oil pump issues - would like some advice

[Yves40]

I'm turning to this forum since I do not seem to be able to solve the issues I'm facing although it's automotive related.

I have an engine here that turns high rpm's. For several reasons I upgraded the oil pump to a high volume, higher pressure unit. The pump has a 75 psi relief valve spring in it. The relief valve opens towards an internal circuit, resulting in the bypassed oil to return to the inlet side.

What i'm experiencing here is that I have the hot oil pressure going up to around 75 psi at 4000-4500 rpm, but after that it drops off. I was at first thinking that it could have been a windage problem although the oil pan used has both a higher capacity and uses windage screens, baffles and scrapers. I tried putting more oil in and lowering the oil level, but none of it seems to help.

It seems to look like cavitation, but some provisions where made in the oil pump to prevent this, meaning anti-cavitations slots.

It looks as if this happening when the pump puts out larger volumes and using the bypass more, although I would expect that this would cause the pressure to rise sufficiently on the inlet side to prevent this. Maybe it's caused by a temperature rise, causing the oil to go to vapor in the bypass.

As for more specifics, it's a Melling high volume oil pump for a small block Chevy V8 (gen I)

Thanks in advance for any help given.

 

[LittleInch]

Let's see if I've got this right. You have added a high volume oil pump which I can only assume us some sort of gear or pd pump which will put out more and more oil as the rpm increases. However if your engine only takes a certain volume, and you've limited the pressure to 75 si, of oil then more and more needs to go through the relief line. This will vapourise and create more and more oil of lower density due to all the bubbles in it. Internal valves like this are not suited to large volume oil flow and you would be better trying to arrange a separate valve with line back to the sump rather than rely on the internal pump valve.

My motto: Learn something new every day

Also: There's usually a good reason why everyone does it that way

 

[Yves40]

This is in effect a PD pump, 2 gears in the pump body.

Yes, this is the way these oil pumps are setup in general apparently. I'm not sure as to why it's recirculated internally. I assume (not sure here) that it has something to do with preventing cavitation in way because it would put the inlet pressure at a point above vacuum.

I'm more or less going towards what you say, but I'm not sure how to do that. Bypassing it externally can only be done, by modifying the pump in the sense that I would have to block the internal bypass to the inlet side, drill throught the body on the other side in order to make the oil return to the sump. If I was right with the previous aspect then it would possibly make the pump create a higher level of vacuum in the inlet which could lead to earlier cavitation.

On the other hand, I could also try and create a metered leak on the block side with the oil returning to the sump to prevent excessive bypassing at high rpm. This would however also reduce low idle oil pressure.

Any input is welcome.

 

[Latexman]

The oil pressure at the pump outlet, which is what opens the pressure relief valve, is simply the resistance to flow caused by the bearing clearances and restrictions. Apparently the relief opens at 4000-4500 RPM. It will not keep rising because flow does not increase thru the the bearing clearances and restrictions as RPM keeps rising, and it is not surprising to me that the pressure falls off some after it opens, if the relief has something similar to a huddling chamber. Does it fall off and stop as RPM keeps rising, or does it keep falling off as RPM keeps rising. What's your redline?



Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Yves40]

I did several tests to evaluate what was going on, so this is a short summary of them :

I first used a 10W40 weight oil : hot the idle pressure was lower than now (around 25-30 psi). Oil pressure at below 5000 was 60 psi and dropped thereafter with about 15-20 psi. I lowered oil level to exclude windage, the issue remained.

In the meantime I ruled out a filter restriction an all other things.

Then I went to a 5W30 synthetic : oil pressure was 40-45 hot, rises to 75 (relief valve setting) at around 3000-4000 rpm, goes to 60 psi over 4000 and to 55 over 5500. I only had it up to 6000 and it seemed to remain at 55 at that level.

Lowering the oil level with the 5W30 gave the same results.

So basically the thinner oil seems to keep the oil pressure more stable.

 

[Latexman]

I'd suspect cavitation, despite the anti-cavitation slots. Can you open up the inlet side?

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Yves40]

Not really. It already has the largest inlet for these pumps, .750". The std pump has a 5/8 inlet. The pump inlet doesn't allow a larger tube. I must add in all honesty that the pickup was specific for the pan and from a differenct manufacturer. The screen in the pickup is larger regarding mesh and the turn is shorter but a still fairly radiused. Pickup is already 1/2" from the pan floor and I verified this with several people saying this should be plenty.

The pressure drop seems to come with most SBC running the high volume pump regardless of pickup type.

As for cavitation, the info that I've read seems to point to 3 different ways for a pump to cavitate :
- inlet restriction causing a too large drop in pressure on the inlet side
- aeration of the oil
- recirculation cavitation.

I think by lowering the level most of the aeration (from windage at least) can be excluded. I'm not sure how I can distinguish between recirculation cavitation and inlet cavitation.

 

[Yves40]

I was contemplating on putting an extra external oil bypass to the pan sump that was regulated a little lower than the oil pump's own relief valve (75), say 70 psi. Not sure if this can be done however.

 

[Latexman]

This sounds more than an issue with the relief. You may benefit by Red Flagging a post in the thread and ask to have the entire thread moved to "Engineers with Hobbies" or "Engine and Fuel Engineering".

Good luck,
Latexman

Technically, the glass is always full - 1/2 air and 1/2 water.

 

[Yves40]

I already posted about this same problem over on several Chevy engine forums, but without much succes. Nobody seems to really know what is going on, although more people are seeing the same thing.

It looks like a pump specific problem.

Meanwhile I receive a pump graph for this pump. It seems that the pump increases its volume to about 3000 rpm at which point it flatlines at somewhat above 9 gpm. The graph stops at 4000 rpm. This engine runs 7500, so almost double that. The graph gives me the impression the pump is overworking the bypass.

As posted I'm going to try the external bypass with a relief setting somewhat lower than the internal one and reroute the bypass to the sump. That would put the internal bypass out of the picture. But this makes me wonder if less is recirculated, would that cause any problems on the inlet side, which would increase the chances on a higher vacuum there and therefor cavitation from inlet restriction ?

 

[Yves40]

I have to come back to this. Upon further looking into the graph it appears that it is expressed in pump Rpm. Since the pump is driven by the camshaft in these engines at half engine rpm, 3000 rpm at the pump equates to 6000 engine rpm.

Now this seems to fall right at the rpm zone where i'm experiencing the drop in pressure. This would mean that pump output is insufficient to follow engine demand at that point.

However, I would assume that under normal circumstances a positive displacement pump keeps on building pressure. So what is the limiting factor here ? Inlet restriction ? Bypass ?