Safe maximum oil presure?

[PEW]

I've been having a discussion on another forum about oil pressure limits. Our old 850 engines have a relief valve that is supposed, by design, to open at 45 psi, according to published data. On my engine the gauge pressure shows 55-65 psi at start from cold, then settles down to about 45-55 psi at cruise speed, with about 20 psi at idle. I consider this the normal thing.

One of the other participants in the discussion boasts that his engine is better because it shows 120 psi. I reckon this is likely to indicate a faulty oil pressure relief valve, or a faulty gauge.

If the gauge is correct, I think it could result in crank seal leakage.

Another issue is the possibility of increased wear of the skew gears driving his oil pump and distributor (on these old engines it's not completely unknown for them to let go altogether).

More importantly, I think the excess pressure might push open the bypass valve in his filter and allow unfiltered oil round the system.

Anyone here know at what pressure the average oil filter opens it's PRV?

 

[ivymike]

I'm not sure how the high oil pressure is supposed to make the crank seal leak. I don't recall ever seeing any crank seals with pressurized oil on the wet side. There are some sealed interfaces at the head gasket which see full-pressure oil and might care about the increase. If the vehicle was turbocharged, the turbo oil seals might not like the pressure (assuming it receives pressurized lube).

He would be using more power to pump that oil up to pressure, and there would be a bit more gear load as a result.

If you've got hydraulic lash adjusters, excessive supply pressure can lead to pump up (and then the valves stop seating).

The filter bypass valve works on the pressure difference across the filter, so it doesn't really care what the supply pressure is.

http://www.rycofilters.com.au/products/oil

How many PSI do you think it takes to burst the can? About 500-600 psi, apparently.

http://www.shoclub.com/lubrication-oil/lubrication-oilpart5.htm

oil circuit diagram for an engine available here:

http://www.cumminsfirepower.com/documents/CFP83(06).pdf

 

[PEW]

Thanks Mike,

I see your point about the crank seals and the oil gallery seals in the head gasket. Both are weaknesses on these engines, the latter due to poor manufacturing tolerances on the sand cast aluminium head and block. In retrospect I agree the crank seals aren't directly subject to oil supply pressure, only crankcase gas pressure, so thanks for helping me see straight in that case. We don't have hydraulic lifters or turbos (these are 850cc, pre-cross flow, pushrod engines, 1950/60s technology), so that isn't a concern in this particular case.

Regarding my actual question, concerning the pressure relief of the oil filter; yes, I am aware that it works from excess pressure differential and not an absolute system pressure. I can't see how else it could do otherwise.

However, even on a new filter, excess oil flow / high pressure beyond some upper limit may cause the bypass differential pressure value to be reached, due to the inherent restriction of the filter material. My concern is that this is an unknown value, hence my question.

I do understand the basics of an oil lubrication system. My profession requires detailed knowledge of systems and their limits. For aircraft, information like a filter bypass Dp value is made available, on the schematic diagram of the turbine (or gearbox, or hydraulics) oil system, or in the text of the Flight Manual.

Some aircraft systems have defined limits on maximum engine (or transmission) oil pressure before applying power; in cold weather it's not unknown to have to wait for engine oil pressure to reduce to a top limit before opening the throttle, even on turbines.

 

[ivymike]

excess flow would lead to increased pressure drop, that's right.. I guess you might be seeing double the normal flow, if net restriction was held constant and the pressure doubled...

 

[MikeHalloran]

Your friend's super-high oil pressure suggests:

- bypass relief valve (pump to sump, not filter DP) set high, or shimmed, or plugged.
AND
{
- bearing clearances very, very low.
OR
- grossly oversized pump.
}

If he's not running a super pump, then the high pressure suggests he may not be getting enough flow to properly cool the bearings. ... but that's a self- limiting condition; the bearings will wipe off enough material to bring the pressure down. ;-)

Your friend is also at risk of blowing out the filter gasket. It makes a distinctive sound....



Mike Halloran
Pembroke Pines, FL, USA

 

[rmw]

And noticable mess.

rmw

 

[PEW]

Yes, I agree. He has said "I'll wait and see what happens, then I'll worry about it".

The other thing I've told him to watch out for is the pipe blowing off the back of the pressure gauge. I once had this happen in my Triumph Spitfire. I noticed the oil warning light was on at the same time my legs and feet suddely got very hot. Of course, I was wearing my best trousers and shoes at the time....

 

[evelrod]

The ultra high pressure readings can also indicate blockage and potential failure area in the oil system. The higher the pressure, the more heat generated and the greater the hp requirement. Any pressure over what is required for proper cooling of the bearings and maintenance of the "oil film" is wasted hp.
E.g., our 1600cc Lotus twincam engine as delivered, rated 112hp @ 6500rpm red line developed 55psi hot and 15psi hot idle. It was perfectly fine. Now the same engine is 206hp @ 9500rpm red line and, if left at the ~60psi limit would result in a very short bearing life. In order to get sufficient life for the bearings and valve train, it is necessary to modify the pump to maintain 95psi hot (idle is about 45/50 hot at 1500rpm). Hot for a race engine is 200f to 240f degrees, typically.

I'm not familiar with your engine PEW, but from here it looks like your in pretty good shape.

Isaac and Mike, oil filters with operating pressure ratings in excess of 250psi are commonly available. E.g., Wix 51515R that I use.

Rod

 

[Tmoose]

In the mid 70s a pretty successful local engine builder advised against real high oil pressure ( over 90 psi, maybe). The stated explanation was it would "wash" the bearings. I didn't understand it then, and still don't. I sometimes suspect it is related to the "coolant has to flow slow enough to absorb heat from the engine" theory, which does not hold up well when eating an ice cream cone while riding in a car with a few windows open.

 

[patprimmer]

Once again

Rod Is Right.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[PEW]

"I'm not familiar with your engine PEW, but from here it looks like your in pretty good shape."

Thankyou, yes, I'm happy with my own engine's oil presssure but mine isn't the engine showing 120 psi!

Does anyone know the answer to my question?

 

[ivymike]

I'm not sure that there is an "average" figure that's worth anything. Integral (to the filter) bypass valves are not even universally used. Wix says that 2-4 psi is about the right pressure drop for a clean filter. If you take their word for that, then double it for dirty, then add another 2 psi for good measure, you'd get to 6-10 psi.

http://www.peterverdonedesigns.com/oilfilter2.htm

LBP - 2-7 PSI, Low Pressure By-Pass Valve
BP - 8-10 PSI
MBP - 11-17 PSI, High Pressure By-Pass Valve
HBP - 18-25 PSI, High Pressure By-Pass Valve
SHBP - 30-45 PSI, Super High Pressure By-Pass Valve
XHBP - 50+ PSI, Extra High Pressure By-Pass Valve

(starting to open - fully open psi)
Rod's favorite Wix 51515R has a bypass at 8-11psi.
The Wix 51222R is at 18-22psi.
Some Wix racing filters have no bypass.

 

[patprimmer]

An oil filter pressure relief valve does not open because of absolute oil pressure. They open because of a pressure difference across the filter.

As you already stated excessive oil pressure breaks things like filter, oil lines and oil pump drives.

It also consumes more power to drive them and does no harm, but no extra good for the bearings. Once you have sufficient oil flow to keep the bearing at an acceptable temperature and to maintain a film of oil between the bearing shell and the journal.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[PEW]

Ivymike, Thanks, you have found the finite information I was looking for.

Pat,
"An oil filter pressure relief valve does not open because of absolute oil pressure. They open because of a pressure difference across the filter."

I said I am aware of that, when mentioned previously (see my reply on 19th, at 12:13).

Thanks again, for your time and assistance.

Merry Christmas to all!

Regards, Paul.