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MG MGB Technical - Adjusting engine oil pressure?

I have a modified 18GB (5 bearing) engine in my MGA. As part of an engine overhaul I am re-seating the oil relief valve and thought I would boost the oil pressure a little. My new relief valve has a 3" spring and the standard poppet valve (Moss 460-155). Our local supplier says the packing piece (Moss 460-165) that increases the pressure on this engine is no longer available. Do people normally put this extra packing in or just run with the pressure generated by the spring alone. I see Barney (MGAguru) advises that 1/4" spring washers can be used to add about 6 psi for each washer (.060 to .070 thick washer). http://www.mgaguru.com/mgtech/engine/of111.htm
Any comments?
Mike
Mike Ellsmore

What pressure do you have? Too much pressure can erode the bearings, and isn't a substitute for worn bearings. The V8 runs at 40psi max, and as low as 10 hot idle, so anything above that on an MGB is not going to hurt it.
Paul Hunt

Boosting the oil pressure also increases the risk of more oil leaks at seals/gaskets. I discovered this when I added a washer to the valve.
Brian Shaw

If your engine is reconditioned, and the spring is the correct length, there is no need to increase pressure. As said above, too much pressure is bad, too.

What pressure are you getting?

Herb
Herb Adler

Thanks for your comments gents. I pulled my engine down after a cam follower failure - prior to that oil pressure when hot at idle was just below 40 psi. Rod and main bearings were in good condition (plasti-gauge clearance tests showed clearance around 0.001 - 0.0015" - only one rod and one main tested). As I found there was no packing piece between the spring and poppet as shown in the Moss catalogue for the GB engine I thought bringing it back to standard (increasing the oil pressure a bit) would be a good idea. Another issue is I have a big low oil pressure warning light mounted above the windscreen (coupe) that comes on at idle after spirited driving and I would like to keep it off! I reckon 5 psi would do it.
Mike
Mike Ellsmore

Mike, Like you I found the packing piece missing when I rebuilt my supercharger engine so I used a small flat washer .030" thick in the cap. Ended up with 75lb hot or cold at above idle and 60lb hot idle. I know pressure does not keep the metal surfaces apart but a few pounds makes me feel good. It never pays to go over 80lb hot or cold as erosion is real. I would not have put the spacer in except the design calls for it. Denis
Denis4

Mike,

I'm not sure you are going to achieve what you are seeking from the proposed course of action.

If I understand things correctly.

Engine oil pressure is a function of pump (sufficient volume)/filter condition/clearances in galleys and bearing clearances or wear conditions.

The oil relief valve you are proposing to pack out remains closed in normal operating conditions and only (supposedly) opens when and if pressure reaches an upper limit, factory manual quotes 75lb, as a safety measure to avoid damage which may follow from excessive oil pressure.

If you are running at 40lb hot engine pressure, the relief valve is a long way from being likely to open if factory set up is in place - that does include the spring packing piece - which is in my 18GG block.

If oil running pressures are insufficient I think you have a bearing clearance or component wear condition that will require attention if pressure is to be raised.

Regards
Roger
R Taylor

Hi Roger,

He did state that he had 40 psi hot idle, which I think is a fantastic pressure, at hot idle.

Herb
Herb Adler

Hello Herb,

You are right, I misread that as running pressure.

However, my basic point remains re fiddling with the relief valve spring. I don't see how that will raise idle/running oil pressure. The relief valve puts a 'cap' on the upper limit an engine can develop before the valve releases. In itself that does not determine idle/running pressures.

Certainly you can raise the value of that relief point by upping the pressure needed to pop the relief valve, but that is a different thing to what Mike is seeking to do. Isn't it?

Or have I got this all wrong?

Mike, what condition is the rocker system in? Bad wear between rocker bushing and shaft can have a significant affect on oil pressure.

Regards
Roger
R Taylor

Gents, I agree increasing the relief valve blow off pressure wont increase my hot idle pressure as it was lower than the blow off setting anyway (brain fade)!
As part of the rebuild I am installing new rod and main bearings and a new oil pump. The rocker system is in good condition (Titan Motorsports - Moss roller rocker set up, needle rollers).
My query is now should I add the packing piece or not for a 18GB engine to raise the relief valve blow off pressure? (I imagine oil pressure would still be below 80 psi with it installed).
Mike
Mike Ellsmore

Mike,

Thanks for the clarification. The following may help.

First up a correction. The factory manual states 70lb for the oil pressure relief valve let off point - not the 75 I stated above.

However, it also gives some useful info that would enable you to rig up an experiment to see how closely the new spring you are using matches the factory set up.

It gives:

spring length at 3 in.
fitted length as 2 and 5/32 in.
load at fitted length as 15.5 to 16.5 lb.

So, spring inside valve cap compressed against a scale to a spring length of approx the 2+5/32" (allowing for the depth of the cap head - checking where the spring end binds inside) should give you an approximation of the degree of conformity to the original.

You could make a decision on that basis.

Frankly, I would think an upper oil pressure of around the 70 to 75 (which it probably was originally - factory would have erred on side of caution) would be sufficient.

I think oil quality and frequency of oil/filter changes are probably more important than a few pounds hike in relief pressure.

Of course, what I think is irrelevant - it is you that needs to be comfortable with the end state.

Regards

Roger
R Taylor

Mike, You are not increasing the valve opening pressure with the packing piece as it is suposed to be there in the first place and the opening oil pressure as stated in the manual would be with the standard spacer. MY engine with all new bottom end, spring, and .030" spacer is 75lb. My OE manual and my OE special tuning manual give the pressure as 80lb max. Its not going to make a lot of difference eitherway. I have done a fair few of these engines over the years and I recon about 50/50 spacer or not and it does not seem to be model related. Maybe is was done on OE assembly to make up for variations in pressure output and keep it in spec. Denis
Denis4

If the spring is in spec then there is nothing to be gained by fitting packing.

I use 6mm flat washers to shim up new springs when necessary to achieve a hot running pressure of no more than 60 psi on a rebuilt engine.

Higher oil pressure than that only wastes power and generates wear on the drive and camshaft leading to ignition problems as the dizzy is driven off the same gear.
Chris at Octarine Services

Hi again,

I have just looked up the MGB driver's handbook, and the quote oil pressures are:

Idling, between 10 and 25 psi
Normal running between 50 and 80 psi.

Herb
Herb Adler

This thread was discussed between 22/05/2013 and 15/06/2013

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