I’ve been looking at “numbers” a lot while working with my GPAS 2180 test engine. One problem the engine has had since day one is “Low Oil Pressure”. I tried to fix it last Winter and ended up with a blown oil cooler. This Fall I replaced the oil cooler, removed the adjustable pressure relief gadget I had installed and replaced it with the original spring. And of course, my low oil pressure problem was back.
So, I ordered an EMPI VW Oil Pressure Booster Kit and, as a precaution I (tested and) installed a new oil pressure sender. The Booster Kit netted a 25 PSI boost in oil pressure. I’m thankful for that, but what I found interesting is this. The 25 PSI increase in Oil Pressure resulted in a 30 Deg. increase in Oil temperature. Those who believe in Conservation of Energy won’t be too surprised. What surprised me is just how much of a rise in oil temperature I got.
The pressure still decreases more than it should as the oil heats. I’ll check the clearance of the oil pump face next.
We have low oil pressure problems when hot and pulled back to the idle stop. Generally we do power off 180s to land, as specified in the ancient scrolls. But it’s a pain when the red lights are flashing and you are trying to land. My work to control oil temperature has helped a lot but I’m interested to see what you figure out.
At the risk of stating the obvious;
An oil pump (& its suction / delivery system) will have a maximum pressure, that it can deliver. The only variables will be oil viscosity, engine & pump wear & rpm.
The pressure relief valve is a safety devise whose primary function is to prevent unacceptably high oil pressures from damaging the engine.
A pressure relief valve can only moderate pressure below the above maximum. For the most part, the oil pressure relief valve will operate when the oil is cold (start up) and at unusually high engine rpm, having less relevance as the oil heats.
Oil viscosity will change as the oil gets hotter.
In a mono (single grade) oil, the oil starts thick, thins as its temperature rises, consequently, the ability of the pump to maintain pressure diminishes.
Multigrade oils have been chemically engineered to do the opposite. The oil starts thin, to aid in engine cranking / starting & speed circulation. As the oil heats up it effectively thickens to maintain pump efficiency / pressure & separation of moving parts.
The engine maintainer, wishing to improve/raise oil pressure only has the following options;
Reduce engine moving parts tolerances (oil leakage) by reconditioning the engine.
Replaced the oil pump with one that will deliver a higher maximum pressure.
Replace the pressure relief valve (spring) with one that will stay closed until a higher pressure is reached.
Replace the existing oil with one that has a higher viscosity at normal running temperatures (usually a multigrade).
The last option is the one that most maintainers would start with.
Replacing the pressure relief valve with one that opens at a higher pressure has implications for engine damage that should not be underestimated. The pressure relief valve should be matched to pump pressure capability.
Replacing an old warn oil pump, should be considered where pump wear is a significant contributing factor to low oil pressure. Be very cautious about replacing with a higher pressure pump than than standard.
Reconditioning a high time or damaged engine, returning the whole to its optimum specifications, is the most comprehensive)costly) solution
Our pumps are fixed displacement and as such create a fixed flow depending upon RPM and pump losses. Pressure is a secondary effect based on viscosity and flow through the engine.
I think his point is that the premise of your previous post is incorrect, or at least misleading.
Like Bryan said, the pumps are fixed displacement. There are many a busted oil coolers that stand in testament to their near unlimited pressure potential. I think the most likely culprit for pressure dropping too much as the oil heats is that oil is bypassing in a place where it shouldn’t. Maybe the face of the oil pump is too far from the “gears”.
BTW, what’s up with the . Exactly what does that mean?
If you think I am incorrect, in whole or part, make the appossing/clarification point(s) don’t just reiterate your own position on the matter ie where/how am I wrong?
Most engine oil pumps are “Fixed Displacement” that is they will deliver a fixed quantity of oil per revolution.
Pressure in this context is a factor of how fast (flow rate) that volume is delivered (RPM) for a given supply/delivery restriction and the viscosity of the fluid (oil).
If the oil pump has the capacity to “..busted oil cooler” it is unlikly that the pump is your problem.
I suggest that a combination of oil bay pass valve setting and incorrect oil viscosity is your most likely culprits.
If I was in your shoes, assuming you don’t have an engine cooling problem, I would start by researching & then using /assessing, multi grade oils.
Should a change of oil not fully address the problem, then look at the oil pressure relief valve setting.
Note: If your engine is warn or has been assembled with excess bearing clearances, you will get “…oil is bypassing in a place where it shouldn’t.” giving a low pressure indication (restriction to flow has been reduced) . A higher viscosity oil may help to address the matter.
I’ve got 90W at the hangar. Should I run it? Even though it goes against what Sonex specifies for the AeroVee? As a mechanic changing viscosity to increase oil pressure would be my last thought.
I’ve got nice new bearings so I don’t think I have excess wear. I also know other AeroVee people who see low oil pressure at idle. I believe what we are seeing is inherent to the design.
I have 2 constructive thoughts on solutions. One would be to minimize pump losses, which would be driven by the end gap of the gears. This is probably the most sensible fix. The other would be to get a 30mm pump instead of 26mm, but that would be way too much flow at higher RPMs. And I’d need to split the case again to enlarge the relief hole.
I have no specific knowledge of AeroVee engines and their oil preference but have worked on air/liquid cooled engines for most of my adult life (I am an amature mechanic).
In my experience 90W are gear/transmission oils - not crankcase.
My first instinct would be to try a higher viscosity MULTIGRADE synthetic oil.
Multigrade - starting at or slightly lower the Sonex/AeroVee base viscosity and rising to say 10 above eg if base is a 30W single grade, you could try a 15W-40 or even 10W-50
Synthetic oils are reputed to handle heat better than mineral oils - countering this, they do not accomodate lead residue from AvGas - the only exception, that I know of. is the Rotax approved synthetic XPS 5W-50 Full Synthetic Aviation Engine oil “…AeroVee people who see low oil pressure at idle”
All/most engines will see lower pressure at idle. In most cases the oil pressure will rise as engine rpm increases - this is a factor of using a fixed displacement oil pump. That the oil pressure is lower at idle is not usually a problem as long as it rises to an acceptable reading at engine flight speeds. “One would be to minimize pump losses, which would be driven by the end gap of the gears.”
I don’t know your pump but would speculate that this is an unlikely fix.
Wear is predominantly to the working surfaces (gear teeth/lobes/vaines) rather than “end gap”/sides. “The other would be to get a 30mm pump instead of 26mm, but that would be way too much flow at higher RPMs”
Too much flow would raise the potential pressure above safe specification.
May be controlled by the pressure relief valve, subject to it having sufficient capacity (“enlarge the relief hole.”) and the spring rate (that controls opening/closing).
It seems to be a factor of human nature, to address any problem presented, by seeking the most complicated/expensive solution - my advise go the KISS rout first. You are less likly to waste money/time. If it doesn’t solve the problem, you have lost little, as the complicated solution is the next step
We run Valvoline VR1 20W50. Synthetic oils are no good with 100LL.
[quote=“SkippyDiesel, post:10, topic:5290”]
I don’t know your pump but would speculate that this is an unlikely fix.
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Too much gap and a portion of the flow goes backwards to the low pressure side. That’s how it is with gear pumps. There is always some leakage there. The question that Wes and I are raising is if that is a significant factor at low flow or not.
As mentioned above“… Rotax approved synthetic XPS 5W-50 Full Synthetic Aviation Engine oil” is specifically formulated to be able to used by engines burning AvGas (100LL)
*“Too much gap and a portion of the flow goes backwards to the low pressure side. That’s how it is with gear pumps. There is always some leakage there. The question that Wes and I are raising is if that is a significant factor at low flow or not.”
This contradicts the Wes observation;“There are many a busted oil coolers that stand in testament to their near unlimited pressure potential.”
It would seem that on the one, unlikly, hand your oil pump is delivering too little pressure and on the other ,equally unlikly, too much.
At idle, an engine can quite happily run on very low oil pressure.
What is the target (recommended/specified) Min oil pressure for engine at flight power?
Are your engines achieving this pressure?
If yes, what is the problem?
If not achieving recommended pressure, by how much is the pressure falling short?
Have you verified oil pressure readings with a “master gauge”?
What are your oil temperatures 1. In Climb? 2. In Cruise?
When I was worried about low oil pressure after rebuilding my Revmaster with an Aluminum case that had larger oil galleries, Steve Bennett told me 5-6 psi at idle hot and 10 psi per 1000 rpm is acceptable and nothing to worry about.
I suspect that’s what I have, but my MFD shows zero and it blinks red lights at me. One of my buddies out west reports about 8psi. I’ve never actually seen it indicate below 15psi, unless it showed zero. Not sure if it is an MFD issue or the VDO sender.
I don’t think my statement contradicts Wes. There is probably some fixed flow going back across the gears which is insignificant when you are busting oil coolers at high RPM with cold oil. But at the other end of the spectrum, low RPM and hot oil, it is probably a meaningful portion of the flow.
Oil pressures and temps are all over the place depending upon temperature and RPM. Runup when cold I’ll see 90PSI when below 80-90F. Climb and high cruise I’ll see 50-60 at 3200 RPM and 190F. Over 200F and idle it can indicate zero, but I suspect that’s an instrumentation issue like I mentioned in my reply to @q2rd2100 .
As it is really cold here at the moment, no low oil pressure problems. More of a summer thing.
In terms of keeping it simple, that’s my plan. Let @WesRagle figure it out and then copy what he does!
Many have quoted the 10 psi per 1000 rpm and that’s a pretty good standard. I was surprised to learn (at least from a source quoting others) that this number is actually rounded up and that the standard amongst race engine builders (I know these ain’t race motors) was 7 psi per 1000 rpm:
Uncle Tony’s opinion
(Hopefully the time mark is captured as he gives this at 4:15 in the video).
FWIW … my Corvair idles hot at 26 psi and cruises ~3000 rpm at 42 psi. I use Rotella 15W40 and CamGuard for diesel engines (for compatibility with the Rotella). Only one oil analysis returned since using CamGuard for ~30 hours but the results were stunning.
In full disclosure I don’t believe I’d be comfortable with (7 psi X 3x) 21 PSI for cruise oil pressure and even 30 would give me pause. Currently I’m at 14 per 1000 (except for idle which is higher).
So far:
You have made no comment on my suggestion of using an appropriate, full synthetic multigrade oil ie Rotax XPS 5W-50
No one has put any (pressure/temperature/ flight regime) figures on this matter. Without data, how can a meaningful discussion take place.?
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I think your thoughts on the larger oil pump are valid. Higher volume should produce more volume at all RPM’s, and you can still relieve the excess pressure with your current PRV. Given the loose tolerances that the VW possesses, I think this could help, has there been anyone else that has tried this and have some data on the results?
It would also be very useful to have a gauge that reads down to zero or an absolute style gauge, so you know if you actually have a problem or not. I know I would be very uncomfortable not knowing if the pressure is just under 15psi or perhaps Zero…
We used to install high volume oil pumps on performance engines in the auto world, and they were fine at high rpm without any increase in the previous set pressure.
As for the question regarding the Rotax full synthetic oil, in short, it is designed for Rotax and its wet clutch/gearbox arrangement (ie. Motorcycle oil) and doesn’t have the required Zinc content that the old school VW engines need. You wouldn’t put motorcycle oil in your car, would you?
Anyhow, let’s just stay focused on a solution for now instead of having me banging on about oils that don’t suit the engine which could be a whole other thread
Going from a 26 to a 30mm pump would increase flow by about 15%. To keep that extra flow from having too much pressure I’d have to split the case and drill the oil relief passage to a larger size. It’s out there on thesamba.com somewhere. At idle, the piston would likely be all the way up and no oil going through that relief hole. And with 15% more flow, the hot idle oil pressure would go up. It has to work.
But there is a downside to more flow across the board. The fluid equivalent of power is flow rate times the increase in pressure times a constant based on what units you are using. When we pressurize the oil, we are putting work into it like we learned about work in physics class. If the pressurized oil does some work, like move a hydraulic ram under load, that basically converts the work into a different mechanical form and the only heat generated is due to inefficiencies. But when we pressurize the oil, run it through an orifice and dump it back to ambient pressure, all that work becomes heat. With 15% more flow, and the same pressures, you can approximate that as 15% more watts going into the oil to heat it up. And this is directly - not heat transfer from hot pistons, just heat from pressurizing and depressurizing the oil. And given that I’ve had to move heaven and earth to control my oil temperatures when it’s warm, that would be a bad thing.
I think the best bet would be to have a better transducer. If I had 5psi at idle I’d be happy. I also think that blueprinting the existing 26mm pump would give a little more flow, and that could tick the scales on getting idle pressure up. Blueprinting is basically minimizing the gap between the gears and the case.
[quote=“Ausparry, post:18, topic:5290”] As for the question regarding the Rotax full synthetic oil, in short, it is designed for Rotax and its wet clutch/gearbox arrangement (ie. Motorcycle oil) and doesn’t have the required Zinc content that the old school VW engines need. You wouldn’t put As for the question regarding the Rotax full synthetic oil, in short, it is designed for Rotax and its wet clutch/gearbox arrangement (ie. Motorcycle oil) and doesn’t have the required Zinc content that the old school VW engines need. You wouldn’t put motorcycle oil in your car, would you?
I would, if there was a chance it would solve , what still seems to be a perceived, rather than actual problem.
In general oils are not "designed " for a specific engine ie any oil meeting the required specification will do.
Rotax were looking for an oil that would help with the high temperatures, that their injected /turbo engines were developing (sound’s a bit like the VW’s being discussed)- Synthetics seemed to be the solution however were not suitable for engines burning AvGas. XPS 5W-50*, a full synthetic, that will scavenge/containe/hold lead residue, was developed for Rotax. XPS 5W-50* can be used in any engine that can benefit from a full synthetic 5w-50.
While Zinc is considered to be a helpful ingredient, especially for air-cooled engines , its use as a significant additive, is relativly recent, in the history of VW boxer engines ie its presence or lack of should not be a “deal breaker” when looking for a suitable oil.
My first car, several others, was a Beetle - I have no recollection of Zinc figuring heavily in the promotion of crankcase oils of the era.
Similarly I have serviced air cpp;ed mower engines, for probably 50 years - Zinc in mower engine oil, is the latest magic ingredient.
