Oil Pressure drop with RPM?

[BGG]

I have a question for you more knowledgable Vintage FF folks. I have a 69 lotus 61 with a vintage motor that seems to run good. I do not know the history, but it did pull strong at the track last summer. It had oil pressure of 50 psi and would drop with RPM's down to almost 20 Psi at 6k+ RPM. The oil light came on that has a separate sensor indicating the gauge was correct.
I have adjusted the oil pressure screw on the oil pump and now I have 75psi cold and 50 Psi at 6K rpms. Better, but why the drop with RPM???
Thanks-Brian

[Jon Jeffords]

I would make sure you have no inadvertent restriction between the oil tank outlet and the oil pump (aka 'feed pump') inlet. Any restriction in this line could cause the feed pump to cavitate at higher rpms. I have heard of instances where an old suction hose would have an inner liner that collapsed due to the suction of the feed pump, but look fine externally.Are you still having this issue after making the adjustment to the oil pressure screw?

[Hawke]

Problem with hose between tank and pump. Looks like it's collapsing.

[romoman]

And this is all at the same engine temps (or at least warmed up), correct?

[kea]

If you are not monitoring the oil temperature, it may be a case of it just getting thinner at high temps, thus lowering pressure.
Some petroleum based oil were known to exhibit high oil pressures when cold.

[BGG]

The new numbers are cold, 75 at 1500rpm and 45-50 at 6k after I adjusted the oil pressure screw in. Temp is not the changing factor. The drop in PSI with higher RPM is the question. I will replace the suction line from the oil tank to the pump today and report back. it makes sense if it is sucking some air or blockage it would cause cavitation and psi drop. Thanks for the help here!!-BGG

[DanW]

The new numbers are cold, 75 at 1500rpm and 45-50 at 6k after I adjusted the oil pressure screw in. Temp is not the changing factor. The drop in PSI with higher RPM is the question. I will replace the suction line from the oil tank to the pump today and report back. it makes sense if it is sucking some air or blockage it would cause cavitation and psi drop. Thanks for the help here!!-BGG

In addition to the suction line from the tank to the pump, replace the short line from the dry sump pan to the scavenge fitting of the oil pump. That short hose can collapse and the oil in the pan accumulates and does not return to the oil tank. That hose must be rated for vacuum. Also remove the drain plug in the pan and check the screen for an obstruction.

Regards,
Dan

[BGG]

Thank you for all the help here!! I replaced all the suction lines (from sump to pump and pan to pump) with new double walled line. I removed the screen from the pan, it look very clean. I cleaned it and reinstalled. I even took the oil pump completely apart and inspected it. It looked new inside, no ware on the surfaces at all.
NO change. 80 psi at idle cold and 50 psi at 6,000rpm coldish.
If it was bearing going out wouldn't it be low across the board?
Very Perplexing.

[kevskisps]

75 psi a idle is crazy. is that with hot oil?

I'm amazed that the pump can create that sort of pressure at that speed unless it not being allowed to flow the oil over the bearing surfaces.

Is the oil filter plumbed in the wrong way round and the check valve causing this some how ??

[BGG]

75psi is COLD at idle. It is way up there due to the me adjusting the Pump up so it had 45psi at 6k RPM.
Before it was 50psi at idle cold and below 20psi at 6k Rpm. I adjusted it up to fix the symptom, still wondering what the problem is to make the pressure drop as RPM increases.
I double checked the oil filter flow and it is correct.

[Jon Jeffords]

I wonder if the pressure relief valve piston is sticking.

During your oil pump disassembly and inspection, did you look closely at the pressure relief components? Any concerning wear (or modifications) to the piston, piston bore, or spring? Is the spring still meeting its spring rate, or has time and temperture caused it to relax? Any edges or burrs built up in the bore that the piston can get caught on? Do you know for sure the spring that is in it is the "factory spring" for your brand of pump?

A good investigative experiment would be as follows:

1. Start cold engine and idle for two minutes.
2. Record the oil pressure at idle, then rev engine to 4500-5000 rpm and again record oil pressure.
3. Allow engine coolant to warm up to 180F.
4. Once again record the oil pressure at idle and at 4500-5000 rpm.

I'm curious to see the 'droop' of the oil pressure relief valve from low rpm to high rpm, and from thick oil to thin oil.

[BGG]

It may be a week until I can get back to it and test what you are saying. I do like the Faulty pressure spring/piston idea. It actually makes sense with the symptoms as you say. When I had it apart it looked ok, but it is a cheep fix if it works. I will order the new part from Pegasus in the mean time.

[Roland V. Johnson]

What oil filter are you using?

[BGG]

Valvoline 20w50 racing

[scorp997]

Where are the sensors located talkative to the pump/filter? Any chance you have the filter hoses on the filter bracket backwards? (I have seen it) Brand of filter is useful as well

[Fmracerbob]

I experienced just such an issue. Turned out the gear on the pump input shaft that engages the cam was slipping when up to temp and as higher rpm's meant more resistance from the oil in the pump. It was a press on gear without a roll pin. But I have seen sheared off roll pins in the past also.

[Roland V. Johnson]

I am not commenting on the accuracy of his results. However, there must be a reason I have been using WIX or NAPA Gold filters (made by WIX) for the last 15+ years. (At Jay Ivey's strong suggestion) Some filters are highly restrictive in addition to his results.

https://www.youtube.com/watch?v=iTNJLEV8CG8

[stonebridge20]

I can't remember when I used another filter besides a NAPA gold on any car in the shop.

[KodaBear]

I am not commenting on the accuracy of his results. However, there must be a reason I have been using WIX or NAPA Gold filters (made by WIX) for the last 15+ years. (At Jay Ivey's strong suggestion) Some filters are highly restrictive in addition to his results.

https://www.youtube.com/watch?v=iTNJLEV8CG8

Wix filters are now owned by Mann Hummel. The last “wix” filter I bought for my Porsche was made in Poland in the Mann factory. Mann makes first grade filters too, point is just that the name on the box does not mean much anymore.

[Steve Demeter]

Right75psi is COLD at idle. It is way up there due to the me adjusting the Pump up so it had 45psi at 6k RPM.
Before it was 50psi at idle cold and below 20psi at 6k Rpm. I adjusted it up to fix the symptom, still wondering what the problem is to make the pressure drop as RPM increases.
I double checked the oil filter flow and it is correct.

If the relief spring allows 75 psi at idle, then if all is good with the pump ( no deterioration inf function at temp, no excessive thinning of oil), then in theory it should be the same 75 psi at high rpms. The spring simply opens when the pressure hits 75 psi. If you are not making that at rpm, the adjustment of the plunger spring makes no difference as long as it is set higher than what the pump is making. I would carefully examine the whole system for something goofy out of whack.. Are the bearing clearances correct?

[BGG]

Thanks for all the input here, I have had a hard time getting to work on it with all the holidays. Originally I had a Napa filter and went to a Wix filter, both large so they should have worked find. I replaced the plunger and spring in the pump and had no effect on the performance.
With all that I have focused on the oil pickup line. I fear is was collapsing or causing a blockage in the system not allowing enough oil to the pump. The level in the remote tank does not go down drastically at rpm so I know the pump is putting the oil back there, but I wondered about cavitation. Once I get the lines replaced I will get back to you.
The bearings are a logical question and I have been trying to avoid that at all cost!! It maybe the only option left standing.

[scorp997]

Don't worry about the COLD pressures... What is the idle pressure HOT, and what is the running pressure (6k) HOT. You adjust the bypass for running pressure while hot, if you are setting it for cold idle you will be low on track.

My car will routinely run well above 75psi (can get close to 90psi on really cold mornings) when cold but will drop down to 55psi when up to running temp. At running temp my idle pressure is 10~20psi when coming off track. Set it for track temp and running speed then make sure you warm up your car before going to pregrid so that your pressures aren't too high when first lap on track.