Guys I need some advise on bearings..

[Kevin Johnson]

guys what effect do you think running a vac. pump or dry sump has on bearing clearance?

You would marginally increase the flow across the bearing -- the scavenge or vacuum pump would assist the pressure pump in drawing out the oil from the edge of the bearing.

Decreased oil aeration would improve the oil film.

I am sure there are other effects.

[Kevin Johnson]

Super important information, especially about the pressure differential affecting the bypass.

seems to me the most overlooked aspect of running lots of clearance is the flow of oil in gallons per minute through the engine .. meaning more windage (HP loss) and the most important the oil filters ability to filter that volume of oil , most filters wont , and the pressure differental usually opens the bypass valve and you end up with bearings that have lots of trash in them .... .0025 has always been a compromise of the bearings ability to carry a load and the oils ability to dissapate heat .. .002 being the maxium load carrying using a large amount of bearing surface area .. .003 removing the most heat from the journal/bearing ... .0025 being the logical compromise ... I have had the wonderful experience of tearing down many engines , circle track and drag race .. When you get close to .002 the bearings have a shine, almost to a wiped lok , but not failed ... past .003 most all have trash in them unless the vehicle ran a dual filter....... at .0025 you wont need your HV pump (dry sump excluded) and your cam gear will be all the happier....not knowing the application , but suspecting a SBC small journal crank , that is where I would be .. if it were a BBC 632 or so , I might be a bit on the bigger side...A longer stroke and a higher HP I tend to think the crank is going to bend and flex a lot more , then experience is the best teacher..what ever works .....PCI

[MadBill]

[quote="Kevin Johnson..You would marginally increase the flow across the bearing -- the scavenge or vacuum pump would assist the pressure pump in drawing out the oil from the edge of the bearing.
...[/quote]
Actually, the flow is likely to decrease. Remember, the vacuum is also acting on the pump inlet side. It is mostly pressure differential between the sump oil and the pump gears that moves the oil into the system. With a perfect vacuum in the pan, only gravity would be (possibly) at work. A typical dry sump pump is lower than the pan bottom (and the sump tank), but many wet pumps are above the oil level, at least some of the time. Also, at extreme low absolute pressures, I don't know if the volatility of hot oil comes into play, possibly exacerbating cavitation.

[Kevin Johnson]

..You would marginally increase the flow across the bearing -- the scavenge or vacuum pump would assist the pressure pump in drawing out the oil from the edge of the bearing.
...

Actually, the flow is likely to decrease. Remember, the vacuum is also acting on the pump inlet side. It is mostly pressure differential between the sump oil and the pump gears that moves the oil into the system. With a perfect vacuum in the pan, only gravity would be (possibly) at work. A typical dry sump pump is lower than the pan bottom (and the sump tank), but many wet pumps are above the oil level, at least some of the time. Also, at extreme low absolute pressures, I don't know if the volatility of hot oil comes into play, possibly exacerbating cavitation.

Yes, you're correct with respect to the wetsump pump and dedicated vacuum pump but not with respect to a vented dry sump tank and scavenge pumps drawing partial or full vacuum.

Also, the marginal decrease in the wetsump pump flow would only occur prior to the wetsump pump pressure relief valve coming into full play which would be fairly constant in a racing engine -- point of inflection -- that's assuming less than a total vacuum is drawn; I think 15" is common.

[MadBill]

Yes, I was looking at the wet sump part of Justin's question. You are right about the dry sump!

There is one more factor though, namely the indicated oil pressure. Since the gauge itself is referenced to atmosphere but a wet sump relief valve would 'see' the partial vacuum in the pan, a wet sump engine would show reduced gauge readings, e.g., ~ 7.5 psi less @ 15" H2O vac, even if the pressure increase through the pump was unchanged.
Conversely, a dry sump engine, with a relief valve referenced to atmosphere, would actually have a larger effective pressure, since as you point out, the leakage path out of the bearings would communicate with the partial pan vacuum.

[Kevin Johnson]

Yes, I was looking at the wet sump part of Justin's question. You are right about the dry sump!

There is one more factor though, namely the indicated oil pressure. Since the gauge itself is referenced to atmosphere but a wet sump relief valve would 'see' the partial vacuum in the pan, a wet sump engine would show reduced gauge readings, e.g., ~ 7.5 psi less @ 15" H2O vac, even if the pressure increase through the pump was unchanged.
Conversely, a dry sump engine, with a relief valve referenced to atmosphere, would actually have a larger effective pressure, since as you point out, the leakage path out of the bearings would communicate with the partial pan vacuum.

I think there is at least one more factor and that is that pressure at the pump and flow at the bearings are related but in many ways independent. I think flow at the bearings will increase despite the indicated pressure being lower since the flow is into a region with fewer mols of molecules bouncing around in an absolute sense.

Yes, the flow derived from the dry sump pump will be greater still.

At high rpms also there is often a point at which flow from the bearings decreases but indicated pressure from the actual pump is a constant -- wetsump or dry. I have heard this explained as the result of centrifugal forces of the passages in the crank acting upon the oil -- I am thinking aerated oil and Ricardo mentions that general issue in a 1935 patent. Greater clearances should yield more flow even if pressure is held as a constant in this case as well.