Viscosity VS Pressure

[UrbanM]

Quick question; I just had my 0-320 E2D 8.5 CR overhauled with an entire new top end from Superior. The engine builder put in 40WT AeroShell. During a flight this morning my oil pressure settled at 50PSI at cruise (75%) power. 81F OAT at 6500MSL Oil Temp 210. I wasn't happy with this so I cut the flight short to do some further investigation. My first inclination was to try 50WT AeroShell and see if this would result in higher pressure. I have looked around the internet for some type of graph or formula to see what changing viscosity might do to pressure readings to no avail. I spoke with the engine builder and he did not seem to concerned. He suggested I run the engine at full power on the ground at operating temp and see what the pressure reading is before I change to a heavier oil. The oil pressure was set during the test run at 79PSI per Lycoming instruction. I am running a brand new Mitchel electronic oil press gauge (Model number D1-211-5056 with the appropriate sending unit). I am going to test it for accuracy tomorrow.
I'm thinking since I have already seen the lower viscosity in action I might just pump out (Or pull the cowl and drain) the oil and start over with 50WT.
Anyone have practical experience with this or any real world results of changing oil weights vs Pressure?
Thanks!

 

[rmarshall234]

All other things being equal, changing from 40 to 50 wt will increase your oil pressure. 81 degrees at 6500 ft is a pretty warm day and 210 degree OT will drop the pressure.

 

[BillL]

SAE50 might restore the margin.

According the the chart below, it would be about 10C difference between the same viscosity for the 40 and 50.

According the Lyc specifications document 55-90 psi is the "normal
" range and SAE40 is OK to 90F ambient. While you are not out of range of oil specs, the pressure is a little low for the recommendations.

Honestly, to allay any concerns about the engine it is not expensive to drain (or siphon) the oil out and add SAE50 mineral oil.

A cooler day with 190F oil temp should provide the same result as the oil swap.

 

[lr172]

I doubt a change from 40 to 50 wt is going to add a lot to the pressure and even if it does, that is not a fix as it restricts you to pretty high ambient temps for start up. Guessing that either there is an error with your instrument, the builders instrument or they incorrectly set the pressure with the oil cold instead of hot (straight wt. oils have a pretty wide viscosity change from the cold to hot extremes, which is why they really aren't used much anymore outside of lawnmower engines and some aviation hold outs). I would first verify that your instrument reads correctly and if so, call the builder and explain the problem - either they set it wrong or some other flaw has caused the pressure to drop 30 PSI since they delivered it to you (both of those on on them). I would not accept 50 PSI @210* for a variety of reasons, not the least of which is it is below or right at the minimum level.

Does your engine have the adjustable OP relief cap? If so, the builder can walk you through how to adjust it.

 

[UrbanM]

Thanks for the responses. I checked the pressure gauge today and it is accurate so I will adjust the oil pressure tomorrow before I fly. Not sure which type of adjuster this is but I will check with the engine shop tomorrow.
Kirk

 

[bruceh]

All you need to do is remove the adjuster and add some washers under the spring. More washers = more pressure against the relief spring/ball inside.
You should be able to have up to 9 or so washers stacked inside, otherwise you need to buy a stiffer spring.

 

[lr172]

A
You should be able to have up to 9 or so washers stacked inside, otherwise you need to buy a stiffer spring.

O/P: Read the manual or ask the builder. Early versions of the cap limit washers to three and then a spacer is required. Later versions of the cap allow up to 9 washers. Don't remember which is which, only that different specs exist. WHile your in there, look for debris, as it is possible the builder set it up correctly and piece of debris has since stuck to the seat. Not uncommon for a new build to have a variety of small metal pieces floating around the oil system in the first couple of hours. Generally, idle level oil pressure gets real low when debris is the cause.

Larry

 

[UrbanM]

Thanks for the replies. Quick update. I changed oil to 50W, pulled the old silver spring and installed the white spring and 2 washers and the oil pressure is spot on.(!) I will probably switch to AerShell W100 or W100 Plus in a few hours and run XC 20 50 in the winter.