SOHC.co.uk Forums > CB500/550

Oil pressure.

<< < (3/5) > >>

McCabe-Thiele (Ted):
My late father used to tell me never fit an oil pressure gauge if it isn't fitted as standard son as you will be disappointed with the readings.

Matt_Harrington:

--- Quote from: McCabe-Thiele (Ted) on July 18, 2023, 07:40:39 PM ---My late father used to tell me never fit an oil pressure gauge if it isn't fitted as standard son as you will be disappointed with the readings.

--- End quote ---
I was told the same - you only start to fret about the engine....

Johnny4428:
That’s interesting Trigger to get some kind of comparison, thanks for that.Ted and Matt, I think you’re spot on! Need another project so that I can stop looking for issues that are not there!🤣🤣

K2-K6:
The confusion (in generally discussed observations) comes from the pressure not being an absolute measure of what the oil system does.

Like blood pressure, it tells you a fair bit, but with the need to interpret what the system in that engine is making use of. Essentially it tells you the pump is working and there is resistance in the distribution system to give pressure by restriction against pumped volume.

On the 750 (unsure of location on others) the oil pressure switch is absolutely perfectly placed in crank supply gallery and after the oil filter. That tells you all that's needed really without extra monitoring. 

A scenario though, if the small restrictors to head supply are blocked, you'll have perfect (possibly even higher ) pressure but with no supply to cam ! Starts to show the limitations of pure pressure report. The camshaft etc is effectively open bath so you can't monitor pressure there anyway.

Many views seem to assume that the pressure recorded is that which keeps the crank bearing and big ends from wearing. Particularly for big ends it would be nowhere near enough to perform in that way. The bearings operate by being flooded with oil, the film strength and shear properties of which then prevent the shells touching the metal. That oil film has to be changed, hopefully for each revolution, by the quantity that's supplied.
If you up the pressure of the system by going to a thicker oil viscosity, then the replenishment rate of those bearings reduces but pressure remains the same. In extreme cases the pressure in system (leak down performance set against pump supply) will blow open the pump pressure relief valve and now stall the supply pace to the crank. Most of the oil now returns to sump while relief valve is open.
You can now see that a maximum pressure (60 psi) that looks healthy, is getting the system close to a problem, similar to blood pressure in fact.

The pressure relief valve on all system is there to prevent pump and pump drive failure (you don't get one on a heart supplied system, well apart from elasticity in arteries etc) ultimately to make sure its reliable in service life, ideally the system needs to be run away from this risk.
Thats done by matching oil viscosity with the typical temperature its used at in service with engine warm.

Too thick a viscosity (with perfectly healthy sysem leak down performance) will raise risk, if oil supply volume stalls then oil film can break down locally through too much heat at crank bearings from not being replaced fast enough. The "classic" film failure on plain bearings is to get pickup of shell material onto journal as the oil doesn't keep them apart. Too much of that and a shell spins with obvious effects.

Very few discussions on engine involve the rate of oil replenishment, but just focus on pressure, the two go hand in hand with people worrying sometimes about the wrong one, or having confidence that the higher the pressure the better.

These engine are generally very good with the design they have, and very low problem occur. But it shows that oil pressure is not the full picture.

McCabe-Thiele (Ted):
Too true in the past Jaguar & MB were what I thought were low oil pressure systems with 40-45 psi considered very  good.

You make a good point about replenishment rates - I'm thinking also that to save weight using narrower journals might be the reason that pressures might have increased on more modern engines.

Navigation

[0] Message Index

[#] Next page

[*] Previous page