what's good oil pressure?

I just replaced the oil pump on my 74 MKII with a new one from Norvil. I fitted a pressure test gage and get 50 psi cold at 3000 rpm, 40 psi at idle. It drops rather quickly as the engine warms up. After about 5 minutes it was down to 35 psi. Is this normal?

If oil viscosity (20/50) rises with temperature, why does pressure drop so?

Good news is that oil is returning to the tank at idle, which it didn't do before.

The oil viscosity will drop even on a 20/50W, I use 10/60W and the difference is a lot less but it does still thin out a bit at full temp, this lower difference was one of the reasons for moving to 10/60w. What revs are the 35psi at after the 5 mins ?

maylar said:

I just replaced the oil pump on my 74 MKII with a new one from Norvil. I fitted a pressure test gage and get 50 psi cold at 3000 rpm, 40 psi at idle. It drops rather quickly as the engine warms up. After about 5 minutes it was down to 35 psi. Is this normal?

If oil viscosity (20/50) rises with temperature, why does pressure drop so?

Good news is that oil is returning to the tank at idle, which it didn't do before.

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It's normal for oil pressure at idle to drop once the oil heats up. In fact, if the oil gets really hot the pressure will probably be less than 5 psi at idle. The general rule-of-thumb is 10 psi/1000 rpm with oil at normal operating temperature. The maximum pressure is limited by the pressure relief valve. A new pump is capable of delivering over 100 psi.

maylar said:

If oil viscosity (20/50) rises with temperature,

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"20w/50" multigrade oil has the viscosity properties approximately similar to SAE20w (W = winter) oil when it's cold, but also has a similar viscosity to 50 grade oil (at 210°F, 100°C) when it's hot, which means miltigrade doesn't get thicker as it heats up, it just doesn't thin out at the same rate as a mono oil would over the same temperature range.

Look at the graph below, and you will see that the SAE20w and 20w/50 multigrade start off at the same viscosity, but as they heat up, the SAE20 monograde thins out much more rapidly than the multigrade.

The SAE50 starts off very thick when cold, but at 210°F/100°C the viscosities of the SAE50 and 20w/50 are both about equal.

http://www.realclassic.co.uk/techfiles/oil030319.html

maylar said:

If oil viscosity (20/50) rises with temperature, why does pressure drop so?

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Who told you that Dave?

Oil viscosity drops as the temp raises so the pressure drops, its just the rate of the drop that changes between single grade and multi grade.

I wouldn't hurt to get the relief valve checked it may well be blowing off a little too early.

Cash

cash said:

I wouldn't hurt to get the relief valve checked it may well be blowing off a little too early.

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As maylar is getting a reading of "50 psi" cold, then the relief vave would appear to be opening within its normal operating range of between 45 to 55 psi?

A shim kit is/was available to adjust the relief valve blow off pressure by adding or subtracting shims under the spring, however if the oil pressure drops below the blow off figure, then theoretically the valve should not open?

I'd agree with LAB here. One thing I don't see folks mention is that pumping oil takes hp. You want adequate pressure but no more than you need.

As maylar is getting a reading of "50 psi" cold, then the relief vave would appear to be opening within its normal operating range of between 45 to 55 psi?

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Hi LAB,
But the book doesn't say hot, cold, what fluid, or even what running pressure the PRV is set up with, and it should. During my training too many years ago, I was told in the case of motors, if the set up info' wasn't available other than the relief pressure to set/check the PRV with the motor and oil hot.

Clearly if maylar packed his PRV with shims when cold it will, or should rocket dangerously off gauge. However, if he fitted a thin shim into the PRV while the motor and oil are hot I would expect him to see the pressure rise above 35psi at 3000 or more rpm. If it doesn't and if he feels 35psi is too low then he's got a problem as supply isn't meeting demand. (Duff gauge, worn motor, getting too hot, thin oil, etc)

My Mk3 hits 70/75psi cold and 45/50psi hot when running at 3000rpm and above, but never exceeds those pressures. It does not leak, burn oil or blow seals and is a very high milage motor, I use synthetic 20/50. (I've got to say I only use the gauge for testing and remove it for normal use)




Cash

I'd agree with LAB here. One thing I don't see folks mention is that pumping oil takes hp. You want adequate pressure but no more than you need.

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Yes Cookie I agree, but what is adequate pressure? Norton have left it wide open to interpretation. Perhaps it doesn't really matter providing the pump pressure doesn't blow gaskets or pop seals, and flows enough head to supply the motor. Personally with an engine I would err on the side of slightly higher than slightly lower at normal running and temperatures incase the unusual happens.

Cash

kommando said:

The oil viscosity will drop even on a 20/50W, I use 10/60W and the difference is a lot less but it does still thin out a bit at full temp, this lower difference was one of the reasons for moving to 10/60w. What revs are the 35psi at after the 5 mins ?

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Initial startup was 40 psi at idle, increasing to 50 at 3000 rpm. I didn't take it much above that as it didn't seem to increase (maybe the pressure relief is doing it's job?).

After 5 min it dropped to 35 at idle and 40 at 3000. I'm using Valvoline VR1 20/50.

LAB that's a great chart. I knew that multigrade changed over temperature but wasn't aware that 50W was thinner hot than 20W is cold. Interesting.

My reason for replacing the ($$$) pump is that the return side wasn't up to snuff. Checking the oil level after riding on the highway the tank was full but cruising around town the oil level dropped way down, almost off the dipstick. Now the tank remains full even at low speeds. I'm not afraid to ride it any more.

I used to err on the high side, way high, until I did a diesel training course where we pulled a rod bearing cap off a 6 71 and just put a piece of wire there to hold the rod during start up. This demonstrated two things, the two stoke kept constant pressure on the rod, and how little oil pressure it could run on.
The next thing that began to convince me was a course where we used a dyno. We were finding hp by reducing internal pressure, drag, seal pressure, bearing width, and oil pressure.
It all adds up and modern car engines take all this into effect. Oil pressure on my BMW at idle would have been scary a few years back but now it's good engineering.

I took off the timing cover and found that the circlip , holding the oil seal in the cover had come loose ( must have been entirely my fault ) and that the seal was pushed out .

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Years ago I rebuilt a T100 motor for a friend. The bike was one of six Triumph took off the production line and fitted with swinging arms and tuned for the IOM in 1952. It ran like a train and the front forks put my Mk1 Commando's to shame. Later my mate removed the outer engine covers and sent them off for polishing. After refitting he noticed the oil tell-tale wasn't working and assumed it was faulty. Months and many miles later he fitted a new tell-tale and it didn't work either.
After pulling the timing side I found the crank seal bush was missing, it had likely fell out during polishing. And that motor showed not a sign of wear and ran no better then before.

Norton reckoned their Commandos while racing flat out down the mountain on the IOM would often show zero oil pressure with out harm. Rumour has it that's why they never fitted an oil pressure gauge.

Cash

I use Kendall 50 in my MkIII and get 100 psi at start up when cold . Outside air temp. 25 C , 3000 rpm gives me 55 psi.
Idle 1000 rpm , pressure 20 psi. I also use the anti wet sumping valve and these numbers don't change with or without the valve installed.

Ron,

According to the Norton workshop manual, section C22, oil pressure should be no more than 55 psi. This limit is set by the pressure relief valve. Yours may not be set correctly or your gauge may not be accurate. I've been told by a Norton expert that excessive oil pressure is detrimental to the bearings. He claims it washes out the bearings. I'm not certain what that means, but I conclude it's bad. You mention 20 psi at 1000 rpm. Is that with hot or cold oil? There is a vast difference in oil pressure with hot oil vis-a-vis cold oil. When hot, 220°F in the oil tank, my Combat will produce about 5 psi at idle, whereas when cold it gets about 30 psi at idle. This prompted me to install an oil cooler. Now the oil temperature rarely gets over 200°F in the tank and the oil pressure at idle is around 10 psi at that temperature.

I had an interesting (to me) experience the other day. I was riding the Norton, fairly long trip, close to a 400 mile day. For a goodly portion of the trip, with the oil and engine already fully warmed, I was on a really nice rode riding through southern Massachusetts, up and down, in and out of the sun and through very rural areas and less so, very varied. Temperature was varying significantly - as much as, I would say, maybe 12-15 degrees F depending on whether I was in foresty areas in the shade or less rural ones in the sun.

Anyway, I was getting a very predictable 6 PSI variation in oil pressure at cruising speed (45-55 MPH). A few minutes in the hotter temps and I would get 37 PSI. Move into a cooler area and in a few minutes, 43 PSI. Back and forth for miles.

This is on a mix of 20W-50 pure synthetic oil (installed at oil filter change) and straight 50W non-syn (topped up from time to time between changes to make up for what's getting past the left cylinder rings and being used to ensure no internal corrosion to the drive-side exhaust pipe).

FWIW.

PS, ya think I spend too much time looking at my oil pressure gauge? :?

This was on a late Matchless G80CS which also uses the Norton pump, but I did some testing with the pump mounted on the engine but driven by a drill. The max speed was equivalent to about 4500 rpm engine speed. I was pumping 20W50 multigrade in and out of the sump with clear hoses into a clear container. It amazed me how much air becomes entrained in the oil. The gear pump really whips it into a froth.

The air is much more compressible than oil and I think that is why the pressure drops so much. The viscosity of the oil would seem to me to have much less of an effect.

Russ