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Re: Oil pressure valve -reply

Subject: Re: Oil pressure valve -reply
From: "A. B. Bonds" <>
Date: 12 Jul 1995 09:54:56 -0500
In <>, craig wiper wrote:
>>It could be that the  oil relief valve and/or spring are in a sort of
>>oscillating mode, alowing the oil pressure to vary, but I really know
>>NUTTING about that little devil...anybody out there care to explain
>>the oil relief valve theory of operation?  Anybody ever change
>>springs with good results? TIA
>I changed the spring and valve on my '74B's engine a couple of years ago;
>used an uprated spring from Moss I think. The gauge reads about 20LBS
>more pressure than with the old valve and spring.

I think that it is important to understand what makes oil pressure and
a lack thereof.  The pump, driven off the camshaft, is rated to
move a certain _volume_ of oil per turn, thus the volume is (somewhat)
proportional to engine speed.  Since oil is more or less
incompressible, it has to go somewhere.  There is some leakage in the
pump and leakdown in the crank journals, chain tensioner and rocker
gear.  This rate of loss depends strongly on oil viscosity, which in
turn depends strongly on temperature.  When the oil is cold or the
volume is high, the pump is making far more than the system can
accommodate through leakdown.  Instead of letting the engine explode,
the British (PS) Leyland engineers wisely added the oil pressure
relief valve, which is nothing more than a spring and ball that, when
triggered, diverts excess volume back into the sump.

The point of this rave is that the relief valve is meant to open at 75
lbs more or less.  If you start with 75 lbs of pressure and it drops
to 50 lbs (a) your relief valve is working correctly, leave it alone,
and (b) the pressure loss is due to leakdown, most likely at the crank
or rockers (= loose bearings).  This is made evident as the oil warms
and viscosity decreases.  If you _never_ get past say 50 lbs, then
either (a) your relief valve is indeed malfunctioning or (b) you have
_really bad_ bearings.  Note that pressures in excess of 75 lbs are
probably unnecessary (save perhaps in highly prepared engines), and 50
lbs running pressure is quite adequate.  20-30 lbs at idle is OK as

The erratic surging of oil pressure at idle is most likely the result
of sloshing in the sump.  There are no baffles.  Losses of pressure
with hard turns are also common.  This has been discussed.
                                A. B. Bonds

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