I’ve been reading the exchanges between Ed, Bobby and now Tracy.

Presently my system measures both oil temperature and oil pressure in the line between engine (high on rear iron, near filter) and PSRU. I don’t see what Bobby reports;, rather I see 70 psi when cold no matter what rpm. As oil temperature rises I see steps in oil pressure rpm-dependent.

If I understand correctly, to see the steady-state 70-80 psi oil return, I would have to tap into the one-piece 5/8” braided hose running from my oil cooler to back to the front cover?

Jeff

 

 

Hi Bobby,

 

Ok, if I understand your set up correctly, you are measuring oil pressure BEFORE the oil filter or the oil cooler – right out of the engine?  If that is correct continue to read – if I misunderstood then this may not be applicable. 

 

Your oil pressure sensor location may account for what you are seeing.  Most oil pressure references are measured as the oil pressure goes back into the engine.  You appear to be measuring it at the opposite end of the lubrication system compared to where most measure it.

 

 The reason for choosing the oil inlet to the engine point for pressure measurement is that point is where lubrication system is providing pressure for the main bearings – so an important reason the pressure is of most interest at that point. It really doesn’t matter (not quite true – you do need the pump to be producing pressure)  what your oil pressure is on the output end of the pump – if the oil pressure is  not sufficient on the end where it flows back into the engine -  where the lubrication system feeds the bearings – then you could  fry your bearings. 

 

An example – lets say your pump is good, but your oil filter or oil cooler becomes partially blocked impeding flow severely.  If you are measuring pressure before those points you would likely see your oil pressure increasing (due to the blocked flow) – which could give you a false sense of security – because your bearings could be starving for oil due to the blocked flow.

 

But, if your pressure sensor is not on the inflow end of the lubrication system – you would have no direct indication that your oil pressure has drop dangerously low.  The higher reading you would be getting from the other end would probably convince you that all is OK.  At least that is a scenario that I believe is possible.

 

So,  In the traditional set up that means that you have pressure drops (Losses) through your oil filter and your oil cooler and the normal oil pressure after that (assuming a stock oil pressure regulator) is around 70-80 psi above 4500-5000 rpm (or in that vicinity). 

 

The oil pump end has a pressure controller that is supposed to pop and release pressure if it senses pressure above 150 psi.  That is NOT the controller that regulates pressure for the system – only a safety release say should your oil line becomes blocked (of course your bearings wouldn’t live very long if that happened).  The oil pressure regulator (normally set for a max of 80 psi in a stock engine) is on the other end of the lubrication system at the point the oil goes back into the engine.  The point that provides oil to the bearings – so perhaps not surprising that the regulator is located there.

 

The pressure is suppose to be maintained steady by the oil pressure regulator– so no matter what volume(flow) of oil (based on rpm) is going through the system the pressure is held steady by the oil pressure regulator on the inflow end.

 

 So I suspect that because your oil pressure  sensor appears to be on the pump end of the system and not on the end where the pressure regulator is located that you are seeing perturbations to oil pressure (and because you don’t have the oil filter and cooler drops) and higher oil pressure than most of us do.

 

Now the pressure drops through the oil filter and oil cooler may not be linear with flow rate (which would dependent on oil pump/engine rpm ) or temperature (viscosity) of the oil.

 

I suspect that the cooler the oil is, the more resistance it finds in flowing through the oil filter and oil cooler.  Therefore you would see more pressure on the pump end due to this resistance, but as the oil warmed up and flow through the filter and cooler easier then the pressure build up before those components would drop as would your oil pressure. 

 

Because of your oil pressure sensor location and the factors I mentioned above, I would expect you to see higher oil pressure and a wider range to the perturbations to your oil pressure based on temps than most of us see.

 

Since you are not using the stock Rx-7 oil cooler, then my comments about the effect of the thermostat on oil pressure may not be applicable.

 

At least, this is the way it appears to me.  Others may have a different view point

 

 

Ed

 

 

Tracy,

 

With the Advanced EFIS / engine monitor all of the instruments are factory calibrated and so far have been correct. Oil pressure is being read directly out of the engine before the oil filter and oil / water exchanger. I made a quick run to the airport this afternoon and made another 15 minute ground with the inlet ducts blocked. I was able to get the water up to 200F and climbing and oil to 165F. OAT was 75F Oil pressure varied from 65 at idle to 105 psi at 6000 rpm. On earlier ground runs to 7500 rpm I was only able to get the oil to 144F and water to 177F. OAT was 65F or less. and oil pressure in the 110-120 range. Warming the oil up did seem to make a difference.

 

Ed,

 

"Bobby, depending of where your oil pressure sensor is located (before or after the oil cooler – assuming a stock Rx-7 oil cooler with internal thermostat).   IF you have removed the thermostat in the oil cooler and have not blocked that thermostat opening, then a lot of the oil will not flow through the several passages through the cooler core.  This could mean you are not getting the normal pressure drop across the oil cooler which if your sensor is down stream would mean higher pressure at the sensor – but probably not more than 5 – 10 psi."  

 

No thermostat in the oil or water circuits. If the sensor placement is making a 10 psi difference then the problem may not exist. My remote oil filter adapter has two inlets and two outlets. I am using the second inlet for oil pressure and the second outlet will be used to monitor the before oil cooler temp with the EM3. No room at the engine oil pad adapter block for the pressure sender so this location was easy. It's visible in one of the photos.

 

Bobby

 

 

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Tracy Crook
Sent: Wednesday, December 03, 2008 11:31 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: New Engine Break In with Gearbox -- for Tracy

Odd problems.  Peak Oil pressure should not be significantly affected by temperature or oil weight.    What is it?  Lower viscosity oil is not a solution.  Are you sure the instruments are right / calibrated?

Tracy

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