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MG Midget and Sprite Technical - Oil pressure

Low oil pressure ?
A well worn topic.

My 1098 runs nicely but when fully warmed up it is only showing 40 psi on the gauge at 4,000rpm. (20psi at 800rpm) Compared to my 1275 when last running gave 60/ 65psi. So it seems a bit low but I don't want to fret over it. For starters, the guage is old and untested, so it could just be that. Or it could be a tired relief valve spring - I like easy fixes! I am using Duckhams 20/50.

Then thinking about the oil function, especially on the crank bearings and big ends. The pressure needs to be high enough to prevent the oil film breaking down so you don't get any metal on metal action. But the other, equally important oil function is to carry away the heat from the bearing. Excessive oil pressure at the bearing contact face will increase the heat being generated and also presumably reduce the volume of oil passing across the bearing and taking that heat away. So there must be a sweet spot for oil pressure for the A series engine design, dependant on the properties of the oil being used.
GuyW

As I recall, for the 1098 it should be 60 psi at start up, and 20 at idle once warmed up.

But I guess you already knew that.
Philip Sellen

Thanks Phillip, I wasn't sure.
I probably need to swap over the oil pressure gauge from the other car, as a first step. At least that would eliminate the possibility of it being a faulty guage.
GuyW

Hi Guy, try a full length spring and ball bearing as an oil pressure relief valve set up
Peter Burgess Tuning

Thanks Peter. I will check the gauge first but that is definitely a good next step.
GuyW

I had similar thoughts, Guy, after seeing 35psi after an hour at 70mph on the way back from the MASC event at Gaydon in May. The car was also running hotter than normal, but not overheating. That has been sorted with a coolant flush and new anti-freeze.

I tried the 'Cooper' spring and ball and the following may be of interest, though I absolutely would not challenge Peter's knowledge of the subject:

Before - cold 80psi, hot normally 40psi, idle 20psi

Cooper - Cold 100+, hot 85, idle 50 + bad rear leak

1 coil removed from the Cooper spring - Cold 100, hot 80, idle 40 + bad rear leak

2 coils removed - cold 90, hot 70, idle 30 + bad rear leak

I ended up using the short Cooper spring and the original bullet valve - cold 80, hot 40, idle 20 + worse leak than I had before I started.

In addition, while constantly removing the distributor to get at the relief valve I dislodged something in the discributor which eventually burst out of the side of the distributor.

So, a new distributor and a worse rear leak later, I am at the same oil pressure I had when i started, which was actually in spec for a 1275 engine.
Karl Thompson

Karl,
that made me laugh more than it should have ;O)
A classic case of "if it ain't broken, don't try to fix it".

Best of...
MGmike
M McAndrew

If replacing the original valve with a ball the spring needs to be somewhat shorter and probable better used in conjunction with an adjustment feature but would really need to be set up using a pressure gauge of known calibration.
S G KEIL

Guy, is this a change? Is the pressure gradually dropping, or has your 1098 always given those pressures?

Says Kieth Calver on the A series.

"High pressure isnโ€™t important anyway. FLOW is. Folk panicking because they didnโ€™t have 80psi oil pressure in their race engines when hot always filled me with mirth."

" - 70 to 80 psi oil pressure is NOT necessary, and excessive oil pressure can raise the oil temperature. 50psi hot and flat out with 25psi at idle is all you need. I have used as little as 40psi with no problems."

https://www.calverst.com/technical-info/lubrication-oil-pumps/

---โ€-----------

If hot, your 1098 doesn't drop below 40/20, I say don't worry personally, as long as it's always been doing this. But if those figures are reducing over time/mileage, that's a concern.

Also, I run the Duckhams 20/50 too. I've found it DOES causes the oil pressure to drop quite a bit from its cold values when hot, compared for example to Halfords 20/50, or Comma classic 20/50.

My fairly newly built crank reground 1275, often drops to 45/50 when hot but never below 20 at idle, using Duckhams. But around double those figures when cold.

Using Comma, idle never drops below 30 plus and 70 at higher rpms.

I've stopped worrying, but when I run out of Duckhams (I bought the 20ltr tub) I'll revert to Comma.

Or maybe the Smith Allan classic 20/50 oil I used in my Capri, which was absolutely fine at decent oil pressures, and is way cheaper.๐Ÿ˜ ยฃ66.65 inc vat and inc delivery.

https://www.smithandallan.com/product/smith-allan-select-xl-20w-50-4165










anamnesis

MG Mike, that is why I wrote my post. Everything I learn I learn the hard way. The devil certainly made work for my idle hands.
Karl Thompson

Karl, exactly why I wrote " I don't want to fret about it" and "I like easy fixes"
Pressure looks low but this needs checking, and for gentle road use it may not matter too much anyway.

Anam, its the engine I have in my frogeye so I haven't done much mileage anyway, but I haven't noticed any marked change in pressure. I posted as much to see what comments I got on the topic of oil as a coolant, so volume and flow rate being at least as important as pressure. If it were just for lubricating the bearings you wouldnt need that huge sump full of oil!
GuyW

If a 20/50 oil is properly rated, why should one brand produce different oil pressures than another? I'm currently using "Torotec" from Aztec oils, sold by my local factors and it produces the same oil pressure as Halfords Classic I.e. 60/65 revving hot, can't remember tickover.
Bill Bretherton

Started using this in an air cooled VW engine because of its high zinc content (1500 per million) and have continued to use it in the A series.
https://www.ryeoil.co.uk/shop/classic-20w50-engine-oil-2/



Jeremy MkIII

"If a 20/50 oil is properly rated, why should one brand produce different oil pressures than another?"

Bill, I have no idea why, but I found it does.
anamnesis

Good discussion on oil cooling the engine here Guy.

https://bobistheoilguy.com/forums/threads/motor-oils-cool-your-engine.331750/
anamnesis

Bill, check this out.

"A customer changes their brand of oil and then complains of low oil pressure --".

"-- Do not be fooled by those oil companies building a 'thicker' oil under a standard viscosity rating --".

https://www.synforce.com.au/oil_pressure.html

So it does happen.


anamnesis

With the ball and spring we sometimes reduce the spring length by 3 mm once the spring has bedded in.
We do not always have to shorten the spring hence starting with full length. The problem we have with replacement bullet valves is the poor quality. Originally steel valves were copper plated and then flashed with hard chrome and worked really well. Modern ones tend to be mild steel and prone to galling.
Peter Burgess Tuning

Jeremy. The Rye Oil you linked to is even cheaper than my offering. 4 quid saving with free post. And spec looks good too.

I'll give that a try when my Duckhams runs out at my next oil change.
anamnesis

back in my old race days we used to work on 10lb/1000rpm so with the racer it'd be set at 80lb blow off hot. You'd have to be real carefull dead cold though not to give it any revs much untill the oil loosened up--the pressure would be off the end of a 100lb gauge cold and any revving at all would see the oil filter blow to pieces. --typical bl@@dy chev.who'd have one-?
William Revit

Hi Willy.

Just some daft questions.

I've never given much thought/worry to oil pressure as long as I never dropped below the book pressure values, and figured if there was enough oil pressure, oil would always get to where it's needed.

But I've been reading more about flow. Oil pressure high, means flow is reduced, since oil pressure is a measure of the resistance to oil flow. But if oil pressure is too low, there can't be any flow.

So is there an optimum balance and can it be measured? Are there such things as oil flow gauges that for example could be used in the piping through an oil cooler curcuit?

Like I said, just some daft musings. ๐Ÿ˜„


anamnesis

Umm, where to start -aren't you on holiday---- this could end up big, I'll start with what you read/wrote-

"But I've been reading more about flow. Oil pressure high, means flow is reduced, since oil pressure is a measure of the resistance to oil flow. But if oil pressure is too low, there can't be any flow. "

Depends entirely where you're looking at this from-
"IF" we're talking a 'set' oil supply volume then yes-

If the bearing clearance is reduced then the pressure will be higher with less flow with more heat and
If the bearing clearance is increased then the pressure will be lower with more flow with less heat

Looking at it another way--if the bearing clearance is a set thing then what you've read is wrong--If you increase the pressure you automatically get more flow.

It all depends on bearing clearance and what grade oil is being used -- there is a direct comparison between the selected oil grade and the bearing clearance required for that clearance

Back years ago with a propper NASCAR engine it'd be running 50 weight race oil and have something like .003" clearance on the mains and depending on what rods you had around .002"-.003" on the bigends

Nowadays they run 0/20 oil and the clearances are that tiny that 50 oil wouldn't get through to the bigends

eg.-I have a latemodel NASCAR Mustang engine here on the floor that will hardly turn over by hand dead cold and needs a coolant heater to get it up to temp before you even think about starting it---If you cranked it up dead cold it'd fizz the bigends before you got out the door and yet it'll spin to 8-90000 rpm all day on 0/20 oil

Now go and take your wife out for a feed
lol
willy

William Revit

Yep on holiday Willy. But being retired, it's all a holiday. Lol.

Thanks, that's interesting and explains a lot. I never thought about such greater bearing clearances.

As for lunch; well as it happens, I've just digested your reply over lunch. ๐Ÿ˜†.


anamnesis

Dug this old chart out for you to have a look at-
Just for an idea, the std bearing clearance for most road cars used to be .0005"(half a thou.)per inch of shaft diameter, and for a sportier engine you could go up to double that- A full out racer would/could have even a bit more.
So for say a 2" main bearing you'd be looking at .001" clearance or up to .002" for a sportier/racer engine.
Nowadays with better oils the clearances are down to minimum and running the appropriate oil for the clearance/temp that they run at.

This is why we don't run modern thin synthetic oils in our old MG things, the clearances are too wide , the oil gets hammered and gets hot and thinner,hotter/thinner causing low oil pressure.



William Revit

Thanks Willy. Seems every subject gets more complicated the deeper you dig.
anamnesis

This thread was discussed between 05/10/2024 and 07/10/2024

MG Midget and Sprite Technical index

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