[TR] TR3A oil pressure

[Jim Muller]

On 17 Mar 2009 at 15:25, DLylis at aol.com wrote:

> I am  befuddled by the increase in pressure as the oil warms.

I confess to knowing little of the TR oik filter, gauge pickup, and 
relief valve (if any) arrangement.  And I just barely remember the 
details of the original post.  However this is such an intriguing 
puzzle that it tripped my how-does-it-work glands.  I can hypothesize 
two mechanisms for this behavior.

The least likely is that you are running a peculiar oik, something 
with a viscosity rating of maybe 3w90.  It just gets thicker as it 
gets warmer!  Then again, maybe not.

Try this.  Suppose the oik pump, positive displacement of course, 
feeds into a journal which has two exit orifices.  One is a pressure 
relief valve which dumps oik back to the sump.  The other is the main 
lubrication route to the bearings, etc.  Suppose too that the 
pressure gauge sensor is downstream from this second orifice.  Cold 
oik would force the pressure relief valve open more.  This means the 
pump is effectively not positive displacement any further downstream.

The pressure downstream from the main journal and detected by the 
gauge would be a sort of equilibrium of the volume coming through the 
orifice and leaving for the bearings.  An electrical analogy would be 
that of voltage drop along a circuit which contains two resistors 
with a voltmeter attached to sense the voltage between them.  It is 
different from electricity in the sense that as more oik goes through 
the orifice the pressure on the opposite side goes up.  If the relief 
valve opens too much, then less oik goes through the main lubrication 
orifice, and could show lower pressure behind it, especially if there 
wasn't much viscous "resistance" from the remaining route to the 
bearings.

Other arrangements could possibly be made to produce a similar result 
but this is the easiest for me to visualize.  I would start by 
checking the relief valve behavior.  But what do I know?

-- 
Jim Muller
jimmuller at rcn.com
'80 Spitfire, '70 GT6+