Erratic Oil Pressure

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MadBill
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Erratic Oil Pressure

Post by MadBill »

A dry sump SBF road racer I consult on developed fluctuating oil pressure during a recent test day. The driver was really on it and turned his best ever laps, but the normally rock-steady 80 psi pressure started jumping between 50 & 80. The obvious culprit would be higher g loads causing oil surge away from the tank pick up and accordingly the level will be upped next time out, but in the interim, any other possibilities come to mind?
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Re: Erratic Oil Pressure

Post by Racer71 »

Was it only fluctuating in corners? Does it have filters on the sump lines?
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Re: Erratic Oil Pressure

Post by MadBill »

Good question; I wasn't there and forgot to ask. It does have coarse screens on the pickups but they caused no previous problems.
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Re: Erratic Oil Pressure

Post by Warp Speed »

Look for a restriction (soft hose ect.) on the supply side of the pump.
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Re: Erratic Oil Pressure

Post by Kevin Johnson »

Possibly the design of the tank pickup (some designs vortex more readily than others). That would be related to your original thoughts but might obviate the need to adjust oil level.
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Re: Erratic Oil Pressure

Post by Roundybout »

Anything changed at all to account for the issue? Possibly a different oil or filter? I'd confirm the issue with a different gauge then look at the supply side to the pump. Where are the scavenge sections located? I'd imagine what makes it back to the sump tank would not have too much air entrained in the oil to be an issue.
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Re: Erratic Oil Pressure

Post by MadBill »

Some good suggestions we'll check out. Same brand oil (Driven), filter (Wix) and tank (Petersen) for six seasons. Fluctuation is less erratic in a straight line, but still not real solid. The Oberg was clean, but JIC we'll look at scavenge screens as well as lines. Scavenge pickups are L&R rear and center front of pan plus rear of valley.

I suspect the high-tech fix will be to keep adding oil until it starts to spew excessively into the catch tank, then suck out a quart. One car I worked on a few years back needed 18 qt. to maintain pressure but with 20 it would fill the catch tank.
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Re: Erratic Oil Pressure

Post by GARY C »

How long has the oil been in the car? Some high tec oils break down fast and start aerating.
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Re: Erratic Oil Pressure

Post by swampbuggy »

Bill, has the supply line from the tank to the pump been on the system for six years, and is the inner part that sees the oil rubber ? Mark H.
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Re: Erratic Oil Pressure

Post by Kevin Johnson »

Was there a different driver?

The interior design of the dry sump pan is not mentioned but an inference can be made that the oil was free to migrate due to the left-rear, right-rear, center-forward location of the scavenge ports. The valley could also be involved.

Depending on how a car is driven (longitudinal attitude or yaw), the reaction force of the constraining walls during lateral acceleration will cause a different distribution of free oil prior to it being scavenged (catenoidal surface). This could allow the rotating assembly to whip more air into the oil. If this is the case, it suggests that the tank deaeration setup was right at the limit of air-control.

The single forward pan scavenge point could possibly allow more prolonged contact/interaction with a greater migrating volume of oil if a reduction in oversteer-biased driving technique was employed to decrease lap times.
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Re: Erratic Oil Pressure

Post by MadBill »

The oil has only ~30 min. of track time and the supply line is aluminum all but ~ 24", but a good point. Operating at some level of negative pressure, it too merits inspection.
I'll have to further ponder Kevin's remarks, but the pan has been in service for 10+ years on two vehicles, although not at quite the current g levels.
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Re: Erratic Oil Pressure

Post by Kevin Johnson »

The migration of oil was a big issue with the shallow 928 sump. The factory supported race teams noted that although the car had 50/50 weight distribution, once the fuel weight decreased the rear would lighten up and come around and for the 928 this was fatal.

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