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Thread: C-90 oil pressure too high?

  1. #1

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    C-90 oil pressure too high?

    I am seeing 42psi at 600rpm and 59psi at 2300rpm. Can this high of oil pressure actually reduce the power output?

    John

  2. #2

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    your spring relief valve is suppose to let off pressure at 35psi unless its washered up, thats the brass cap with the spring and check valve underneath it. not very many continental guys talk about high oil pressure. was it this way for along time, new motor or what? electrial start engine or not? oil screen plugged? i worked with a guy for about 10 years always knew him as johnnyvon.
    Last edited by tempdoug; 02-03-2019 at 11:39 AM.
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  3. #3

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    Initially after overhaul and having the accessory case welded and machined in the oil pump gear pocket by a nationally known provider of this service, I kept loosing oil prime right from the get go. I replaced the accessory case with a new one and the pump gear was hand lapped to minimal tolerances. Never a prime problem since and never less than 55psi at cruise. 500hrs later still no problem but i always have felt it is not putting out as much power as other C-90's based on static rpm (new prop) and side by side aircraft performance. Compressions all about 78, new plugs, clean air filter, etc.

    So maybe I'm all wet on this one but could the higher pressure being developed by the pump create a load on the engine?

  4. #4

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    go to page 27 and start reading half way down on the left side. why 55 ive got no guess.http://veteranflyg.se/wordpress/wp-c...l-Aug-2011.pdf

  5. #5
    BC12D-4-85's Avatar
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    An experienced engine builder told me excessive oil pressure was bad for bearings. Somehow caused extra wear and friction in the engine. But maybe the pressure gauge should be calibrated before you adjust things. And...variances in air intake and exhaust systems will affect power output. So will camshaft profiles and engine timing. If you're using a Brackett foam air filter remove it and compare the static rpm with and without it installed. There's an inch of manifold pressure and 50 rpm lost with that product compared to a Donaldson and Challenger.

    Gary

  6. #6
    Cub Builder's Avatar
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    To answer to your initial question; Yes, It takes more HP to drive the oil pump to maintain a higher oil pressure. It's about the equivalent of having a 50 amp alternator fully loaded for drag. Not a lot, but noticeable. I had a mechanic put one too many washers under the spring in a C-85 once. You could hear the engine laboring to drive the pump. Don't know what the pressure went to as it destroyed the mechanical gauge from the overpressure.

    -Cub Builder

  7. #7

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    Mine will go to 50 psi at idle when it is cold. With new crank and new bearings, when the oil hits 180 degrees, pressure is at 15 when the tach says 550. Relief valve is definitely closed at that point.

    reset your relief valve so it says 50 at idle, cold, and everything else should fall into place.

    We had a 90 Cub that was marked 40. the DPE wouldn’t fly it. She saw that needle hit 50 and headed for the bomb shelter, I guess.

  8. #8
    BC12D-4-85's Avatar
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    In addition to potential instrument error and internal engine tolerances oil viscosity plays a role in indicated cold vs warm oil pressures. For example take a single viscosity oil grade like 50W and compare the pressure cold and hot to a multi-grade product like 20W-50. Multi-grade aviation oils are thinner and have lower viscosity than singles when relatively cold (for aircraft oils typically 100*F), yet due to additives are designed not to thin below a 50W's viscosity when hot (typically 212*F).

    Not a mystery just another factor to consider. The multi-grade additives can shear and lower hot oil pressures may be indicated with use. When they do it may be time to drain and replace.

    Gary

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