[Healeys] Oil pressure

Joe and Lenore Armour sebring at hotkey.net.au
Wed May 15 20:23:59 MDT 2013

Magnus Karlsson wrote:

>I have a problem with an engine that I just restored completely. The oil
>pressure gets too high when revved over 2500. The pressure reaches all the way
>up too 80 psi, which of course is way too much. The strange thing is when the
>engine is cold and you rev it, the pressure will not rise above 60 psi.

BMC pressure relief valves are very basic.  They are constantly open it 
is just a matter of how far.
Remember that the 'relief' valve is a flow ( volume) control valve  
which results in a system pressure.  You do not mention the oil 
viscosity being used .   In my 3000 which has a 15 litre oil sump which 
takes a long time to pass all the oil thro the engine and therefore is 
slow to heat the oil, the viscosity stays high even though I warm the 
engine up for some time.  When I drive off at revs below 2500 rpm the 
pressure remains at say 60psi, if I accelerated early with more revs the 
pressure drops to 40 - 45psi.  I then reduce the revs and wait for more 
time and heat and the oil pressure returns up to 60psi

My theory is that when the oil is cold and you have new and therefore 
close tolerences the volume of the pump output far exceeds the amount of 
oil that is being passed through bearings and  rockers etc.  Consquently 
a large volume of oil is being by-passed back into the sump  and at 
lower revs and pump output the relief valve can still regulate . When 
higher revs and pump volume occur the relief valve is pushed right back 
in the bore against the spring and the now higher volume of thick oil 
by-passing holds the valve back and in effect it is no longer regulating 
the pressure which drops as you are seeing.  In my case when I reduce 
the revs the pressure increases to the nominal 60psi and when the oil 
temp/viscosity changes the relief vale can then regulate the pump volume 
to provide the rated pressure. 

By design the oil pump output at all revs and temperatures should exceed 
the volume of oil that is being accepted into the engine lubrication 
galleries and therefore the relief valve is regulation the oil volume to 
control the pressure.  High oil pressure does not guarantee oil is 
getting to the metal requiring lubrication.

Pressure ,  Volume  and Temperature  are related.

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