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MG Midget and Sprite Technical - Oil pressure conundrum

The oil pressure has been taking a long time to come fully up, especially in cold weather (eg last winter). I wondered if the relief valve wasn't seating well, so invested in the ball type. This comes with its own spring and after fitting indeed the pressure sprang up - all the way to 90psi. I recall that in the 70s uprated relief valve springs were all the rage, and boy racers delighted in impressive gauge readings. On my engine now it never goes below 80psi at running speed even when toasty hot.

But the downside to all this is that the rear main bearing oil leak that I have been meaning to fix for some time is now far worse. So what I want to do is to retain the ball-type relief valve, but reduce the pressure to a sensible level. Even after I've renewed the oil seal (which was never very good) I'm concerned that it won't stand up to 90psi. Does anyone know what length spring I should use? The standard spring is much too stiff as the ball takes up more space than the cup-type valve.

Regarding the leak, we discussed this in another thread recently and the provision for drainage from the bearing cap was mentioned. I checked a spare cap and it already has 2 drain holes that go back to the sump, so no improvement possible here. Careful attention to the fit between the lower half of the seal carrier and the bearing cap will I think be the primary focus, although I still don't much like the design of the Morris Minor Centre kit. There seem to be better kits available now, eg one with a split seal that goes behind the crank flange instead of running on it.

Les
L B Rose

Basic physics states that high pressure = low flow. So the higher the oil pressure that you have in your engine, the less oil that there is reaching the bearings. Doesn't sound like a great idea to me.
Simon Wood

I fitted a ball and new spring to a couple of engines, but found I had to trim the spring to get the pressure down to an acceptable level.

It didn’t take much - possibly only 2/3 to 3/4 of a coil IIRC.

I know that the first time I tried it, I cut too much off and had to get another spring.
Dave O'Neill 2

Do you know that your gauage is accurate?

As I understand it the pressure needs to be high enough to maintain an unbroken oil film over the bearing surfaces. But too high a pressure has negative effects. As Simon points out, it may mean less volume of oil passes the bearing and a significant job of the oil beyond lubrication is to transport the excess heat away from the bearing. At the same time that oil being squished in the bearing at high pressure will itself create extra heat (remember the bicycle pump end that gets hot from pressure?) so too high a pressure is not good.

One way of reducing the relief spring pressure would be to experiment with extra washers under the screw cap.
GuyW

ETA: typed whilst Guy posted, I can't remember if Peter uses a washer or not.

First quick thoughts, Peter Burgess has previously posted about the relief valve so it'll be in the Archives.

Have you tried putting the previous valve back in to see if it changes the readings again.

Are you sure your gauge is accurate.

Have you changed to a different oil recently.

(IIRC) you've got a thermostat on your oil cooler.

Neither the long wait for pressure last winter nor the idea of 90psi or 80psi when hot sound good but the gauge showing 80 at revs before getting fully warm wouldn't worry me though.

I'd have though, subject to your oil not being over-cooled, that your engine running hot as you previously posted that this would warm and thin the oil unless it's fresh and/or oil designed for high temperature use.

Nigel Atkins

Simon,

I would like to see your logic for that statement as an engineer I don't see it and would expect higher pressure to result in more oil flow through the bearings.
David Billington

Les,
I just had a very quick look in the Archives for Peter's post but could only find that Peter had put modern valves are made/finished of the wrong materials so can cause problems, and fitting a 9/16" ball bearing, and alter spring length (not sure I've remember that one right though).

I keep forgetting this isn't a 1275 engine - what pressure should a higher powered 948 run at(?) (standard 948 has 15 idle and 60 running).
Nigel Atkins

I'm as one with David here. The oil pump is a fixed displacement gear pump so, setting aside internal losses, the delivery volume is controlled by the pump rotational speed.
The higher opening pressure of the relief valve means that less oil is bypassed through the valve and more is available to circulate through the engine. This is borne out by the increased loss at the crank seal.
An interesting experiment would be to isolate or just blank off (cover) the oil cooler temporarily and see what effect it has on the pressure.
I can't help with the question of the crank seal but I don't think you need to worry about the pressure being excessive otherwise.
Greybeard

I think the conundrum is that the pressure is a function not just of the volume of oil pumped, but also the amount of restriction to flow. This will depend on the oil viscosity and how freely the oil passes through the bearings (bearing clearance or wear)and other oilway routes. If pressure is too high it will generate extra heat at the bearings so the excess pressure needs to be limited by allowing part of the volume of oil being pumped to by pass via the relief valve.
GuyW

Guy,

That's not necessarily true as insufficient flow through the bearings can lead to excessive heating due to oil shear if the oil stays in the bearings for too long. I know that was a common problem with the old Riley engine as used in the Healey Silverstone when raced as the large bearing journals when ran at higher speeds in racing led to excessive oil shear and heating, IIRC the standard cure was to open out the oil passages and increase the oil flow through the bearings and reduce the oil heating problem.
David Billington

Absolutely David. As I said:
" pressure needs to be high enough to maintain an unbroken oil film over the bearing surfaces"

And the volume of oil flow also importantly acts as a heat transport system to remove excess heat from the bearings. So both volume and pressure are important, and the relevant contribution of these factors at the bearings needs to be balanced.
GuyW

David Billington - Google Bernoulli's Principle
Simon Wood

What have I started here? No I don't have a thermostat on the oil cooler. I usually blank it off for the winter, but haven't done so yet as I was running hot anyway. I am using ordinary 20/50 oil. I haven't calibrated the gauge, but with the old valve and spring it read steadily at 60 cold and only slightly lower hot. All I can do is to try another gauge. I have one that came with an air con refill kit but not sure that will work on a liquid instead of a gas.

Anyway the leakage tells me that the pressure has gone up a lot. The extra washers under the cap sound worth a try.

Les
L B Rose

Just mind the extra washers still seal properly!

I am out of my depth here, but wonder why the rear scroll seal would be affected by higher oil pressure? Surely it isn't pressure fed? Maybe it's just that more oil fed to the rear main means more oil around that end of the crankshaft.
GuyW

Yes the bearings will leak oil either side, and the rear side is next to the scroll seal - which doesn't work now as its top half is replaced by a plain piece for the lip seal carrier. And the sump is pervaded by oil mist of course.

Les
L B Rose

Simon
Your theory amuses me as well

The 'relief' valve is adjusting the flow of oil 'from' the system not in the system
If the exhaust oil flow from the relief valve is reduced by fitting a longer or stronger spring,it results in more flow (and pressure) to the gallery/bearings

There is no way on the planet that increasing oil pressure to the bearings would result in less flow to the bearings

The only downside of high oil pressure is that of oil leaks from rear crank seals that
aren't up to the job (possibly caused also by worn main bearings with larger than perfect oil clearance)and maybe premature wear on oil pump/camshaft drive gears

Personally if I had a midget with 80psi hot I'd be a very happy traveler
willy

Les--what oil pressure do you have hot idling
William Revit

.... and to chuck in another theory, the heat generated in a bicycle pump is down to compressing a gas. Liquids don't compress so that doesn't apply.

I did hear once that high oil pressure can cause the filter cannister to rupture.
Graeme Williams

Graeme that's why some versions of the oil filter housing have a bypass valve inside them
David Smith

Les

'Conundrum?' - would that be a Rabula Conundrum perhaps?

See: https://vimeo.com/150602227

Cheers
Mike
M Wood

Mike, so you mean if we keep guessing someone will eventually get the right answer?

Graeme, I presume you are right as liquids don't compress, so why would it heat up? But although I can't recall where I picked up that particular "alternative fact", I do remember it was from what I considered to be a reliable discourse on bearing lubrication; somewhere like Calvers site. Just goes to show you shouldn't believe everything (maybe anything) you read on the internet!
GuyW

Liquids heat up due to viscous shear.
David Billington

Now we're getting there David
That's one reason why racers run slightly more bearing clearance compared to cabs---
Graeme
On the bursting oil filter comment our Chevs had 100psi op gauges and at startup they would wind off the clock, you couldn't touch the rev pedal until it got a bit warm and got below 100 or the can would just burst, warm they would normally run 80 running flat out and around 50 idling, any lower than that and bearing wear was a problem
David--with all due respect, the bypass valve doesn't really dump pressure it diverts filter inlet oil to the filter outlet port bypassing the filter element if it can't flow enough
William Revit

Willy - yes; but I see it that if the filter housing bypass valve was absent, and the filter couldn't flow all the oil presented to it then the pressure inside the filter would rise and it would be likely to burst, no?
David Smith

yes--100% agree
William Revit

I am having fun trying to remember my school and college physics and thermodynamics. What's the equivalent to Boyle's Law (which just relates to gases for pressure, volume and temperature?) for liquids?

The assertion that liquids do not compress does not make much sense - they may have a general tenancy to be less compressible than gases, but can't a liquid be, and be put 'under pressure'?

There must be some lovely liquid formulae and characteristics related to viscocity, temp and volume. And such fun stuff as the science of rheology and phenomena such as 'shear thinning' (is that the same as 'viscous shear'?). A nice common example to demonstrate viscous thinning is observing how thick emulsion paint can visibly thin when you stir it vigorously, especially when close to the stirring stick (can see the same with custard and also when whipping cream).

Cheers
Mike
M Wood

I think, Mike, that under 'normal' ranges of temperature and pressure, liquids don't compress as there is no ( less?) space between the molecules than in a gas. And as regards hydraulics in a car you can obviously apply pressure to the brake fluid, but you hope that it doesn't compress ( reduce in volume) or your brakes aren't going to work very well!
GuyW

With the custard and whipping cream theory
I just tried vigorously stirring my warm rice pudding, what a mess, it went everywhere but the skin still stuck to my spoon the same protecting it's delicate surface
I'll try again with a cold bowlfull but I suspect the result will be similar
Didn't see it thin at all--
lol
willy
William Revit

Mike,

You're thinking of a thixotropic fluid https://en.wikipedia.org/wiki/Thixotropy
David Billington

Thinking about the bearing issue reminded me of the discharge coefficient https://en.wikipedia.org/wiki/Discharge_coefficient and would be used to say calculate the flow from a hole in a barrel. The oil pressure provided by the pump is equivalent to the static head of fluid in the barrel and calculated from rho * g * h rho = fluid density, g = acceleration due to gravity, h = height of fluid above discharge point. So higher pressure results in increased flow through the bearings.
David Billington

Forgot the important flow equation https://en.wikipedia.org/wiki/Orifice_plate#Incompressible_flow
p1 would be gallery pressure, p2 sump pressure so as pressure increases so does flow.
David Billington

William

Sorry only just saw your question. The hot idling pressure oscillates around 40-45psi. I am just going out to the garage to trim bits off the spring until I get down to 60 max. Annoying job because I have to remove the tacho drive and dissy every time to get at it. At least I have the latter marked with correct timing.

Les
L B Rose

Les,
you may have already noted this but just in case not, from DaveO's post earlier on trimming the spring to get the pressure down -
"It didn’t take much - possibly only 2/3 to 3/4 of a coil IIRC.
I know that the first time I tried it, I cut too much off and had to get another spring."
Nigel Atkins

Cheers Les
The 100thou spacers in MGBs give around 10-15psi each if that's any help for a rough guide on how much to chop--I'd leave the untouched end of the spring against the round ball so the spring can't get jammed down beside it though
William Revit

The oil flow will take the direction of the least resistance. So if, for instance, one big end is very worn out, the other "good" big ends get less flow.

Flip
Flip Brühl

I have trimmed a coil off the spring and pressure is down to 70 max. Drops to 60 hot which is where I want it.

Les
L B Rose

Les,
is the car still running at a hot temperature generally?
Nigel Atkins

No Nigel I have changed the thermostat to 75ºC, and it now runs just over 190ºF. I tested the old 83ºC one and it didn't look as if it opened very far. I don't have a bypass hose and I have the 2 holes drilled in the thermostat to compensate. But that would not be enough for motorway cruising on a hot day if the stat failed.

Les
L B Rose

Les
Just out of curiosity
Now that you have reduced the max. oil pressure,by reducing the length of the relief valve spring, has this had any effect on idling speed pressure-
In theory it shouldn't as the relief valve is a max. pressure blow off valve, but--
Interested to know what your car did when doing this mod.
willy
William Revit

Willy, so far I have not done a high speed run but at 800rpm hot idle it hardly drops at all - still about 60. I do know oil temp rises a lot at high speed judging by pressure drop with the old valve. I'll report back after a motorway run.

Les
L B Rose

Les, having just posted about 1275 HG " ... even though these new black type have been known to act as a thermal barrier between head and block which can tend to increase engine temperature" I just wondered if your high temperature running originally started after a HG change (that's IF you can get the black type of HG for your engine size and IF you fitted one).
Nigel Atkins

Les,
I know you won't like this which is why I've resisted putting before it before but it's only an idea so can be rejected.

190f (88c, passed the N) seems a bit high still to me - not that I've run a 950cc and I know yours is souped up and I think used to run at that before(?).

I know things change and there's always a range in real use but the book shows the standard 950cc as "Normal operating temperature 164'F. (73'C.)".

As a precaution and for the fact that the engine is running warmer and you've covered the water coolant side I'd use an engine oil that offers a bit more margin and use at higher temperatures. Just one example, as it's made for a high performance classics, 20w50, has ZDDP and for temperature, not expensive and is by a well known British company - Millers Classic Sport 20w50 Engine Oil - https://www.millersoils-shop.co.uk/classic-sport-20w50-engine-oil

Many other alternatives are available.

My simply thought is that, within a range, engine running temperature has certain temperature for power and higher up for economy and souping-up is more about the power end.

There, I've typed it, I can have my bath now.



Nigel Atkins

This thread was discussed between 16/10/2018 and 01/11/2018

MG Midget and Sprite Technical index

This thread is from the archive. The Live MG Midget and Sprite Technical BBS is active now.