Cooling is a rather
controversial subject, certainly one that has caused more consternation than most. We have
done a lot of testing recently using several different VariEzes and Long-EZs.
This is a brief summary of what we found: If you only have one
CHT probe, install it on the most forward cylinder, cylinder #4 on a Lycoming, cylinder #1
on a Continental. In a normal EZ per plans engine installation, the forward two cylinders
will invariably run hotter than the aft two. We have consistently found the most forward
cylinder running as much as IOO'F hotter than the most aft cylinder.
If you have a four probe CHT system, one you can rely on, that
has been calibrated and is known to be accurate, you can experiment with "ramps"
on the floor of the lower cowl, as shown in CP42, page 3. The deflector ramps will
dramatically change the cooling pattern of your engine, depending on the shape, size and
position of these ramps. It would be difficult and maybe even foolhardy to try this
without good instrumentation.
The way the cooling in an EZ cowl works is apparently not the way
it works in a Cessna 150. For example, in an EZ, ram (male) scoop or NACA flush scoop, the
high velocity cooling air enters the cowl and most of this air runs up the slope of the
lower cowl, hits the aft vertical baffle, and squirts up through the fins of the two aft
cylinders.
Since most of the incoming air has gone, at high speed, through
the aft two cylinders, in most cases overcooling these two cylinders, the forward two
cylinders do not get enough cooling air, so it follows that they run too hot. This is the
way it is, at least, on the several EZs we have closely examined.
If you accept the above scenario, it follows that a ramp or
several ramps installed on the lower cowl, positioned and shaped to deflect the
high-speed, incoming, cooling air and cause it to go up and through the forward two
cylinders, should do the trick. In all cases we have tested, we have been able to reverse
the hottest and coldest cylinders! This is pretty significiant and what it tells you is
that with some experimenting, you can get all four cylinders running within just a few
degrees of each other. Everyone who has seriously tried this has reported the same
results. This has included some real skeptics.
In the last two weeks, we have tested 6 EZs, two VariEzes and
4 Long-EZs, using a water manometer. We used a stock Cessna 150 as a kind of
"baseline". We found that a standard ram inlet EZ compares very closely to an EZ
equipped with a flush NACA inlet, provided both have well-built, close fitting baffling
and both have the same size, stock cooling outlet. Changing the size of the outlet will
change the pressure drop across the cylinders.
Of course, there is a lot more to cooling than the pressure drop
across the cylinders. "Blockage", or the resistance to the flow of cooling air
caused by the baffled engine is a big driver. Very poor baffling or no baffling at alI ,
obviously will result in a very low pressure drop. Very tight baffling forcing the
incoming high pressure air to slow way down will obviously result in a large pressure
drop. This differential is called the delta 'p' and is measured in inches of water.
Lycoming says that for a Lycoming 0-235 engine, you need a delta
'p' of about 4" of water. The curves shown below are the results of our recent tests.
Click to enlarge
Note that the two Long-EZs with the lowest delta 'p' across the
cylinders (only 3" delta 'p' at 160). do in fact, have good cooling. Both are well
equipped with 4 probe calibrated CHT gauges. What does this prove? Only that if the
baffling is excellent, tight with absolutely no leaks, the cylinders will cool acceptably
even with only 3" of water delta 'p', also, note that both of these Longs have
smaller than normal cooling air outlets.
The temperatures on the above airplanes are measured at the
bayonet cylinder head fitting on the Lycomings and on the top spark plugs on the
Continentals. One of the Lycoming engines is really well instrumented, with probes on all
four cylinders at the bayonet fittings, and on the bottom spark plugs as well as on the
top spark plugs. The results of this test are as follows. Maximum power setting (mixture
slightly rich for the climb) results in the bayonet probes averaging 360' 380' F. Bottom
spark plugs average 400' - 420'F. Top spark plugs average 440' - 460'F. At 10,000 feet,
OAT= +10', in level flight at maximum continuous power, (mixture leaned to best power max.
EGT), the bayonet probes average 330' - 350'F. The bottom plugs average 360' - 380'F and
the top plugs average 430' - 450'F. Inspite of an average difference of 70' - 100'F from
the bottom to the top of each cylinder, this is probably about as good as you can do and
is quite acceptable, according to Lycoming.
The optimum baffling for an EZ engine is probably not possible
due to the mechanical difficulty of building it, but you can come close. For a Lycoming
0-235 or Continental 0-200, try to baffle as close to the sketch below as you can.
