Oil pressure.

[Johnny4428]

Ok here’s one for engineers and mechanics of which I am neither. I have had a bit of niggling doubt about the oil pump on my 550. So have had a gauge fitted to the oil gallery plug underneath the points cover and have done an initial low revs check at idle to 2500rpm. So far I have had 40psi at idle and 60psi at 2500rpm. With nothing to compare against, and the general question asked on mr Google, coming back with a very vague answer, any guidance would be much appreciated.

[Nurse Julie]

By what I remember, anything much over 60 psi at the oil pump, the pressure relief valve on the pump should open, thus stabising the pressure. If its valve is opening early, the spring could have gone weak or the gap is too large between the giroters, or there is pitting on the giroter surfaces. How accurate is your gauge? Have you got oil flow at the camshaft?

[Bryanj]

40psi at cold idle is good, i think the pressure switch trips at 7psi so anything over that at hot idle must be ok by mr honda.
As said i think 60 is about where the relief valve opens

[Trigger]

What is your RPM at tick over ? .
The only way to get a accurate reading is removing the oil pressure switch and screwing in a pressure tester at the pump. Bryan is correct, the oil pressure switch is set at 7.1 psi 

[Trigger]

While we are on the subject of oil pressure. A couple of years ago i bought 5 NOS oil pumps in sealed Honda boxes , fitted one to a engine, started the bike and bang. After replacing the oil filter and a new housing I found that the pressure relief valve in the wrong way.




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[Johnny4428]

Holy cow Trigger! Lot of pressure to do that. Thanks all for reply’s, the tick over speed is around 1100rpm.
I’m quite sure all is well as far as oil pump goes, but I’m having doubts now about aftermarket oil pressure switch. (too tight to by an OEM one at the time)😆 Don’t know how accurate the gauge is Julie. I will see if I can test against some other known pressures. That explains why the gauge went up to 60psi and didn’t go above although I only tested up to 2500rpm. More testing to come.

[Trigger]

You can get a Honda genuine switch from Julie at about £25.
Maybe it is the aftermarket switch is at fault. Don't know if you read this, but i was getting different pressures with a aftermarket brake switch >>> https://www.sohc.co.uk/index.php/topic,29459.msg283073.html#msg283073

[Johnny4428]

Yes, read that indeed Trigger, what a bummer. That would have pushed most folks patience getting to the bottom of that. Yes definately going to change out pressure switch. I’ll give Julie a message. Thanks.

[Johnny4428]

Just to follow up a bit, I carried out an oil pressure test on the 750 with similar results to the 550 maybe a tad higher pressure at a similar idle speed. Have ordered another oil pressure switch from Nurse Julie just for peace of mind. I think it is safe to say that the oil pressure anxiety is easing.

[Trigger]

I was testing some oil pumps today for a 550 and was getting 45PSI at the oil pressure switch port at 1200 RPM. Yours seems to be with in spec   

[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]

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.

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.