressure relief during normal operation either, it just comes closer because of the increased flow.

A high pressure pump has a higher pressure relief than stock, but it has a higher volume as well because it couldn't increase the pressure in the system over stock without increasing the volume or decreasing bearing clearances. I have yet to see an oil pump that decreases bearing clearances.

[This message has been edited by Cameron (edited 11-24-2001).]

So basically the high volume pump is a little bigger and would flow more if clearance allowed it to but if it doesn't it's flowing more through a by pass which is useless. The high pressure pump is basically the same size as stock with a different pressure relief spring. The aftermarket pumps usually come with an assortment of pressure springs that are color coded for different pressures (not necessarily chevy V-8's but a lot of different engine oil pumps come this way) So you could take a stock pump remove the pressure spring and stretch it and have a high pressure pump. Then find out you stretched it too far and bust the oil filter! Sounds like the same result between the two but if in a lab. testing the pumps the high volume pump would flow more oil when tested to the max because it's a little bigger. But in the real world in an engine the end result is not much different between the two except maybe on a fresh engine the high pressure pump will show a very high reading on the guage. A high volume will show basically the same as a stock pump. In time as the engine wears the h. pressure pump may drop pressure due to increased clearances and the h. vol. will remain the same. This is because the oil the h. vol pump was by-passing to start with can flow through the engine because of increased bearing clearances. This sound about right?

Right-on, Bedanobub,

He, He!! And I thought my stuff was long-winded!! It took me a minute ta get my tongue outta the way of my eye teeth so I could see what you were saying.. LOL. pdq67

Sounds pretty close. I don't think that you could bust the oil filter with the stock pump though. More than likely, if you were to install a stiffer pressure relief spring in a stock pump, you would see no increase in pressure when the engine is at operating temperature. This is because most stock pumps never make enough pressure to open the pressure relief during normal operation. However, if you did install a stiffer spring, you would likely see an increase in pressure at high RPM and when the engine is cold. These are about the only times that the pressure relief is opened in stock applications. The statement that you made about both a high volume and high pressure pump being pretty much the same is mostly correct. The high volume pump has larger rotors and the high pressure pump is a high volume pump with a stiffer pressure relief spring. In order to make more pressure all of the time (like when the pressure relief isn't open) you have to be pumping more oil. Also, like you said, most performance oil pumps come with several different pressure relief springs so that you can customize the pump to your engines specific needs.

Sorry about the long winded posts. Ok I admit it I may have had a few holiday cold beverages!

Don't feel to bad. You aren't the only one who is long winded.