That's a good write-up, as usual - I would like to make the comment that
the 16X has narrow rotors because of that problem of losing heat in such a large
rotor area, resulting in incomplete burn. This supports your comments I
believe.
OK here you go...... The rotary has problems with unburned hydrocarbons at
all RPMs. The reason is the area of cooled combustion chamber available to the
fuel air charge. The mixture tends to condense back into droplets and that
reduces the area of the total number of gasoline molecules available to latch on
to an oxygen molecule so more than we would like never combine with oxygen and
do not get burned in the chamber, but get excited by the heat of combustion and
mate up further town stream about where your EGT probe is installed. Many other
problems can be traced right to this same problem. No starts due to low cranking
speed is one. No heat of compression equals no excitement and no combining and
no start.
You can see Mazda working to reduce this problem by getting the engine to
warm quickly, and thus reduce the fleet HC output and be allowed to sell them
in California. First you see that god awful stock thermostat, with its two
plungers and plugs. When cold the lower hole is open and the upper hole
is closed. So as the engine starts up cold, water can circulate inside the
engine only. So, the engine warms uniformly and when at full operating
temperature the upper hole opens to the cooling system and the lower hole is
closed off.
That is the water trick. The oil trick is the balls and springs in the
crank. If the engine is started and left to idle, the balls remain seated and
no oil sprays into the rotors to cool them off. From a cold start they want
the rotors to heat up ASAP to reduce the HC. So if you drive away, you are
heating the rotors with the combustion process, and the balls unseating from
RPM changes makes little difference. If left to idle for long periods, only
bearing spill will enter the rotors to cool them, but that is plenty.
There have been and are other schemes involved as well but the ideas are
the same. Rapid heating of the chamber on startup.
So limiting cooling during warmup is to reduce HC and improved mileage
due to the lower viscosity and less drag of warmed oil. Simple.
To meter the amount of oil there is a specific sized hole in a bolt head
that holds the springs and balls in the gallery in the crank. The down side of
removing the balls and springs is that the rather small oil pump cannot
maintain enough oil pressure to keep the idiot light out at hot idle. The
missing balls produce 2 big oil leaks in the crank. Not a thing to worry
about, because the bearings are huge for the loads involved, but it worries
people to see the light on, or the gage below 20 pounds on a hot day.
Racers increase the oil pressure in order to move heated oil off of the
bearings quickly to prevent melting the soft grey stuff on the bearings. Ay
higher revs the stock pump has enough output to do this. The higher oil
pressure (say 100 pounds instead of the stock 71 pounds) then causes the two
cooling jets in the crank to put out a bit too much oil to the rotors. This
results in plenty of cooling of course, but it also adds to the foaming of the
oil that then drops back into the sump through that hole in the center iron.
Foamed oil has trapped air mixed up in it, and air is an insulator, so the
effect is that oil coolers start to act smaller than they are. So if you have
jacked up the oil pressure on an early engine, then you probably need to add
the Weber air correction jet to replace the ball and spring. The amount of oil
supplied is reduced with RPM when you scream the engine because of the
centrifugal load of the oil column being pushed into the crank. So the FD or
twin turbo engines used 115 pounds of oil pressure. So at modest speeds you
are putting a bunch of oil through the rotors.
I suspect, but do not know if the later engines have smaller jets
to account for the higher oil pressure. Typical jets are.220MM for turbo
engines or .200 MM for NA engines. (Racing Beat). I use .180MM and 100 pounds
of pressure but I have very good oil cooling. Oil temps above 160 degrees
cost HP according to Daryl Drummond who built our first race engines. Also all
of the star Mazda and Formula Mazda series engines.
At airplane RPM there is just no problem with rotor cooling that can be
attributed to the balls and springs or lack thereof. Oil cooling is a function
of cooler size and airflow. Some improvement might be seen if a straight
weight oil is used over a multi grade to avoid some of the foaming. Or if a
racing oil is used that has anti foaming agents and additional anti scuffing
agents. Mistral found that to be the case as they used airplane oil at first,
and kept overheating the oil. It was an airplane after all.
Although it reduces power, the hotter rotor would produce better mileage
due to better fuel burn. So if you lean it way out anyway, it is off of full
power, so it might be worth some investigation, since there is much whining
about fuel consumption in rotaries.
Lynn E. Hanover
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