need advice ona strange oil pressure problem

[scootergt3]

engine details:

2.7 77s case with bypass mod ( factory )
66mm crossed drilled crank
new bearings
997 turbo pump
rsr thermostat bypass
993 oil filter console
large front oil cooler with oil tank up front.

The issue:
at idle/low rpms, oil pressure is perfect, around 6,000 rpms and above, the pressure drop quickly to around 35-45 psi

we replaced the previous gt3 pump with a 997 turbo pump, confirmed all the crank plugs are intact, all oil lines(-16) are free of any restriction filters not clogged. We have also been experimenting with shimming the pressure relief valves.
with the pressure relief valve ( vertical ) shimmed with about 8mm of shims, no real change, with the safety valve shimmed, the pressure is higher across the rpm range but still drops to 40psi or so after 6,500 rpms. any thoughts on what could be the issue?

thanks in advance!

PS, all pressures are confirmed with both mechanical and electrical gauges.

[blue911rsr]

What size is your line from tank to engine and dose it have
a spiral in it to keep it from collapsing internally

[scootergt3]

all lines are -16. I have the spiral part but not installed.. while revving the engine, we didn't visualy see the any of the feed line collapsing.

[safe]

Is there any oil in the tank at 6000 rpm? Add a quart or two as a test.

[winders]

I have a front mounted oil tank and a GT3 oil pump in my race engine. All the oil lines are steel-braided AN-16 and do not have the inner support spring. My engine sees oil pressure as high as 120 psi and I have no issues with oil pressure going lower when RPMs are up .

[dannobee]

Possible restriction somewhere. And I'm with Magnus, add a quart or two to the oil tank and see if it changes. If it does, bypass the cooler with an AN-16 union temporarily and see if it improves. Is there a gauge or sight glass in the oil tank that someone can watch while you rev it up?

[winders]

Also, what does the breather hose setup for the engine and oil tank look like? If you take of the cap of the tank, what happens?

[scootergt3]

the oil tank has a sight tube, so i can and did watch the oil level( tank is a little less then half full when running. dannobee, great suggestion on the cooler( i ordered a union yesterday)

winders, the breather run from the engine to the top of the cage then down to the tank, the tank is vented with a -16 into a catch can with a large filter to atmosphere.

my other race car has the same setup and no issues. keep the thoughts coming guys... thanks

[winders]

Do you have a thermostat between the tank and the cooler?

[scootergt3]

no thermostat in the cooler lines

[Henry Schmidt]

Sounds like a pick-up issue. Your oil pump has a screen attached so what sump cover are you using?
There are some aftermarket venturi style sump screens that were manufactured poorly. The pickup is actually restricted when installed producing a similar issue.

[scootergt3]

using this sump plate

https://www.pelicanparts.com/More_Info/MCD99310700100.htm?pn=MCD-993-107-001-00&SVSVSI=0782&DID=43861

[Henry Schmidt]

Originally Posted by scootergt3

using this sump plate

https://www.pelicanparts.com/More_Info/MCD99310700100.htm?pn=MCD-993-107-001-00&SVSVSI=0782&DID=43861

I've used many, That is not your issue.

['78 SC]

How is the line from the tank to the oil pump inlet routed? Any high points? If there's air anywhere in the line that is not getting flushed out when the pump runs, then the flow to the pump is going to be reduced. Kind of like a car with a cooling system that has a high point that needs to be "burped" when the system is filled.

No matter how good the oil pump is, it can only pull down to a near vacuum (vapor pressure of cold oil) so the there's only about 14 PSIA pressure differential to push the oil, so any obstruction (even a bubble) can limit flow.

[gled49]

What oil filter are u using on the 993 console. Rothsport found oil pressure issues with some filters in retro applications.

[scootergt3]

no real high spots in the oil line. runs along the floor and right out the inlet on the case. I was running a 944 oil filter and also tried a 993 oc 229

[winders]

I have used the 944 oil filter (Mann W 719/22) that was used on the 965 Turbo and the 993 (Mahle OC229) filter with equal success.

[gled49]

I ran the same filter setup on an engine, front tank, 930 oil pump, I did use -20 supply hose with support coil, and didn’t have any issues. But Gamroth did warn me about possible OP problems with the small 229 filter that I was forced to use because of space. His choice for no issues was the System One (I think).

[cgarr]

Sounds like when you hit 6500 the relief opens then never really closes as it should as the rpm drops? Do you have the guide tube on the plug to keep the spring straight?

[scootergt3]

yes, guide tube is installed, I also made sure the galley was clean and full travel of the relief pistons.