Mike, open and reseat pressures differ. A typical relief cartridge,
the Sun RDDA (www.sunhydraulics.com) says that reseat is 90% of set
pressure. They don't say, but I suspect this is a Bernoulli force
thing. Also, there is "override", the increase in pressure with
increase in flow. I've not seen a response such as you describe on a
direct-operated relief (spring/ball). I have seen gauges and
transducers show one when there was air in the sense lines. Air other
places could cause that, as well. Specifically, air in the oil stream
would cause changes in flow through the relief valve in the
setting-reseat region.

On Tue, 2 Nov 2004 20:18:10 -0800, "Ron" no one @home.com wrote:

On my O320 the pressure is on the side away from the ball, trying to open
the bypass valve. The ball stays seated until pressure builds to the level
of the bypass setting, then the bypass is forced open, allowing the excess
pressure (oil) to be relieved back to the sump.


"MikeremlaP" wrote in message
...
Okay, here's the deal. Lycoming O-320 in a Glasair. On full power,
whether
oil is hot or cold, oil pressure will be higher than the regulator is set
for.
(Shoots to top of yellow arc - 90 psi. I think all Lyc's do that.) Upon
power
reduction, say turning downwind, you can "see" the ball jump off the seat
when
the oil pressure gage goes "sproing" and the pressure suddenly drops to
the
regulated 60 psi in a classic 2nd order damped sinusoidal response. It
will
stay at regulated pressure until the ball seats itself again, say on final
approach with the engine at idle where the pump isn't making as much
pressure.
Then the process starts over again, on the next touch 'n' go. I'd like to
have
regulated oil pressure all the time.

I'm theorizing that there's some kind of venturi effect, where the ball is
actually sucked down harder on the seat when lots of oil is flowing past
it on
the engine side. Seems counter-intuitive, I know, but what else could be
holding the ball down?

Anyone got a simple fix I can try? Larger ball bearing? Smaller ball
bearing?
Cutting groves in the seat to help ball break free? Different angle on
the
face of the seat?