Too much oil pressure?

[HCA]

This is a good post to jump in, if I may please, and ask a question that I will one day be asking - what is a good oil pressure on the twin cam to ensure everywhere that needs oil gets a good supply - for road/normal use please?

At what pressure, without the return spring ever been tampered with, is it obvious that bearings are nearing the need of attention?

[512BB]

A twin cam in good condition, with a standard original oil pump, should start off running at between 45 - 50 lbs pressure when cold. If it runs at 50 lbs pressure when cold, that will quickly drop to about 45 lbs as it warms up. Please don't anyone tell me that my spring has sprung, instead go and find your grandmother and start teaching her about eggs. Never race the engine when at that pressure, or when cold.

When hot, ALL my engines run at between 38 - 45 lbs pressure with 20 / 50 mineral oil at 3000 rpm and above, and at about 15 lbs when at idle. I have never owned a knackered twin cam, but if one ran at anything under about 33 lbs when hot at 3000 rpm and above, I would consider it requiring work. Oil pressure at idle, when hot, is pretty unimportant, so long as there is some, say 5 lbs minimum

Now just a heads up gentlemen. I have heard that there is a shortage of oil filters due to the virus, and that people are starting to panic buy. Shame on them

[69S4]

Don’t tell me - people are using them in place of toilet rolls?

[Davidb]

FRAMM!!!

[HCA]

Thanks for the info. I shall file it away in 'things to know'!

For a good road engine, I take it then that the high pressure or high flow oil pumps are not going to add much benefit?

[rgh0]

For a road engine use a standard volume standard pressure ( 40 psi relief valve) pump. Even for race engines there is debate among builders I know as to whether you really need a high volume high pressure pump or just high pressure or just a standard pump. To some degree it gets down to the bearing clearances and type of bearings and type of oil you prefer to use and how much you worry about idle pressure versus 8000 rpm pressure and how long races you do and what ambient temperatures you race in etc etc . Part of the debate is a high pressure high volume pump uses more engine power to drive so a standard pump is worth a bit of HP at the back wheels through lower pump driving energy even if it may mean more frequent engine bearing refreshes and people spend a fortune to get a couple of HP

cheers
Rohan

[HCA]

We had the same debate in the Cooper S fraternity, they are probably still having it now!

I shall follow your advice, also save a bit, in listing a std pump on the shopping list...

Thanks!

[nmauduit]

To some degree it gets down to the bearing clearances and type of bearings and type of oil you prefer to use and how much you worry about idle pressure versus 8000 rpm pressure and how long races you do and what ambient temperatures you race in etc etc . Part of the debate is a high pressure high volume pump uses more engine power to drive so a standard pump is worth a bit of HP at the back wheels through lower pump driving energy even if it may mean more frequent engine bearing refreshes and people spend a fortune to get a couple of HP

Rohan

how hard/accurate would it be to estimate the actual power use of an LTC oil pump at a given rpm? I tend to think that it would also depend on actual pressure (at the specs level via the relief or below at max revs) and oil viscosity (temperature dependent as well) at a given time, so possibly quite build dependent...

Then is there a lot of that power "wasted" after the relief opens (in oil shear back etc)? if not it would make sense to have a HV version with regular pressure for a mild tune, as a insurance to maintain regular pressure at high revs without the loss/strain of the higher pressure all the time... Any mod you know of that would let one set the pressure relatively easily (e.g. via some screw acting on the relief spring preset)?

[Chrispy]

A very rough back of the envelope calc gives me around 330watts. That's assuming 50psi pressure, 0.33L/s of flow and 30% efficiency. Not accounting for viscosity, only density. Will be in the ball park, but of course when speeds get high, things get all weird with losses.

Edit:

I know you asked for power at each RPM, you could do it with some flow tests of the pump, or careful measurements of the gears. A lot of stuffing around for what may be 1-2hp in total at full blast.

[2cams70]

A high oil pressure gives you more oil flow at the bearing and hence reduced temperature at the bearing. The oil as well as preventing metal to metal contact acts as a coolant removing heat from the area.
A high capacity pump (i.e high volume) is only required where high oil leakage is a factor due to large bearing clearances and/or low oil viscosity. Bearing clearances may need to be larger at sustained high RPMs due to the greater deflections that occur under these conditions.
A high capacity pump will maintain a given set pressure at any given engine RPM with larger bearing clearances and more oil leakages better than a standard capacity pump.

You don't need anything other than the standard pump for a road/normal use engine

[Davidb]

I find the whole oil pressure thing fascinating-as you may have realised!
Fifteen years ago I had a lot of correspondence with Keith Dixon, an English engineer/designer, who, in partnership with Peter Lings [designer of the Jag v12 crank I think] designed racing crankshafts for historic Aston Martins:-DB2, DB3, DB3s. As a consequence I obtained one of these wonderful crankshafts, made by Arrow Precision in the UK for my DB2-it was a work of art!

Subsequently Lings/Dixon designed a racing crankshaft for the "E" Type jaguar and this has been instrumental, I understand, in the speed of the "Lightweight E Type" at the Goodwood Revival and elsewhere-Crostwaite and Gardner use these cranks... (They allow the long stroke Jag engine to rev to 7,500 rpm!)

(Edit: I should add that it is the attention to eliminating/reducing/controlling harmonics in these cranks that allows the high revs-getting the oil to the bearings means they keep running!)

Keith Dixon, who I got to know well enough that he came and stayed with us a couple of years ago, tried to explain to me the theories and processes they used in designing/manufacturing these cranks. While I have excellent mechanical aptitude apparently, I have minimal scientific knowledge/understanding so much of it is lost on me! I have just found an explanation from Keith that he sent me fifteen years ago but it is on my old computer and I cannot get it to transfer to my new computer so you will have to do with my pre'cise of it.

"Lings-Dixon crankshafts embody a form of crankshaft oilfeel drilling, believed to be unique, which substantially reduces the oil pressure required to feed oil into the main bearing from the oil feed groove in the bearing shell and reduces the centrifugal pumping effect out at the pin exit point. This is the Low Pressure Offset Compound (LPOCA) form of drilling.

Unnecessarily high oil pressure is detrimental because:

It absorbs significant power.
It causes unwanted wear of the skew gear oil pump drive.
Flow is far more important than pressure - it is flow that takes heat away.

The reduction in oil pressure required by these crankshafts depends on the precise angles of the drilling which for unavoidable reasons can vary according to crankshaft dimensions."

and continues later:

"As a rule of thumb, it is reasonable to allow for a pressure drop of 10% for each right angle change of direction {of oil flow} ."

There was also much change/modification to both the oil pump and the block oil passageways on these engines {Aston Martin} that were developed by Rex Woodgate who you may have heard of. The goal being to improve volume/flow but not increasing wear on the skew gears driving the oil pump.

I have applied these methods to my Lotus Twin Cam engine-hence my grinding the outlet port on the block, the intake and outlet ports in the pump and the entry port back into the block from the pump to remove right angle turns and "flow" the passage of oil into the block. I can do nothing about the flow of oil down the (Laystall made but Cosworth designed) crank of course.

2cams (and others): Do my modifications to ease flow increase the oil pressure without increasing drive gear pressure (by reducing back pressure)? {Keith is sick and I can't ask him}

[Donels]

Just to throw another thought into the discussion. I'm pretty sure a high pressure oil pump does nothing for the big end pressure. That is dictated by the pressure generated by the centripetal force of crank rotation. Some Rolls-Royce Merlin engines had very low oil pressure, sufficient being generated by the rotational force. If I remember correctly Tony Rudd's V8 Lotus engine initially had very low oil pressure using the Merlin idea. The Austin 7 oil system also used this with its 'spit and hope' system.

[Davidb]

OK, I may have made my story/post above too esoteric. (ie too much bs)

So my question to those who understand fisics much betterer than what I do (which is probably most of you)-

If we smooth out the flow from the oil pump pick up, to the oil pump and the into the block, eliminating resistance wherever possible, will the oil pressure be higher due to less back pressure?

[rgh0]

A long way to try to answer a simple question that has lots of complexity around it.

The oil pump is a positive displacement pump so the pump will displace a fixed volume of oil per turn. Thus more engine revs more oil pumped in proportion. This is not precisely accurate as the amount of "slip" in the pump where oil leaks back past the rotors will also vary with revs and pump back pressure and degree of wear on the pump but its an OK first order of magnitude statement,

As revs rise the amount of oil pump turns per minute rises and the flow rate of oil rises. As the flow rate rises the pressure drop increases from the pump to the oil leaving the bearings at atmospheric pressure and returning to the sump under gravity. This pressure rise is seen on the oil gauge as it rises from around 20 psi at idle on a hot engine to around 40 psi at 3000 rpm. How rapidly the pressure rises depends on many factors such as type of oil used, type of pump, bearing clearances and pressure drop in the passages due to restrictions or misalignment of the various passage joins

Once the engine revs rise above idle and thus flow rate commences to rises and thus pressure rises to reach the pump relief valve setting which is around 40 psi in a normal pump and 60 psi in a hi pressure pump, the relief valve opens and the pressure is held at this point by the relief valve dumping the excess flow back to the sump or back to the pump suction depending on which type of pump you have. With a high pressure pump the engine revs will be higher and the flow rate to the bearings greater before the relief valve opens at 60 PSI compared to a normal pump where the relief valve opens at 40 PSI. With a High flow / hi pressure pump with its relief valve at 60 psi the same flow will be going to the bearings as with a standard volume hi pressure pump however the point at which the relief valve opens will be at lower revs.

If the measured oil pressure is below the relief valve setting it means all the oil being pumped is going to the bearings. If the measured oil pressure is at the relief valve setting it means that you have reached maximum oil flow rate to the bearings for your particular setup.

Cleaning up the flow passages will increase the flow at which the pressure reaches the pump relief valve setting as the pressure drop will be less at the same flow. This will increase the oil flow to the head cam bearings as well as to the crank main bearings. The con rod big end bearings are lubricated by oil being pumped from the crank main bearings by the spinning crank itself. This flow will also increase slightly as the pressure inside the crank main bearings will have gone up a little with the increased flow but this is a second order effect.

so to answer your specific question around cleaning up the passages - the pressure will be lower and the flow rate the same at the same revs before the relief valve opens all other things being equal. However the maximum flow reached will be higher when the relief valve opens at slightly higher revs.

clear as mud ??

cheers
Rohan

[Davidb]

Thank you Rohan!

From Keith Dixon's paper:
"As a rule of thumb, it is reasonable to allow for a pressure drop of 10% for each right angle change of direction {of oil flow} ."
That is what I was trying to overcome/avoid.

So Rohan, it does mean that the flow is increased-which is a good thing-but pressure would be unchanged?

I am still trying to understand why I am getting 60psi oil pressure with the new standard pump and was getting 55 psi with the old, standard pump (measured with a stand-alone gauge)?