Cooling a Rotax
912
in a Rans S-7
2011/06/26
2004 S7S on 1260 Lotus
Most of this material applies to any year S-7 and S-7S (or even other aircraft for that matter).
Not many people will want to make the drastic change to air flow over the cooler that is shown here but a simpler method of installing a larger cooler is also covered.
Background:
First, it is important to
keep in mind that there are many variations of the S-7 equipment depending on
the kit year and subsequent updates. Coolant rads started out under the pilot’s
seat when the 582 was the engine of
choice then were moved backward a couple of feet under the passenger seat to
compensate for the 912 weight and then were mounted forward of the firewall on
the S model. Late 90’s models also have the rad under the pilot.
Oil coolers in the early 90’s
were the S tube type and were mounted horizontally above the engine with no
direct air hitting them. Because this type and placement was quite ineffective,
some people added a second one in series but that also was inadequate. By the
mid 90’s Rans was shipping an Earls style cooler which is one of the better
designs mounted vertically in the left air inlet nostril like the one on the
right below.
Here is a picture of the three
common oil cooler designs:
The S-7S cooler is a little wider than the one on the
right
There is information on the
Earl’s (or Jegs Racing) site showing the heat transfer capabilities of each and
the one on the right is the best. The left type was installed on early S-7 912
installations; the one on the right was used until the S model when a larger
one of the same type was used .
Keeping the oil
cool For
more discussion on keeping oil and coolant hot, click here
This information is dealing
with the mid 90’s style (and to some extent the S model) having the Earl’s cooler like the one on the right above in the
left nostril of the cowl.
This location has several
disadvantages:
1.
It prevents the
cooling air from hitting the cylinders like the right side allows.
2.
Heated air from
the rad is allowed to enter the engine compartment.
3.
Putting a rad
perpendicular to the slip stream without any ducting is inefficient.
4.
The back side of any
rad should be in an area of low pressure, not the high pressure created by air
entering the right nostril.
5.
The rad almost
touches the exhaust stack below it.
Some people have found that
providing additional air entering the engine compartment contributes to overall
lower temps. The builder of one S-7 chose to leave off the large spinner and
its’ back plate to allow air to enter around the prop hub. Another builder advocates cutting two tennis
ball sized holes below the spinner. See below for pictures.
While these methods help, a more elegant
approach is to re-position the rad to allow more air in where it was designed
to enter.
Removing all of the above
disadvantages can be achieved by housing the rad in a duct and mounting it
flush with the top cowl then putting a louver on top to provide a low pressure
path for the exiting air.
This shows the rad in the
fiberglass duct in position with the cowl off:
With the cowls mounted and
the rad fastened down there is good clearance from the manifold and
throttle/choke cables.
Below is the front view:
Here is the finished cowl:
The duct occupies a little
over ½ of the air inlet opening leaving a good blast of air for the cylinders.
This picture also shows top
rear exit louver which helps after shut down cooling.
Monitoring temps inside the
cowl showed a significant temperature increase after shutdown. The louver at
the top rear of the cowl allows this excess heat to flow out.
This position for the oil
cooler does create extra work to remove or replace the top cowl. Using ½ turn
fasteners instead of bolts would reduce this extra effort.
The smaller black scoop on
the side directs air over the muffler and the top yellow scoop provides air for
the cabin heater using the same size Earl’s as the oil cooler.
Before the changes and with
an oil and water thermostat, on a 30+
(90+) degree day oil temps were 270 or so and water (cylinder probe) 215. Now without thermostats and with the new set
up, temps are 230 and 180. Probably ½ this benefit was due to removing the
thermostats (The Permacool thermostat has a problem in hot weather) and the
rest due to the changes.
Larger oil cooler
All things considered a much
easier method of reducing the oil temp in the earlier S7 is to just install and
S model rad.
This will drop the temp by about
25F. The main steel mounting bracket
needs extensions as do the other 3 clamping rods and outer S bracket. It will
also sit closer to the exhaust pipe so a heat shield baffle is important.
On this 94 S7 the oil cooler
developed a leak, probably because it was mounted without the encasing
brackets, so I replaced it with the larger one off the S7S:
Rather than the AN3 bolts
tapped into the aluminum spacer tubes, I use larger tubes and 10/32 threaded
rod right through.
The other suggestion for
helping to keep temps down is to add more inlet air either by taking off the
spinner and back plate to open up the flow to the existing gap around the
crankshaft or additional holes as in this mod:
The round holes are 2 3/8”
This cowl is on an S model
with both rads packaged below the muffler, thus the air scoop (which isn’t
quite adequate)
Rans uses a shallow cowl air
exit flange (around the exhaust) but most feel that enlarging it (like on the S
model below) increases air flow without enlarging the inlet openings.
Here is an interesting look
at airflow mods on an Avid: http://avidflyer.wikia.com/wiki/912_Cooling
which directs air right to the
cylinders. It started out with some different cooling issues than we have on
the S7 however.
See details on cabin heat
and thermostats
Peter