I'd like to discuss oil pressure. A previous thread had a post about running higher oil pressure at 90 (Walt). Mine is 73-75 as delivered by the factory. Who adjusts theirs up and why? What's the downside of adjusting the pressure up? What does it actually do to raise the oil pressure? Does it reduce oil flow?
Van's Air Force
The definitive Van's Aircraft support community! Buying, building or flying an RV? Join our exclusive family of mentors and enthusiasts!
Oil Pressure
- Thread starter JDA_BTR
- Start date
sansoneservices
Well Known Member
I don't see an advantage to turning oil pressure up. It should be within factory specs
A known flaw in the lyc design is too little oil flow through the lifter and the resulting limited amount of oil getting to the valves. Oil on the valve stems helps with cooling the valves and we know that lycs suffer from heat issues on the exh valve. Some believe that increasing the oil pressure will help increase oil flow to the rocker box and thus help with exh valve cooling. Don't believe anyone has ever proven that this helps though. I run OP in the mid / upper 80's in the hope the above helps.
most every oyher lifter out there has a small port that flows oil out of the lifter and into the pushrod. The lyc lifters only sends oil that bleeds past the interface of the two plunger pieces (no dedicated passage for oil bleed). These are tight fit, hand lapped pieces. I suspect there is decent variability assy to assy and may explain why some engines have less flow than others, making it difficult to showcase this as a problem in the exh valve area.
most every oyher lifter out there has a small port that flows oil out of the lifter and into the pushrod. The lyc lifters only sends oil that bleeds past the interface of the two plunger pieces (no dedicated passage for oil bleed). These are tight fit, hand lapped pieces. I suspect there is decent variability assy to assy and may explain why some engines have less flow than others, making it difficult to showcase this as a problem in the exh valve area.
Last edited:
There are lots of people here with better memories than me but here's a shot.
Lycoming quietly increased their OP and OP reline settings (95 and 115psig?) some years back. Additionally, some engines had their measuring points moved from the pump discharge area to the end of the oil galley (front of engine) for a better reading of overall OP health. This would mean an even higher pump pressure on top of the increased values due to the losses. Most speculated it was because of the issues w/valves as Larry stated. Higher OP readings mean less flow is going through the bypass/higher flow to the components. I'll definitely be aiming for the upper limits when it's time.
Don’t know where it is in the docs, but can be done. Most modern engines have an external adjuster on the relief assembly. Older versions require the removal of the relief assembly and add washers or change springs. Raising pressure will not increase temperature in any meaningful way.
The oil pressure is raised by increasing spring pressure on the pressure relief valve causing excess oil to dump back to the oil pump inlet at a higher system pressure. It's just a ball on a seat with a spring. It won't reduce oil flow, it will increase it. When the bearings become worn and clearances increase, oil pressure will go down. It's a classic sign of a worn out engine. Increasing the spring pressure will possibly bring the pressure back up, (and maybe make an unwary buyer think the engine is healthier than it is), but the wear is already there and sooner or later the bottom end will let go.
Bill Marvel did a study on this some years back. Might still be out there in googleland. The Lycoming mushroom lifter and hydraulic unit design was lifted from flathead auto engines of the 30s and was never designed to do more than pump itself up for zero lash while operating. Lycoming decided to use the bleed down oil from the hyd unit to feed through hollow pushrods and drilled rockers to lube the rocker and help cool the valve guide. The quantity of this bleed down oil doesn't really seem to be sufficient to do that job so increasing the oil pressure should, in theory, help with that. Another issue is that the cam and crank both get their oil pressure from the oil galley on the right side of the engine making for lower oil pressure on that side. This is a contributing factor to #3 cylinder having more valve guide problems than the others. The barrel lifters they used in the AD engines seem to be better in this regard, though they had a lot of other problems in development which led to some, well, AD notes.
A good question is: do the hydraulic lifter units in the newer roller lifters do a better job of passing oil to the top end? Other than the possibility of blowing up an automotive oil filter (not that likely) I don't see why raising the oil pressure some will do any harm.
Ed
Bill Marvel did a study on this some years back. Might still be out there in googleland. The Lycoming mushroom lifter and hydraulic unit design was lifted from flathead auto engines of the 30s and was never designed to do more than pump itself up for zero lash while operating. Lycoming decided to use the bleed down oil from the hyd unit to feed through hollow pushrods and drilled rockers to lube the rocker and help cool the valve guide. The quantity of this bleed down oil doesn't really seem to be sufficient to do that job so increasing the oil pressure should, in theory, help with that. Another issue is that the cam and crank both get their oil pressure from the oil galley on the right side of the engine making for lower oil pressure on that side. This is a contributing factor to #3 cylinder having more valve guide problems than the others. The barrel lifters they used in the AD engines seem to be better in this regard, though they had a lot of other problems in development which led to some, well, AD notes.
A good question is: do the hydraulic lifter units in the newer roller lifters do a better job of passing oil to the top end? Other than the possibility of blowing up an automotive oil filter (not that likely) I don't see why raising the oil pressure some will do any harm.
Ed
I suspect lyc put the same poorly designed plungers in the new roller lifter bodies. They just can’t help themselves. They just won’t accept that as the root cause.
Certainly not in writing. They are afraid to redesign anything that could be construed to make them liable for not doing better in the first place.
Another thing that I didn't mention is that the ports on the lifter bodies don't register to the annulus in the bore all the time which reduces flow. The Superior roller lifter engine lifters register full time - the annulus is wider. Can't say about the Lyc roller setup.
Ed
Another thing that I didn't mention is that the ports on the lifter bodies don't register to the annulus in the bore all the time which reduces flow. The Superior roller lifter engine lifters register full time - the annulus is wider. Can't say about the Lyc roller setup.
Ed
I have been dealing with sticky valves (#2 cyl io390) and have raised the oil pressure from 75 to 80 thus far. I think that maybe 85 might be my goal and will note impact on temp and ultimately on the sticky valve interval. Other threads on this site go into the sticky valve stuff and higher oil pressure is a possible help.
The basis of the sticking valve problem is sodium filled valves and lack of cooling oil flow. Marvel and Scott - "When Lycoming adapted the automotive mushroom style hydraulic tappet assembly to their engine, they failed to provide a dedicated, unrestricted oil flow path to the rocker boxes that could carry sufficient oil for adequate cooling as well as for lubrication."
More reading material:
Ed
More reading material:
Airframe and Powerplant
Understanding valve malaise Among the piston designs to power light aircraft, a handful of Lycoming's engines have gained the distinction of being the most reliable and durable powerplants around. In particular, certain versions of the O-320, O-360, and O-540 engines reign supreme in hangar talk...
www.aopa.org
Ed
Cessna 172 since the restart of production take the oil pressure reading from the front of the oil gallery. 80# reading at the front will provide 90# at the rear gallery. I ran my 0 360 parallel valve in a Pitts for 1000 hours in high 80's. No valve problems. I also believe the higher pressures result if faster recovery time for momentary loss of pressure doing acro.
The 115 psi limit is for cold starts, not for continuous operation.
I don’t believe the sodium filled valve's are part or the problem, it is just too little oil flow. The sodium valves was one of lycs misguided changes to address valve and guide over heating and suspect that they do marginally reduce heat transfer to the guide. Less mass = less heat storage = less heat transfer. They seem to keep trying to stop heat transfer instead of improving cooling via oil. It seems so basic, but they just don’t seem to accept it.
There are a number of different spring weights and lengths as well 2 different housing lengths so it would be difficult to say how many turns you would require. I have the long housing with the heavy spring and as I recall I turned it in 2 turns to go from 70 to 85. FWIW.
Sodium filled valves aren't the whole problem, they just made it worse by transferring more heat from the valve head to the stem and into the guide. The molten sodium in the valve stem moves more of the heat higher up in the guide and into the head. The cooling fins can't necessarily remove enough of the heat to keep the guide from wearing excessively. This is why sealing up the baffles is so important. Lower CHTs help reduce valve guide wear. If the guide were being bathed with sufficient oil it would run cooler and there would be a lot less wear on the guide. Both air and oil cooling are important for the health of the exhaust valves. I think we agree that what we've been talking about here is the possibility of increasing oil flow to the top end and increasing valve guide cooling by increasing oil pressure with the goal of reducing guide wear.
Continental engines with solid valves do more of the heat transfer at the valve seat and the bottom part of the guide. Their valves stick a lot less often and the guides tend to wear less. Their problem is more often at the valve/seat contact area. If the contact area isn't uniform around the whole circumference, the valve tends to burn. This is why Continental recommends/mandates borescope inspections so as to catch the problem early. Not such a bad idea for Lycomings also, though sometimes the guides are already worn enough to cause the bad interface at the seat. Lapping the valves in place will help for a while, but there is no long term cure for an out of round guide short of replacement.
Ed
Continental engines with solid valves do more of the heat transfer at the valve seat and the bottom part of the guide. Their valves stick a lot less often and the guides tend to wear less. Their problem is more often at the valve/seat contact area. If the contact area isn't uniform around the whole circumference, the valve tends to burn. This is why Continental recommends/mandates borescope inspections so as to catch the problem early. Not such a bad idea for Lycomings also, though sometimes the guides are already worn enough to cause the bad interface at the seat. Lapping the valves in place will help for a while, but there is no long term cure for an out of round guide short of replacement.
Ed
