Part of the answer is oil pump wear. I suspect that Mazda does not make
some of the equipment found in Mazda engines. Years ago I found that oil pump
end play could be way out of spec. So I started with new pumps (when we still
used stock pumps) and shortened the pump body to the minimum allowed
dimension.
The other part of the problem is that the rotors in the engine tend to foam
the oil to excess. So from a long period where entrained air has escaped
the oil over a night or a week of no use. Then you see good oil pressure on
start up, then as you see oil temperature come up and oil foaming comes on
during climb out, the oil pressure drops away. Foam in oil is air bubbles.
Air is compressible. So the peak oil pressure is no longer available. Also air
in the foamed oil in the pick up expands to satisfy the lower pressure in the
suction side of the pump. This reduces the volume of oil the pump can acquire
to pressurize.
There are a number of items in play here. One is the oil brand, pour
weight, recipe, and application of the oil.
Racing oils have additional additives that reduce foaming. Racing oils have
additional anti scuff compounds.
Better for cold starts and engines that sit for long periods between uses.
Racing oils have much higher film strengths than conventional oils. Racing oils
absorb heat and give it up more quickly than conventional oils. Do you see a
pattern here? Remember that duty cycle thing? Start the car engine. Drive away
on ice cold oil. Don't turn over 1,800 RPM ever. Listen on shutdown as the
condensate from the warm engine runs down into the sump. The first to be pumped
to the bearings just before the cold oil on the next start.
Or, start the rotary powered plane. Warm the oil a bit. Take off and climb
out with the front rotor never quite full of cooling oil, and the rear rotor
over filled and foaming like crazy. Now turning up 3 times the street RPM
all of this was designed for. Synthetic racing oils can be used in
rotaries. You must premix the fuel as synthetic sump oil will not burn
without gumming. Use a synthetic 2 cycle oil as a premix or inject it with a
modified pump. I use RedLine 50 weight in the sump (Dry sump) and 2 cycle
RedLine in the premix.
While inspecting at annual, you can remove and inspect the stationary
gears. Just do not move the prop while the gears are out (one at a time). Then
at least you can inspect and measure the main bearings, and reinstall the
stationary gears. Metal in the filter makes the hair on my neck stand up.
Not one oil relate failure since 1980.
Lynn E. Hanover
In a message dated 9/30/2017 11:35:07 A.M. Eastern Daylight Time,
flyrotary@lancaironline.net writes:
Has
anyone experienced low oil pressure due to wear of the Renesis oil pump?
My
Renesis engine on my RV-7 has had decreased oil pressure for a while.
I
changed the oil pump and now my oil pressure is back to normal.
I
have 780 hours on the engine and about 2500 engine heat cycles – most of my
flights are very short. On takeoff with relatively cool oil
the pressure went to 80 PSI but as the oil heated up the pressure decreased to
as low as 65 psi on a hot day during climb out.
I
change the oil at 50 hours I always inspect the oil filter material and
had some very small shiny flakes in the pleats. The particles appear to
be non-magnetic
A
month ago I decided to remove the engine and replace the oil pump.
I
found the aluminum oil pump housing had a slight amount of wear where the
steel pump gear seals against the housing. Replacing the oil
pump has resulted in much better oil pressure.
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