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Automotive Why is one of my engine's pressure gauges reading way off?

  1. Sep 6, 2016 #1
    I have an Accusump, which is a mechanism which acts as a reserve supply of oil, if pressure should drop too far, at the end of a hose, connected to the rear of one of the engine's galleries. It is connected by way of a Peterson adjustable remote pressure relief valve. The hose is on one side of the in/out ports. It typically reads 35-45 PSI regardless of how I adjust the bypass valve. It hardly moves regardless of higher oil pressure elsewhere. In the previous engine, I had the Accusump T'd into a circuit that went from the oil pump to an external filter and oil cooler and back. Then, its gauge was responsive to the pressure as it fluctuated due to RPM or heat.
    Is it reading wacky due to the bypass in the Peterson valve which is in-between it and the inlet? In the Peterson unit is a straight-through passage with the bypass port T'ing off it in the middle. When the pressure overrides the bypass valve, oil goes into the crankcase (or vents to atmospheric pressure) through a -10 hose (same as the other to the Accusump). When I put a gauge at the other end of the oil gallery (that the relief valve is on) near the pump, I found I had adjusted the bypass valve up to 210 PSI!

    I am wondering if the pressures in gallery passages near the pressure relief valve are being adversely affected by all the oil that is flowing out. This point is at the rear of the main gallery. Am I upsetting the pressure balance designed into the engine? This is a Subaru boxer style, four cylinder, EJ 257 motor. I have the stock internal oil relief valve disabled in this 12mm, high capacity pump. I am wondering how I should best have the Accusump plumbed in. I have various gallery plugs to choose from.
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  3. Sep 7, 2016 #2

    Ranger Mike

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    we ran accusump on the small block Chevy late model stock car. For those readers following this post, it is a big aluminum tube that stores about two quarts of engine oil. The tube is a spring loaded cylinder that is plumbed into it he main oil passage. There is an on- off lever attached to the big cylinder tube. with the engine running, oil from the engine fills the two quart volume and depresses the spring at the bottom of the cylinder. The drive closes the inlet valve and the oil is “ stored under pressure”. Then the engine is switched off via the ignition kill switch. Next time the engine is to be starter, the driver opens the valve , oil under about 35 psi squirts into the engine to “pre-lube” the bearings then the starter button is hit and we have a pre-oiled engine. This dramatically increases engine life as it prevents dry start of the bearings.

    The Accusump really is for countering oil starvation while racing. On the race track will cornering under Gs, oil can slosh away from the oil pick up at the bottom of the oil pan. This temporary condition will starve bearings and do much damage. The oil accumulated in the accusump will be pushed into the engine oil galleries by the spring if oil passage pressure drops below the 35 psi limit. BTW, this only works if the driver is smart enough to have opened the shut off valve of the accusump before starting the engine.

    I think you need to plumb your accusump as close to the main bearing oil gallery as possible. The oil pump passage works just as well. I think you have it hooked up wrong from what you described. You did not do any harm but changing it to proper hook up will help.
  4. Sep 7, 2016 #3
    Current Accusumps use air pressure behind the piston to determine when the oil is introduced. You are right about using a manual valve! You have to remember to open it and close it. :rolleyes: I like to close it the moment I have revved the motor and have good pressure built in the Accusump.
    Along with a specific answer, similar to Ranger Mike's, I was hoping for a somewhat theoretical answer that might better help me understand the fluid dynamics that are taking place so that I can better determine where to introduce and remove oil. I am not aware of anyone that has attempted what I am doing (with an EJ25* racing engine) so I don't even know anyone to lean on for advice. I feel I have a dynamic situation, as the pump output is RPM dependant and passages and galleries are different sizes as oil moves through the block and into the heads and AVCS units. When I prepped the case and oil pump I matched ports, radiused corners and smoothed passages so oil could flow easier and with less turbulence.
    One of the issues with the stock pump is the bypass is internal to the pump and relieved oil goes immediately back to the pump and can cause a superheated oil condition if the pump is sized too big or the pressure relief is set too low. My objective is to have 85 PSI from about 1,500 RPM up to 8K and use low viscosity, multi-viscosity oil, like a 5-30.
    I am nervous about placing the bypass valve too far away from the pump. It seems to me that if I have too much volume at high RPMs, that the external, nominal .5" internal diameter, -10 AN plumbing could be overwhelmed and the bypass valve could not do its job because of restrictions in hoses and fittings. Also, that if I choose the wrong place in the galleries to draw off unwanted pressure, that I might negatively impact the pressure down the line, into the heads. In measuring ports and galleries I see that they start around .6" diameter and go to .56" for the long, primary galleries and then to .35" as they run to the heads and go to the crank. If you look at the flow diagram you see that the crank is fed from both ends by separate galleries.
    As a side note, I have seen calculators on the internet for determining pressure drop based on size, bends, length, etc. Would this be of any real use to me, given the variables I have? Is there a simple solution?
  5. Sep 7, 2016 #4

    Ranger Mike

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    There is a reason for the pop off valve in the oil system. You don’t want to blow out the cartridge oil filter with ultra high oil pressure. Your 1/2 inch oil lines are fine as we ran these on the 4 cylinder formula car for years. I recommend you carry 10 psi oil pressure per 1000 rpm so you should peg out at 70 psi at max rpm. At idle 3 to 40 psi is plenty. You can do a lot more harm than good with excessive oil pressure at idle. don’t forget, the internal combustion engine must have water for cooling and OIL to cool the piston bottoms and oil the cylinders. you go squirting too much oil in t he cylinders and you risk over powering the piston rings and scraper and lose oil control. Factory engineers spend thousands of hours and millions of dollars designing proper oiling. don’t go over board on excessive oil pressure. it robs horsepower. Use 0-30w Mobile synthetic if you want to. I would add an oil cooler to the engine if you are cranking it to 8000 rpm. Increase the total oil capacity as well as you do not want aerate the oil. Air bubbles caused by foaming is a killer.
  6. Sep 7, 2016 #5
    I agree with all you said and have done all that, too. I have a nine quart capacity system or seven without the Accusump. I have a huge, 40 row Setrab oil cooler and external filter block, running a full-sized racing filter. It is just that I have read that some like to go to 85 PSI for small engines and I have built it to do 8K safely. Clearances are not tight. I idle at 1000 RPM and have 40 PSI, hot. I haven't drained the break-in oil yet. I've taken it to 3,500 RPM and the relief valve keeps it around 80-90. It's just my concern about best placement for the relief valve and why the Accusump was only seeing 40 PSI with the way I had it plumbed that has me worried. I don't want to see it blow up on the dyno!
  7. Sep 7, 2016 #6

    Ranger Mike

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    i would say that it is highly unlikely you will blow her up..you got all the right hardware and looks like you got a winner there!
  8. Sep 11, 2016 #7
    I figured out part of the answer to my question. Venturi affect! I still don't know how to completely eliminate the effects of T'ing a sensor into a pressurized line where there is varying flow rate.
    Surely I was seeing a venturi affect that lowered pressure at the Accusump! I'm guessing if I had a plenum or cavity with the proper shape and volume that effects could be negligible. Perhaps it is related to the orifice size of the sensor/transducer? Some applications, or seeming solutions to the venturi effect, might slow or buffer actual pressure. Perhaps that is necessary (in a non-electronic way) so readings are stabilized and readable.
    I'd really like to know more or get suggestions as to how and where to measure engine oil pressure accurately.
  9. Sep 18, 2016 #8


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    It appears to me that basing your pressure gauge takeoff location by observing the location(s) used by engine manufacturer(s) and trusting that they have determined the best location for that sensor is your best route. This will only give you the pressure in the system at that takeoff point; but, as you have noted, there are several branches to feed the individual elements of the engine and without using multiple pressure pickup points and gauges with intake taps immediately before the lubricated components you cannot know the exact pressure at each component.

    For the total pressure at any point the best arrangement is to place the sensor takeoff in a position that aims it directly at the incoming flow steam, i.e. using a tee with the oil supply coming in one straight through branch, the sensor takeoff on the opposite side and the oil discharge out the side branch; but, this tee arrangement can result is some turbulent pressure loss through the tee at high oil flow rates. I used this arrangement on a fuel line sensor without any delivery rate problems but discovered that it also created a particulate collection point at the sensor takeoff that captured contaminant particles as the fluid turned the corner (similar to a vortex separator) and plugged the sensor port after an extended period of time. Of course, this should be much less of a problem for a clean oil system than it was for my fuel feed system.
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