Oil pressure drops at hi RPM.....Stumped

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Turborick
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Oil pressure drops at hi RPM.....Stumped

Post by Turborick » Tue Oct 24, 2017 8:14 pm

have any of you guys ever seen this? oil pressure starts dropping after 7500 rpm
seems to not be as bad with thicker or colder oil
this run is with 5/30 oil
oil temp was 135 F
Turbo SBC 2.55 stroke 4 bore
.0024/.0028 clearance
piston squirters
drysump
peterson 12 qt tank

Red line is rpm
green line is oil pressure
oil pressure scale is on right side of graph
oil12734.png
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Re: Oil pressure drops at hi RPM.....Stumped

Post by MTENGINES » Tue Oct 24, 2017 8:39 pm

Is the tank mounted lower than the pickup? 90 degree bends in the supply?

Explain the oil pan. Wet dump converted? Segmented.

Give more details.

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Re: Oil pressure drops at hi RPM.....Stumped

Post by Turborick » Tue Oct 24, 2017 8:53 pm

MTENGINES wrote:
Tue Oct 24, 2017 8:39 pm
Is the tank mounted lower than the pickup? 90 degree bends in the supply?

Explain the oil pan. Wet dump converted? Segmented.

Give more details.
The bottom of the tank and the pump are at the same level
yes there are 2 tube 90's one at the tank and another at the pump. the supply line is -12
oil pan is a Dan Olson right side kickout, scraper, windage tray, #12 pickups, separating diverters, and capturing trough
the pump is a peterson 4 stage. 2 to the pan and one to the valley

not sure what other details you need
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Re: Oil pressure drops at hi RPM.....Stumped

Post by MadBill » Tue Oct 24, 2017 9:14 pm

Is that oil temp correct? 135° F. and it drops less if the oil is colder still? :shock:
How much oil do you carry? Can you up it a couple of quarts to see if it's a drainback issue due to the cold oil? (although you say it appeared to be better with higher viscosity..)
Felix, qui potuit rerum cognscere causas.

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Re: Oil pressure drops at hi RPM.....Stumped

Post by Turborick » Tue Oct 24, 2017 9:36 pm

MadBill wrote:
Tue Oct 24, 2017 9:14 pm
Is that oil temp correct? 135° F. and it drops less if the oil is colder still? :shock:
How much oil do you carry? Can you up it a couple of quarts to see if it's a drainback issue due to the cold oil? (although you say it appeared to be better with higher viscosity..)
yea that is correct 135deg F
there is 3 gallons in the dry sump tank
we have held the engine at 7500 rpm in neutral and the tank level does not drop
I was surprised at how watery the oil is at 135deg F
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Re: Oil pressure drops at hi RPM.....Stumped

Post by BlackoutSteve » Tue Oct 24, 2017 9:59 pm

Is 135°F normal for this engine?
If so, you should switch to a 0 or 10 weight like Pro Stock do if you're gonna run so cold.
Too cold for a 30. :)
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Re: Oil pressure drops at hi RPM.....Stumped

Post by Turborick » Tue Oct 24, 2017 10:11 pm

BlackoutSteve wrote:
Tue Oct 24, 2017 9:59 pm
Is 135°F normal for this engine?
If so, you should switch to a 0 or 10 weight like Pro Stock do if you're gonna run so cold.
Too cold for a 30. :)
yea but the thinner or the hotter I go the worse this problem gets.
This is a Land Speed Racing engine it can run full throttle for up to 90 seconds so we run them cold.
this engine has the conventional SBC valve layout with 2 ex valves next to each other
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Re: Oil pressure drops at hi RPM.....Stumped

Post by carmakerevive » Tue Oct 24, 2017 10:48 pm

got a restrictive filter element between the pump and engine? or between the feed and pump? Sounds like restriction or cavitation somewhere.
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Re: Oil pressure drops at hi RPM.....Stumped

Post by Circlotron » Tue Oct 24, 2017 10:54 pm

If the pump flow has a certain amount of pulsation, is there a possibility that the line leading to the pump is a critical length at a particular rpm and you are getting some kind of resonance and consequent cavitation / aeration at the pump inlet?

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Re: Oil pressure drops at hi RPM.....Stumped

Post by Racerrick » Tue Oct 24, 2017 11:20 pm

slow the pump down. too much pump rpm

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Re: Oil pressure drops at hi RPM.....Stumped

Post by Straight8 » Tue Oct 24, 2017 11:54 pm

Suction hose collapse maybe? From tank to pump.
Last edited by Straight8 on Tue Oct 24, 2017 11:55 pm, edited 1 time in total.

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Re: Oil pressure drops at hi RPM.....Stumped

Post by MadBill » Tue Oct 24, 2017 11:54 pm

I expect your HP is well into four digits, but 100 + psi still seems way high. We run ~ 60 in our 750 HP/8300 RPM SBF road racer..

Where are you tapping in for the pressure signal and could you perhaps tee in a second sender immediately after the relief valve to see if there is an excessive restriction?
Felix, qui potuit rerum cognscere causas.

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Re: Oil pressure drops at hi RPM.....Stumped

Post by MTENGINES » Wed Oct 25, 2017 1:33 am

I have seen large main or did clearances cause this. In excess of .028r.030m. It hears the oil up real quick. What's your side clearance in the rods and litter clearance. ?

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Re: Oil pressure drops at hi RPM.....Stumped

Post by MTENGINES » Wed Oct 25, 2017 1:37 am

carmakerevive wrote:
Tue Oct 24, 2017 10:48 pm
got a restrictive filter element between the pump and engine? or between the feed and pump? Sounds like restriction or cavitation somewhere.
Yes. I have seen that also. Most dry dumps I do result in a hp6 filter because even clogged you can't restrict flow through them.

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Re: Oil pressure drops at hi RPM.....Stumped

Post by Geoff2 » Wed Oct 25, 2017 4:48 am

This is only a suggestion. Could it be the centri force at 7500 rpm at the rod brgs is pulling oil through the oil holes, faster than the pump can deliver?
This seems possible because you say that it is worse with thinner oil, which would flow more easily & deplete the oil galleries of oil.

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