a problem, so Composite-ARF had to find a quiet and pow-
erful solution, and we recommend the MTW short mini
pipes with the headers as shown in this installation. They
come complete with teflon joiners and swage clamps and
this package can be ordered from Composite-ARF as an
option. This set-up gives the engine a nice throttle
response, a perfect mid-range, and also increases the top
end power slightly.
The main plywood bulkhead in front of the landing gear
already has the 2 semi-circular cut-outs for the twin mini-
pipe system, so installation is quite easy. Mount the 2 head-
ers to the motor and use these to find the approx. locations
of the cut-outs needed in the bottom of the motor dome. Mill
these in the motor dome for the mini-pipes. Keep them
small at this time, and enlarge them later for a perfect fit
and sufficient cooling. We recommend that you make the
holes in the lower firewall at least 12mm (1/2”) bigger than
the diameter of the mufflers to allow enough cooling air
flow. C-ARF advise that you do not cut through the central
vertical 50mm (2”) wide carbon joining tape between the 2
headers.
The twin mini-pipes seat on a steel strap (supplied with a
template for bending), and are held in place with 2 springs.
The springs are hooked over three M3 bolts that are
screwed into 2.5mm Ø holes drilled in the 3 small plywood
blocks that are securely glued to the front of this bulkhead.
The photos here should make the construction very clear.
You will find that the mufflers must be angled downwards at
the back to make sure that they clear the wing spars when
the wings are in the forward (3D) position, and you may
need to bend the headers very slightly to achieve this,
which is easily done by hand. You might need to enlarge
the 2 semi-circular cut-outs in the bulkhead slightly, as we
have in the photo above, to get this angle.
If you don’t have any suitable springs to retain the mini-
pipes it is very easy to make them. Just clamp a length of
1.0mm piano wire and an old screwdriver (about 5mm Ø
shaft) into a battery drill, and hold the other end of the wire
in a glove while spinning it on
slow
speed. Bend 2 hooks in
the ends to finish the job. Please take care doing this.
To be really sure that the mini-pipes cannot more back-
wards and touch the spar, we install a couple of ‘stops’
made from scrap 6mm (1/4”) plywood, with a small piece of
teflon joiner held onto it with an M3 bolt. Glue these behind
the mufflers, against the carbon-composite bulkhead.
This simple mini-pipe retaining system has proven very reli-
able for several years in all our planes, and holds the pipes
Composite-ARF Extra 330L (2.3m span)
27
(above and below) For additional
security, you can fit ‘stops’ behind
both mufflers, so that they cannot
move backwards. These are easily
made from teflon tube and 6mm
plywood blocks as shown.
(above) View of the muffler mount-
ing from the front, taken through
the firewall.
(below) View of the twin muffler
mounting from behind.
