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Lower vs higher oil pressure question

clam

clam

Well Known Member
Considering a non-inverted oil system on a traditional horizontally opposed, normally aspirated engine, the Sky Ranch Engine Manual and Lycoming both make the statement "All things being equal a low but within limits oil pressure is more desirable than a high but within limits oil pressure."

Why is this? Crankshaft seal, pump wear, friction losses, something else?
 
Oil Pressure

That is counter to the much more recent information that high green oil pressure is very healthy for the valve components on a Lycoming.
 
Oil Pressure

For a more complete explanation search:
Increasing Oil Pressure in the new Cessna 172 and 182
 
Ok.. I read the article.. what do you propose is the ideal warmed up cruising oil pressure reading from the traditional back pressure port?

I used to go by the guidance that if you don’t have an oil filter, change the oil at 25 hours, and if you do have an oil filter ( most of us do), then change the oil at 50 hours.. I didn’t like how black the oil was at 50, and how quick it got there.. I started changing at 25 hours (oil AND filter) and I’m surprised that after a few cycles, it still looks pretty light after 25.. I would like to think that the shorter change intervals may help with valve lubrication.
 
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Oil Pressure

Tom I ran my Pitts with 0 360 parallel valve in the high 80 range. Aerobatics is a whole different issue and I felt the higher pressure reduced the recovery time for maneuvers where the Christen system can't quite keep up. 1000 hours of 3300 r/m plus and the engine behaved like new when I sold it. It probably flew at least 300 more hours on that engine.
In the era where most Pitts with fixed pitch were operated well above the engine red line I never heard of any kind of engine failure of the core engine. Mags, etc but never the core engine.
TBO on aerobatic engine in that era was 1200 hours.
 
Don’t know their rationale but I can say this from experience, high oil pressure robs horsepower. One could argue it makes leaks more likely too but I wouldn’t make that claim myself.
 
It makes some difference whether or not you have extra oil squirts on the bottom of the pistons and /or on the camshaft. The piston squirts cool the piston but increase oil consumption. The cam squirts prevent cam lobe wear.
Both of these modifications usually cause lower oil pressure but are overall considered beneficial.
 
If you take the time to look up the article that JRS14855 mentions, it claims the additional oil pressure leaks out of the lifter body, and up into the pushrod tube housing, then floods the rocker box, providing lubriation to the exhaust valve.. seems like a little bit of a stretch to me..
 
clam said:
Considering a non-inverted oil system on a traditional horizontally opposed, normally aspirated engine, the Sky Ranch Engine Manual and Lycoming both make the statement "All things being equal a low but within limits oil pressure is more desirable than a high but within limits oil pressure."

Why is this? Crankshaft seal, pump wear, friction losses, something else?
Click to expand...

I am sure this recommendation was based on wear. Back in the day, hot rodders building up 350 small blocks would add a high volume high pressure oil pump because more has to be better, right. Well, the oil pump was driven by a little fork in the distributor shaft and it just wasn't that strong. Many of these shafts broke and the engines seized up. The Lyc pump arrangement seems quite robust, but not sure it is bullet proof.

Just an anecdote and not necessarily support for this opinion. IMHO, the fatal flaw on the lyc is the anemic oil flow to the rocker boxes and increased OP can only help in that area, though unsure if it actually does help.

Larry
 
Oil Pressure

Cessna believes that it does help but wanted to keep it secret. Years ago the start red line was moved from 100 to 115 so I don't believe there is any downside to high 80's range.
 
jrs14855 said:
Cessna believes that it does help but wanted to keep it secret. Years ago the start red line was moved from 100 to 115 so I don't believe there is any downside to high 80's range.
Click to expand...

But like the article said, moving the pressure pickup from the back to the front could be a 10psi reading difference, and that’s without actually changing the engine oil pressure.. so moving the limits only to accommodate the pressure location doesn’t change the oil pressure, right?
 
Oil Pressure

On a "new" Cessna the gage is plumbed to the front of the right gallery. So 70# on the gage is really 78 at the old location. My theory is that Cessna did that so owners would not freak out about "oil pressure being too high".
 
Low but within limits for when the oil is warm. What does the oil pressure do shortly after start when the oil is cold? It's going to be higher, but is it still within limits?
 
Oil Pressure LYC 115 (Cessna)

I asked that question at OSH a few years ago (LYCOMING). They moved the value on Cessna to 115PSI, but only for cold engine starts. HMMMM...

Also, on a Pitts (owned three S1's) always ran at least 90 PSI because the
pressure would go down to 60 inverted. And yes, would pull the throttle back
slightly if the RPM went past 3500 RPM (needle pegged). Lycoming told me
don't worry until you pass 4000. Everyone complains about the noise on the ground past 3500 too....
 
When I took the Lycomong courses it was suggested that higher oil pressures on start up would help valve lubrication and help prevent valve sticking.
Came up during a question about idle rpm on a cold start and one consideration is enough rpm to get good oil pressure.
Was counterintuitive to us cold climate flyers who think lower rpm on a cold start is more better.
 
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