oil pressure dropping over 5000 rpm

[010752]

read up on Grumpy's reply in this old thread :

I'm going to assume pumped volume is high enough, so this leaves restrictions in the circuit and these can be :
- oil filter (non bypass filter AC Delco PF2)
- oil cooler
- the 90° bends I had to use

Well the story continues. Although I saw less of a drop with the 5W30 it wouldn't go up to the bypass relief valve setting which is strange since the volume produce by the pump goes up. It either has something to do with the bypass or cavitation.

I got a pump graph from Melling and what i'm seeing on my engine irl seems to be confirmed by the graph. There is a lot of debate on what could have caused this.

The thing is that people running below 5500 will probably never see this problem.

I'm still searching for a solution. It might end up with a dry sump, although on our cars that's a lot of hassle.

That filter does have an internal bypass, not that it matters at all.

The graph is not for the anti cavitation pump from Melling.
The proper pump is a 10555C, hope you are aware of that and have it.

Really not much debate, the loss of oil pressure above 5k with sbc HV wet pumps is well documented with cavitation being the root. It's no surprise that the thinner oil helped a lot.
You seem to have eliminated the other common causes, aeration and oil supply starvation.

Lots of testing has been done Melling, Moroso and others and the pickup is the problem. In fact most of the aftermarket hipo pickups flow substantially less than the oem. Screen size, suction vortexes.

Might want to try and mod yours for less restriction.

Call this guy, he has a dual pickup hv pump that may be of interest. Might be the easiest cure. Treasure chest of info.

http://www.schumannsdynamicperformance.com/dual-feed-oil-pump-small-block-chevy-race.html

 

[Belgian1979vette]

Thanks, I never knew a dual inlet even existed.

An answer I got from someone else. Something to think about :

This chart illustrates my point regarding the operation of the bypass. The test is performed with the required fixed orifice size to achieve 70psi at 4000 engine RPM (given that the pump itself is driven 2:1) on 5w30- at what I can only assume is to operating temperature? 70@4000rpm on the suggested viscosity is a pretty standard condition, nothing outrageous. You'll notice pump output is completely linear before about 1900 crankshaft RPM. That line corresponds perfectly to the pump's displacement output at those RPMs. The bypass valve is on it's seat. Over 1900RPM, the bypass begins to operate. Yes, nineteen-hundred RPM. Flow at the output of the pump begins to plateau, until 6000RPM where FLOW finally reaches it. You'll notice a blip on the graph at exactly 6000RPM. The bypass valve is being effected to it's specified fully-open point (not considering the secondary bypass design, which can open up even more, but without a progressive bleed control). At this RPM,6000, with the standard viscosity and clearances, is where given "bypass pressure specification" seen in manuals should match the PSIG. You'll notice 6000RPM is a far cry from where this graph clearly shows the bypass begin to function.

Belgian, with regard to the "sudden drop" of PSI using higher viscosity oil, which is not seen using a lower viscosity oil, is something a few people are noticing and it's not vehicle specific. Cavitation is one possibility, though it real cavitation is rare IMO. There should be no reason a pickup tube and screen can't flow 10GPM of 40grade, unless there is contamination or a severe design flaw. I've mentioned this to another recent poster with nearly identical concern. I pointed the finger to the bypass design, which is the culprit IMO, but not a flaw. Given that the bypass is a dynamic, progressive orifice, and that it's bleed rate is proportional to its position in the bore, which is determined by the spring rate x backpressure, it regulates pressure but only relative to the fluid's viscosity. Some pump designs use what I'd call a secondary bypass channel, beyond the linear, progressive increase of leakage used in normal bypass operation. When the valve cylinder get's pushed beyond the normal 'primary' range, a second orifice with a much larger, 'uncontrolled' leakage capacity opens up. This is a smart thing to do IMO as it allows wrecklessly high RPMs on cold and/or viscous oil without excessive internal pressure spikes that can lead to damage. I hypothesize that you're entering the secondary bypass range (where leakage is much less controlled, a true bypass mode) when using the 10W40. This is exactly an appropriate time where CATERHAM could tell you that your oil (10w40) is too thick and he would be absolutely right.

 

[010752]

Thanks, I never knew a dual inlet even existed.

An answer I got from someone else. Something to think about :.....

Not really at all.
Don't know the rest of the correspondence, but this sounds like a lot of mumbo jumbo.
First, that chart is flow not pressure, it simply shows that the bypass opened at 70psi within the confines of the restrictions they setup and with continued rpm increase, the flow rate dropped off proportionally until the relief spring was fully compressed. No hydraulic manufacturer would state false pressure. If you buy a 100 psi relief valve, it opens at 100 psi. That guy is probably thinking thermostats.


Don't know what this secondary bypass thing he's talking about, but all hydraulic components have a certain amount of acceptable leakage, a necessity for operation , but it governs their efficiency.
In a gear pump, it's usually around 85%. This leakage behind the gears etc, in the anti cavitation pumps is directed thru the extra slots to feed oil to equalize the suction pressure and eliminate the vapor, thus cavitation.
Nothing at all to do with relief or bypass pressure.

Going over relief DOES NOT result in lower oil pressure.
You said you had a 75 psi relief spring and other than cold startup, your pressures are never over 60 psi, so hot, you are never going over relief, no more complicated than that.

I assume you have this pressure losss on high rpm acceleration. If so, take a look at the oil climbing up the back wall of the pan and leaving the pickup. If it's under normal high sustained rpm, forget that. Miloden makes a baffle to prevent the climbing I think. Or you can construct the pickup to the side or rear of the pan instead of the normal front.

Definitely take the time to call or email the link in Iowa I posted. Think his name is Vern, but he know his oiling and especially cavitation.

 

[Belgian1979vette]

It's doing the same under constant high rpm as well.

I already sent them an email. They suggest the pump exiting to the pan. I find that kind of odd since that would put the pump at more risk of cavitation I would think.

So what you're basically saying is that the pump doesn't produce enough flow at high rpm, even though this is a HV pump ? I would have assumed if this is the case the pressure drop would have been higher with a lower viscosity oil.

 

[010752]

It's doing the same under constant high rpm as well.

I already sent them an email. They suggest the pump exiting to the pan. I find that kind of odd since that would put the pump at more risk of cavitation I would think.

So what you're basically saying is that the pump doesn't produce enough flow at high rpm, even though this is a HV pump ? I would have assumed if this is the case the pressure drop would have been higher with a lower viscosity oil.

I don't know what you mean by that.

I'm saying that you most likely have cavitation and therefore the pump is losing flow output at a higher rpm. Lower flow = lower pressure.

 

[Belgian1979vette]

The situation is this (numbers are purely for the purpose of this and not correct): if the pump puts out 40 gpm at high rpm and the engine only uses 10 gpm, 30 gpm have to go through the bypass and be recycled. The pickup only has to flow 10 gpm....if you exit to the pan you make the pump pull the full flow, 30 gpm through the pickup. This causes a pressure drop and can induce cavitation.

I specifically asked them this and Schumann's reasoning is that the bypass at this amount backs up into the pickup tube, in effect hindering the engine to pull enough oil.

 

[010752]

Your train of thought is confusing a flow control valve with a pressure relief valve. Two different animals.
The Melling pump only has a pressure relief valve.
Anything less, in your case, 75psi and the flow is being circulated completely and properly.

What happens on the pressure side of the pump (including or excluding the relief) is irrelevant to the suction side if the demand exceeds the suction restriction.

No matter how you shake it, it leaves cavitation, as long as you have oil covering the pickup and the oil is not aerated.

So anyway, what did he say about the dual inlet pump?

 

[Belgian1979vette]

Your train of thought is confusing a flow control valve with a pressure relief valve. Two different animals.
The Melling pump only has a pressure relief valve.
Anything less, in your case, 75psi and the flow is being circulated completely and properly.

What happens on the pressure side of the pump (including or excluding the relief) is irrelevant to the suction side if the demand exceeds the suction restriction.

No matter how you shake it, it leaves cavitation, as long as you have oil covering the pickup and the oil is not aerated.

So anyway, what did he say about the dual inlet pump?

The description was something about extra holes in the cover for the inlet side. Apparently on the side....I didn't quite understand that one, aside that it's aimed at an additional oil flow into the pump. But the pump he suggested doesn't have that feature, just the external bypass.

Piece about bypassing althoug in dry sumps. Same principle though.

http://nutterracingengines.com/racing_oil_pumps/about_dry_sump_oil_pumps.html

Some pump manufacturers like to claim that recirculating the bypassed oil back to the inlet of the pressure pump causes the oil temperature to increase. Well, technically that is true. The problem is that the temperature increase due to recirculation is so small that it is difficult to measure accurately.

Those manufacturers who criticize recirculation claim their products are superior because, instead of recirculating the bypass oil back into the pressure pump inlet, they pump it back to the oil tank (or to the sump).

HOWEVER, sending the bypassed oil back to the tank (or the sump) creates an even bigger problem: The output of the pump at high RPM can be 30 GPM or more, while the engine requires only 8 - 12 GPM. That means that the pump will be bypassing at least 18 GPM through the relief valve to maintain the set oil pressure. If the bypass oil does not recirculate to the pump inlet, then the full pump volume (30 GPM or more) must flow from the tank to the pump inlet through the dash-12 inlet line, with only atmospheric pressure (at best) to move it. The reality is that it can't be done without reducing the inlet pressure below the vapor-pressure of the oil, causing pump cavitation, line collapse, aerated lube oil, and all the engine problems that follow those problems.

NRC pumps provide optimal pumping efficiency by recirculating the bypassed oil back to the inlet side of the pump.