Can an oil pump REALLY cavitate?

[John C.]

This isn't specific to Corvettes but I was perusing my latest Summit catalog for an oil pump for my 327 rebuild. Amongst the stock replacement oil pumps I noticed the blue printed "race" oil pumps with their "anti-cavitation" grooves. I have seen these before but I never spent much time thinking about them until today when I found myself with a lot of time on my hands.

I know there are some engineer types on this board that can help me with this so please do!

I'm familiar with cavitation as it applies to a centrifugal pump such as the SBC's water pump. For those with as much free time as I have today but unfamiliar with the phenomenon, cavitation occurs when a centrifugal pump (in this case pumping water) is running so fast that the impeller is slinging the water outward so fast the suction source cannot apply enough pressure to keep the spaces between the impeller blades filled adequately. The pressure in these spaces drops below the boiling point of the water clinging to the impeller's surface and steam bubbles form on the pump impeller. These bubbles are flung from the impeller and as they leave the surface they contact the rest of the water (which isn't steam), are cooled so much they collapse, water rushes in to fill the void created by the collapsing bubble, and an intense pressure shock is created. This pressure wave can and often does cause pits in the impeller and rapid impeller wear. This is cavitation pumping WATER and it can be caused by the water being at too high of a temperature vs. inadequate suction pressure.

Oil is a different creature. I have a tough time believing that oil changes phase from liquid to vapor like water does. I know that at high rpm's oil pumps experience something that causes vibration that can result in spark scatter if the distributor is being driven from the same place as the oil pump (cam). Is the cavitation referred to in an engine's oil pump the result of the pump being run at such a high speed that it is filling the oil passages faster than the oil is being supplied and thus the oil pressure waves in the passages is being reflected back down to the pump's discharge and thus causing vibration? Or is the pump merely pumping so much at high rpms that the suction pipe cannot supply adequate volume so voids are being formed within the pump's lobes and the "anti-cavitation" grooves simply route some of the discharge oil back within the pump to prevent this?

It's just one of those things that's been buggin' me......as always, thank you for your responses!

[Dave Suesz]

Maybe if the intake is blocked. *NM*

[Duke W.]

Re: Can an oil pump REALLY cavitate?

Yes, any pump will eventually cavitate if driven at ever greater speed to the point where when the shear forces in the fluid become so great that it actually begins to tear the fluid apart and create voids. This can also happen if the fluid is a gas and is the underlying cause of compressor stall in a jet engine.

The oil system is one of the limitations of the Cosworth Vega engine. It will make useable power to over 8000 with little change from production configuration, but the OE oil pump, which is the same as on the standard Vega engine has a tendency to cavitate beginning at about 8000 revs. (The coolant pump uses a lower flow impeller than the 140 engine.) When Cosworth developed the racing engine they used an external belt driven pump with one pressure and two scavenge stages. McLaren engines used a similar setup for the CV based Midget engine. Also, because extremely high windage in the crankcase at very high revs prevents oil from returning to the pan, dry sumps are just about mandatory over about 8000 on either a CV or Chevy V-8.

For the street V-8 you do not need a "racing" oil pump since most reasonable street V-8s will not produce useable power above about 6500 to 7000. Below 7000 the OE chevy pumps should not cavitate.

Duke

[John Walker]

Re: Can an oil pump REALLY cavitate?

Are you guys talking about cavitating or simply losing prime. there is a difference!

[John C.]

Ok, so......

...I still don't understand the mechanism of why cavitation in an oil causes vibration and the purpose of the anti-cavitation grooves. Is the re-filling of the voids at the discharge of the pump causing the vibration? Are the grooves a kind of "mini-flow" to allow the pump to internally recirculate the fluid?

I completely understood that I would not need an oil pump of this type, I was just curious about cavitation when pumping oil.

Again, thanks to any and all responders.

[Duke W.]

Re: Ok, so......

You appear to understand cavitation. Liquids cannot exist, steady state, in a vacuum because they all have a vapor pressure and will eventually evaporate in a vacuum. When a liquid caviatates creating a void some of the liquid will vaporize up to the vapor pressure associated with the materials temperature, so you can say that oil "boils" just as hydrogen and helium boil a few degrees above absolute zero.

As far has how the grooves you describe prevent caviation, I haven't the foggiest idea. Maybe it's just hype.

The one caviation issue I have some mild familiarity with is diesel engine liner erosion due to coolant caviation. The high rate of pressure rise in diesels can "ring" the liners and the high frequency vibration causes caviation, and when the voids collapse the shock waves can erode the liners. The same basic phenomenom can occur in a pump, and the solution is usually to make the pump bigger to reduce the average velocities. Pump design is somewhat of a black art.

Diesel engines coolants are specially formulated with "nitrites", which reduce or eliminate the cavitation erosion problem, and good maintenance practice requires that the nitrite concentration be checked between antifreeze changes and maintained at an appropriate level.

Duke

[Clem Z.]

Re: Ok, so......

the grooves are "ball milled" to allow some oil from the pressure side of the pump to be sent to the suction side of the pump. these grooves are milled in the cover and up the sides of the pump cavaity. i think it is more to prevent "spark scatter" by smoothing out the impulses of the gear teeth while pumping the oil. JMHO

[John C.]

Thanks Duke & Clem! *NM*

[Doug Flaten]

Re: Ok, so......

As Clem said, it adds some slippage. If the pump is not as efficient, there will be less tendency to create a localized pressure drop. It is adding clearance to the pump. As with water, the temperature is also going to play an important part in the tendency of the oil to cavitate.