What else effects oil pressure, besides the obvious?

[bpu699]

Lots of posts on here in regards to low/borderline oil pressure.

Typical answer is to check the sender/guage and verify its low. Then, check the relief pistons. Next comments is usually to look at the internals/crank/bearing/etc.

But what else can effect oil pressure? I have looked over oil pressure schematics in the Pelican books and Bruce Anderson's book, and wonder:

1) Wouldn't a plugged/malfunctioning thermostat (in the case) also cause problems?
2) A plugged/crushed oil cooler on the motor
3) Plugged/crushed external lines (in and around the motor, not the ones on the passenger side to the front cooler). I know many of these are on the scavenge side, but wouldn't an inefficient inflow upset the output pressure?
4) Plugged oil filter?

Any of these a possibility?

My 930 is about 4.0-4.5 bar at 6000 rpm. Leakdown 4% or less.... The motor is somewhat apart to reseal it. I don't intent to take off the heads or redo the bottom end, as pressures are "pretty good." But is there anything else to check, to make them slightly higher? I definitely don't have 6 bar at 6000 rpm. But none of my other 911's had that either. Car has 50,000 miles on it or so, top end rebuilt by the PO...

Thoughts?

[Wyvern]

the "weight" of the oil will make a difference.

One thing to keep in mind is that oil pressure does not necessarily mean oil flow ...

[Flat6pac]

Oil pressure is resistance to oil flow.
You might try the cam oil line restrictors, keeps flow in the crank and pressure on center line

[Flat6pac]

Oil pressure is resistance to oil flow.
You might try the cam oil line restrictors, keeps flow in the crank and pressure on center line

[bpu699]

Thanks guys, I already have the cam restrictors installed, 20/50 castrol gtx.

I was just wondering if there is anything else to check. Researching posts, folks imply that nothing on the scavenger side will effect the oil pressure. But, I would think thats not the case...

If the "influx" side has a flow limitation, this would effect the "efflux" side, right? Or is it simply that this just doesn't happen in real life?

I have noticed something interesting also...right after an oil change, pressure is outstanding...

At idle, cold, 5-6 bar...but only the very first time you run the car. Each subsequent start was up to 4 bar, cold, tops. Why? Does the oil "degrade" or thin out with the 1st use?

I also noticed when I put the cam restrictors on, this also happened. Phenomenal pressure on start up the first time. Then lower.

The idle pressure, even warm is quite good. 2.5-3 bar, even hot. But it plateaus at 3000 rpm at 4.5 bar, and stays there up to 6000rpm. I shimmed the pressure release valves, didn't make a difference...

Oil pressure is fine based on the 10psi/1000 rule...but I just want to see if there is anything else to check, as the motor is out...

[Sigurd]

Imho I would say the oil pressure is just fine.

Have you installed Carrera tensioners? That can decrease the pressure somewhat.

The probable causes you mentioned will most likely increase the pressure. Internal leakage, or lower oil viscosity, will decrease it; bearing or oil pump wear, higher temperatures. The Carrera tensioners are another internal leakage.

When the engine is back into the car you could double check the pressure with a mechanical guage.

Why is the pressure high after an oil change?

I don't know. As I mentioned lower oil viscosity will decrease the pressure. If you experienced fuel dilution of the oil this could happen. However the fuel would evaporate when the oil came up to operating temperature.

What is the normal oil temperature?

[bpu699]

Thanks for the input guys. I am still amazed that the world functioned before the internet .

Bo

[lindy 911]

The oil tank is the supply for the "influx" (intake) side of the oiling system. The oil tank is an open circuit as it is vented. The scavenged oil returns to the tank via the filter. The intake side of the system is direct from the oil tank supply via the "S" hose. If there were restrictions in the scavenge side of the circuit from the sump in the case to the tank, the case would fill with oil and the tank would drain. If the tank ran out of oil the pump would starve and you would see no oil pressure. So that isolates any restrictions to the "S" hose from a supply side.

Lindy

[Walt Fricke]

Bo
I'm with the others: you are worrying about something you don't need to worry about. Though I see why you are wondering, and can offer no explanation as to why you get better cold oil pressure on first start up after an oil change than at any subsequent time (other than the suggestion of dilution with fuel). If this were my car, I'd switch to Mobile 1, though. See if that makes things more consistent. Maybe the Castrol viscosity drops a bit after first use. M1 is very consistent in that regard.

Also, you can't easily "plug" any of the things you are imagining. Maybe if you left a shop rag in a hose or something. But nothing short of that. Small crap in your crank oil passages can fry a rod easily, but you will have great oil pressure until that happens. The oil passages to where your oil pressure sender is located are pretty large, so the kind of little stuff that can cause trouble (think cam spray bar holes) is not likely to block them. You could try a new oil pressure sender, though those either quit working altogether, or have a jump somewhere - above some pressure it pegs. You could try a mechanical gauge, but that won't work until the engine is back in the car.

So how about malfunctions: a) the engine thermostat fails in the closed position - with oil not routed through the engine oil cooler. So you'd notice a rise in oil temps to go along with some eventual decline in oil pressure as the oil thins out with heat.
b) how are you going to crush the engine oil cooler without busting it at least a little bit open and leaking lots of oil? It doesn't get crushed, certainly not to the point that oil won't flow reasonably through it.
c) None of the external lines are in the pressure system. Lindy explained what that means. If you had a red rag in the S hose, you would not have the pressure you do have.
d) The oil filter is in the oil return system. If blocked, that means less oil to the tank, etc. Also, the filter (factory recommended types, anyway) has a bypass. If the filter is all clogged, oil will flow right through it. Naturally, whatever caused this is going to mean an engine rebuild anyway.

[Cupcar]

FWIW-I had an engine failure in a racing car using a 911 engine with a GT3 oil pump that was due to the 3 foot long tank to engine suction line sucking shut at high RPM. This was diagnosed post-mortem with in car video of gages showing pressure drop as engine RPM increased - the reciprocal of the desired needle motion!!

[bpu699]

Thanks for the help guys...

I am probably paranoid, and just wanted to check everything while the motor was out. I keep reading about oil pressure in threads . Pressure was checked with a mechanical guage, and is 1 bar higher than the dash guage (at idle and through 5000 rpm).

I am getting 10psi per 1000 rpm through about 5000 rpm...seems plenty...

I just wanted to see if there was any thing that could be impeding flow...to get the pressure even higher ...