INTMD8

So I have read that if you pull too much crankcase vacuum with a wet sump oiling system, that the oil pump can lose it's prime and/or you can have oil pressure problems.

Maybe I'm looking at this the wrong way, but I don't see how this is possible.

If you have the entire crankcase under a vacuum, then the oil pump pickup and the oil pump outlet (oil gallerys) are operating under the same conditions, or the same atmosphere. So to me it seems that a vacuum in the crankcase that is working agains the oil pump pickup, is also working to pull oil out of the outlet side of the pump.

So if the oil entering the pump, and the oil leaving the pump are both in the same vacuum atmosphere, how could that vacuum inhibit the pumps ability to prime/work?

Teach me

81ZMouse

You can't look at the outlet side of the pump as the crankcase. The outlet side of the pump is regulated by the bypass spring to a set pressure, let's say 60psi. So, if the spring is accurate, the pump should maintain 60psi on the outlet side, regardless of the conditions outside of the bearings.

Think of it like this. If you are blowing air into a large balloon, and the balloon has a small pinhole, letting air escape. What pressure are you blowing against? Is it atmospheric pressure, or is it the pressure in the balloon? The answer is, it's the pressure in the balloon. So, you really don't even know what atmospheric pressure is.

Same with the oil pump. It's all about differential pressure. If the outlet side is regulated to 60psi, and the inlet is atmospheric, you have a differential pressure of 60psi. If the outlet side is 60psi, and the inlet is -15"Hg (-7.36psi), then the differential is 67.36psi. So, the pump has to work harder to move the same volume of oil.

Shane

Mr. Z28 73/97

Intimid8 the only thing I recall reading regarding what you are saying is moroso makes both 3 vane and 4 vane Vacuum pumps 3 vane for wet and 4 vane for dry sumps. And some folks were using the 4 vane on wet sump systems and were actually pulling oil out of their engines causing spun bearings as I recall it.

TheNovaMan

The purpose of a crancase vacuum pump is to reduce blow-by. It could be that, as a side effect of the crancase vac pulling on the oil one way and the pump pulling the other way, the result is cavitation and starved bearings.

81ZMouse

A vacuum pump does not reduce blowby. At least what I call blowby; the combustion that gets by the rings and into the crankcase. Because the crankcase has a negative pressure, it will most likely INCREASE the amount of blowby, but actually "pulling" combustion air from around the rings. A vacuum pump WILL reduce combustion contamination for the same reason.

The MAIN advantage of a vacuum pump is the ability to run VERY low ring tensions, and VERY thin rings. You can do this without fear of contaminating the combustion chamber with oil, and leading to detonation. Very high vacuums (over 15" Hg) can actually help to "suck" the oil out of the bearings, and especially the wrist pins. Most catastrophies I have heard of with vacuum pumps are due to galled wrist pins, not spun bearings. A lot of the high-end engine builders are starting to put a Casidiam coating on their wrist pins. It is a diamond-like coating marketed by Anatech. I've never used it, but it's supposed to help in these situations. Without this coating, Titanium wrist pins will not live in a high vacuum environment.

Shane

TheNovaMan

LOL, you're right, I was mixed up. I guess I was thinking of "anti-blow-by," which is oil going up from the crankcase into the cylinders.