Oil pressure reading variation after good run. Should I worry?

[Colonial_1]

@davidsxj6

Viscosity helps of course, but if the bypass were completely open the rise should be linear at some point.
Where does the oil go?
Did you mean the bypass started to open at 20? That would explain it. It’s a crude device, isn’t it…

Hi David,
I wouldn’t expect a linear pressure rise, as flow through the bypass and clearances will increase with pressure, so the pressure will reach some maximum regardless of increasing revs.
Based on spring pressure and valve surface area it looks like the valve starts to move at around 19 psi, and the bypass ports are exposed progressively from 22 to 25 psi.
The bypass system looks beautifully engineered compared to others I’ve seen (American V8s), even though the way it works is the same - maybe this was the standard for British engines of the era.
Cheers,
Simon

[Colonial_1]

@Colonial_1

Based on spring pressure and valve surface area it looks like the valve starts to move at around 19 psi, and the bypass ports are exposed progressively from 22 to 25 psi.

Well having just checked the valve piston area, I have got this totally wrong. Valve movement theoretically starts at 55 psi, and the ports are exposed from 65-73 psi.

Apologies for the careless posting.

Simon

[davidsxj6 (David)]

Thank you. What you said before makes sense, now even more so. No need to apologize. After having the valve stuck on two occasions I am very happy that it is comparatively easy to get to.
Frank, of course the relief valve is supposed to limit pressure. Ideally it would be just about to open, like a thermostat. The confine on the low end is of course hot idle with thin oil. Also, the valve can partly open, like a thermostat, and in the higher engine speeds I imagine it doing just that.

Have a nice day both,

David

[Frank_Andersen]

@Colonial_1

Well having just checked the valve piston area, I have got this totally wrong. Valve movement theoretically starts at 55 psi, and the ports are exposed from 65-73 psi.

Apologies for the careless posting.

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No apologies required, Simon - we need a challenge…

No specs are actually given for the relief pressure, which is awkward - so all feedbacks are useful…

Making assumptions; the pumped volume is linear with pump engine rpms - the leak-out from bearings are linear with oil pressure. With 40 psi at 3000 rpms stated as minimum, and engine requirement for oil is also linear with rpms; one would assume required pressure at 4500 rpms to be some 60 psi - and relief valve should then stay closed up to some 65 psi…

However, assumptions are somewhat fraught, but in principle, with engine hot, reving should not open the relief valve - which would reduce oil flow through the bearings. In any case; relief pressure may be measured with cold oil, increasing rpms; beyond some rpms the pressure should no longer increase - the relief pressure …

The pressure is adjustable by using washers under the spring…

Frank
xj6 85 Sov Europe (UK/NZ)
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