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 m type oil pressure
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john gott

United Kingdom
23 Posts

Posted - 05/07/2019 :  14:13:32  Show Profile
what is the running oil pressure for the m type mg?

tonym

United Kingdom
460 Posts

Posted - 05/07/2019 :  14:54:00  Show Profile
Mine is 55-60 psi
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Keith Durston

United Kingdom
434 Posts

Posted - 05/07/2019 :  16:09:56  Show Profile
Mine is 80psi but I have a Phoenix crank and shell bearings using Castrol XL oil.
Keith
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sam christie

United Kingdom
2158 Posts

Posted - 06/07/2019 :  12:17:53  Show Profile
Is the oil pressure relief spring usually set for 55-60 psi ?

Sam
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tonym

United Kingdom
460 Posts

Posted - 08/07/2019 :  14:55:58  Show Profile
55 - 60 was a top of the head guess.
On a long run, on a hot day, it was a steady 65 = with John Gott looking on for confirmation.
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NeilS

United Kingdom
178 Posts

Posted - 09/07/2019 :  13:01:22  Show Profile
The interesting thing is that more oil pressure is not always better with hydro-dynamic bearings. Around 2.5bar (about 35PSI)is optimum for most and this is what you will find on most small high speed gas turbines and even up to gigwatt scale machines. anything above about 4.5 Bar (say 65 PSI ) can actually scour bearing material from the shells and shorten life, admittedly not a huge problem as most of our cars don't cover huge milages. From a design point of view the MMM releif valve is pretty poor as it does not have enough spill area with cold oil and the seat design is poor and prone to leakagge with hot thin oil.

The other thing to throw into the mix is that the pressur at the big ends will be higher than the supply pressure due to centrifugal effects, so the souring always shows big ends before mains.


Neil
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Oz34

United Kingdom
1743 Posts

Posted - 09/07/2019 :  14:33:29  Show Profile
Neil, while respecting the fact that I think you are an engineer and I am not, am I not right in suggesting that a reciprocating engine needs more pressure than a turbine in order to resist the loads imposed by that very reciprocation?

Dave
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NeilS

United Kingdom
178 Posts

Posted - 12/07/2019 :  09:50:10  Show Profile
The turbine has a slight advantage that the oil wedge forms in a stable position dependant only on shaft angular velocity as the load vector is constant (i.e. what we would classify as "the eccentricity is constant" as no bearing runs centred in the clearance). In the recip the load vector is continually changing on the big end bearing so the wedge has to move around to follow that load (i.e. eccentricity is continually changing.

The oil supply pressure has little to do with the pressure in the oil wedge which will be significantly higher in most cases at least on the power stroke. The supply pressure is only really required to replenish the oil within the bearing clearance as it leaks out. The recip will require a higher oil flow than a turbine per unit bearing clearance due to the changing eccentricity pumping the oil to an extent. The oil also has a second function in providing cooling to the bearing.


With the Phoenix crank setup Mini Cooper shells are used, in the A series normal oil pressure is normally 30 to 45 PSI and in the Cooper versions the power output and therefore rod loading is higher.


Having said all the above the oil pressure on my M is close on 100 PSI cold and runs about 60 when hot. Which is a bit high in theory but I am not too worried. Equally if it was 40 PSI hot I would not be unduly worried.

Neil
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coracle

United Kingdom
449 Posts

Posted - 12/07/2019 :  10:25:48  Show Profile
Contrast all this with the Austin 7 standard "Spit & hope" set up where the oil is simply squirted from the side of the crankcase at channels in the crankshaft webs with oil drillings leading from those channels to the big ends.

Clearly the oil supply to those channels is intermittent and it's flow from there to the bearings is facilitated by the centrifugal force generated by the rotating crankshaft.

Surprisingly it works but if higher performance is required, modification is necessary!
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Oz34

United Kingdom
1743 Posts

Posted - 12/07/2019 :  18:42:10  Show Profile
Thanks Neil for that very interesting little piece. As a lay person I hope I'm right in summing it up for practical purposes as "Don't get too worked up over oil pressure" which I've thought for many years is a valid attitude.

Dave
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NeilS

United Kingdom
178 Posts

Posted - 15/07/2019 :  08:19:57  Show Profile
Yes, entirely don't get too obsessed over it. as Coracle says Austin 7 have a very basic system and I have two big Edwardians that just have dipper rods on the big ends and they do fine but obviously with a much lower bearing loading. As a point of information the hand book for my Delage say normal oil pressur e at 40MPH should be 40PSI but that there is no need to worry as long as it is above 7 PSI!

The onething I did fail to mention is that there are a couple of engines where the oil pressure impacts other areas of the engine than the bottom end. The one I have first hand experience of is the 2.0 Ford Pinto in FF2000 and S2000 (which I used to compete with) that needed a min of 50 PSI to keep the cam from failing but that was running the motor to 7,000 and the Ford arrangement is no where near as good as the MMM so as long as you follow the instructions in one of the Year Books by Foz to ensure


Neil
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KevinA

New Zealand
355 Posts

Posted - 15/07/2019 :  10:36:07  Show Profile
And to add to Neil's comments, the only time to be really worried with a splash feed lubrication with dippers is when the oil pressure is high. Generally that means you've got a blockage somewhere.
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searunner

Italy
24 Posts

Posted - 17/07/2019 :  09:32:54  Show Profile
One other important question is the VOLUME of oil delivered by the pump
Normally at High pressure oil corresponds a lower Volume
On oldest cars, some lubrication are sent by the slamming of the oil by the sump to the internal of the basement
Cooling are one important function of the OIL -normally neglect - on my 911S Porsche engine 13 liter of oil and a big oil radiator under the right front wing was dedicated to cooling- air cooled engine, are euphemistic
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NeilS

United Kingdom
178 Posts

Posted - 18/07/2019 :  13:55:55  Show Profile
"Normally at High pressure oil corresponds a lower Volume"

That is not really correct.


Oil pressure ( neglecting pressure relief valve) is a function of

Pump flow Capacity
Oil viscosity
Flow area of the bearing clearances and other paths

Higher viscosity = Higher flow
Smaller flow area = higher pressure = lower flow
Higher pump capacity = higher pressure

Therfore for any given engine at a fixed level of bearing wear higher pressure = higher flow

So if we thake an M type oil pump and fit the wider gears used on later cars the pump capacity and hence oil pressure increase.

The one problem is the poorly designed MMM oil relief valve. A decent valve should be able to cope with both hot and cold oil and hold the pressure to better than 20 PSI difference hot to cold, not the 50 to 60 PSI the MMM one manages. This is due to poor seat design and insufficient bypass flow area. OK it's not the worst I have seen MY Delage is far worse!


Now the Original Air cooled Porsche is a very interesting engine and very untypical. It should really be known as an oil cooled motor not an air cooled one. More heat rejection is from the oil that the air cooled cylinder heads. With MMM engines this is no the case most heat goes to water.

Neil
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