Sufficient Oil Pressure?

[zalto]

Hey Guys,

Long time, no post. Finished up pilots license back in March with the Colt. I have noticed that while flying on hot days and the oil temp gets above 200 degrees the oil pressure will drop to about 75 psi at 2400 rpms. The engine has about 100 hours on it since Major overhaul. The engine will run about 210 - 215 degrees on a hot day. The oil is Aeroshell 15-40, 5.5 quarts. There is no oil consumption problem. Also after landing and back at idle speed the pressure drops to about 30 psi (still in yellow arc). Assuming that the pressure doesn't continue to drop further as engine accumulates more time, should I worry that the pressure is 75 psi instead of 80?

Thanks,
Dusty

[andya]

I wouldn't get too concerned with 75 psi, especially when hot in the summer. I think 60 is min and you're well above that. Your idle is still above minimum also.

I have an engine that is about 80 or just above when hot and would like to drop it a little because it is pegged at 100 for initial takeoff and climb in winter

[Gilbert Pierce]

Your Airplane Flight Manual should give you the limits on oil pressure. Piper says the O-235 Green Arc: Normal operating Range is 65 to 85 PSI. The middle of the green arc is 75psi. Right what you seeing. 25 PSI is the minimum so at 30 psi at idle you are still in the acceptable limits.
Check your Colt AIRPLANE FLIGHT MANUAL to see if the above is correct. I copied the above off my Clipper Airplane Flight Manual with the same engine.

[zalto]

Yes, you are correct. I am well within the operating limits. In the early 90's I owned another colt with high time on it (probably close to 2000 hours). It would hold a constant 80 psi at cruise rpm. On initial take-off it would "spike" to 85 or 90 psi and then fall back to 80. I attributed this to the cool oil and the pressure relief valve momentarily sticking.

In the log book for the engine, during the overhaul the crank journals were "polished". Maybe there is a little excess clearance with the rod and main journals???

Probably just being a little anal on the situation. I had a 73 mustang that i drove for thousands of miles with the oil pressure below 10 psi ......

Thanks,

Dusty

[Gilbert Pierce]

Oil pressure is adjustable. The oil pressure regulator next to #3 cylinder can be adjusted to increase the oil pressure up to a point by adding #10 washers under the spring or by a screw driver adjustment depending on which type of regulator you have. Your A&P can do it. However increasing it 5 lbs. does not accomplish much and will not result in increased engine life. What it does do is increase what is called windage. The excess oil flying around inside the engine causes friction much like drag on an airplane. It can slightly reduce power and increase heating. Race cars and dragsters use a "windage" try to reduce this effect. It can also erode bearings by having high pressure high velocity oil squirting out from around the bearings.i.e. oil pressure set higher then necessary doesn't accomplish anything and can be detriment. When engineers design and engine they determine what pressure is necessary to adequately lubricate the bearings. They then set the oil pressure limits. In Lycomings case it is 75psi plus or minus 10psi which gives you latitude for hot or cold oil.

[Steve Pierce]

I would switch to straight weight oil in the summer. You can add a washer to the oil pressure relief valve to bump it up but no reason in my opinion. You are experiencing a common relationship between oil temperature and oil pressure in my opinion. I think if you fly it a while you will see a correlation between outside air temp., oil temp. and oil pressure. If you are concerned check the oil screen and do an oil analysis frequently and monitor any trends. Also, it is very common to polish the crank if it is round and dimensionally good. I think if you had excess clearance you would have a lot lower oil pressure.

[Hillbilly]

I would switch to straight weight oil in the summer. You can add a washer to the oil pressure relief valve to bump it up but no reason in my opinion. You are experiencing a common relationship between oil temperature and oil pressure in my opinion. I think if you fly it a while you will see a correlation between outside air temp., oil temp. and oil pressure. If you are concerned check the oil screen and do an oil analysis frequently and monitor any trends. Also, it is very common to polish the crank if it is round and dimensionally good. I think if you had excess clearance you would have a lot lower oil pressure.


I concur, its the first thing I thought when I read you were running multi-vis. Of course I agree with the rest of what he says too :D

[zalto]

Thinking back, I switched to the multi-vis back in the winter to help with cold start up lubrication. Also in the winter, I don't think I've seen the oil temp over 180. Now that I'm still running the 40 weight, and also the multi-vis has a higher tendency for viscosity loss during service, I think that is the reason for my slightly lower oil pressure at summer temperatures. I will change back to straight 50w and check the difference. I can go back to 15w40 at annual time in November.

Thanks guys, always very intelligent responses here,
Dusty

Film at 11

[Hillbilly]

Let us know how it shakes out, always interested.

[Jetfever]

I asked my friendly I/A and an A&P concerning your oil pressure question.

The response from both experienced in aviation was:

:) "Is it in the green"? "Then it is OK!" :D

End of story.

[zalto]

Hey guys. It's me again. Interesting annual this year. Screens were cleaned. No contamination noted. Engine has been running well and holding oil pressure as previously noted. During the static run-up finishing up the annual, the oil pressure would not exceed 65 psi. It would raise proportionally with engine rpm until it hit 65 psi and then it just stayed there regardless of higher rpm. As a test, one washer was placed under the pressure relief spring. The pressure then went to 70 psi which would indicate to me that the pressure was being limited by the spring pressure and not excessive bearing clearances. A different pressure gauge was then connected to verify the accuracy of the one on-board. It read 71 psi. Two more washers later, I was back at 80 psi. A test flight to 4800 msl raised the oil temp to about 170 degrees and the pressure stayed => 75 psi. Upon landing and back at idle (roughly 700 rpm) the pressure was at about 20 psi.

Any idea why I lost the initial 15 psi from the regulator? Is it OK to have idle pressure below the yellow arc (< 25 psi)? The oil is fresh 15w50.

Thanks,
zalto