Pipe tuning
A standard muffler muffles the engine exhaust sounds. A tuned pipe instead
helps sucking gases through the cylinder, making it running
very reliable and giving the engine more power.
However, a tuned pipe has to be tuned correctly, such that the engine RPM is
within the working range of the pipe.
Far below the resonance frequency the pipe works just as a muffler.
So in that case, you could just as well have
bought a standard muffler and saved some money.
If the engine is just below the resonance frequency, the engine will not run properly,
because the pipe will counteract proper gas flow through the engine.
The engine may
show symptoms of running lean such as overheating, not or hardly smoking while at
the same time 'hicking' like it gets too much fuel,
or even 'throw up' every now and then, giving a sudden outburst of power
lasting half a second or so, causing sudden heavy overspeeding of the engine followed
by an equally sudden drop in rpm (Note 19/6/2: this 'throwing up' or 'sneezing'
probably was caused by a broken autorotation bearing).
More nitro, other glow plug etc will not improve very much
on this situation.
In particular, these symptoms occur with the combination
OS50 + Hatori540 + gear ratio 8.5:1.
Pipes have a specified working range specified by the manufacturer.
For instance, the Hatori 540 is designed for engine speeds between 16000 and
19000rpm. Especially if you need top power, you should compare this
with the optimal engine RPM. The OS50 has its maximum power output at 17000rpm,
so this is a good match.
However, you may want to run at different RPM's than the maximum power. For instance,
with the 8.5:1 gear ratio on the Raptor, an engine RPM of 17000 corresponds
to a headspeed of 2000, and 19000 corresponds to an incredible 2235 rpm!!!
With the Raptor, I found the engine running smooth only at 18000rpm, or 2100rpm!
I dont think this to be very healthy for your Raptor, 1950 is about the high end for 3D
maneuvres. If you want a hovering headspeed of 1750rpm, this would correspond to a enginespeed
of 15000rpm. But the engine will not run properly between 15000 and 16000rpm because
of the pipe. The manual says 'Recommendable setting is to adjust between 17000
and 18000 rpm when in flying in the upper sky and below 15000 rpm when in hovering'.
This is an understatement, better read this as obligatory instructions.
In order to use the pipe at lower rpm, you have to increase the pipe length.
For instance, to move the pickup point from 17000 to 15000 rpm,
the pipe length should be increased with a factor 17/15, so we have to add 2/15 to
its length. This length is between the beginning of the header pipe and
the first barrier inside the pipe.
The Hatori 541 header is about 16cm, the first barrier in the pipe is at about 19cm from
the entrance, so the total is about 35cm in the standard configuration (a few, say 6 mm space between
header and pipe, as instructed in the manual). 2/15 of 35 is 4.6 cm, so if we put 4 cm extra between
header and pipe the resonance frequency is approximately brought down to 15000.
As a test, I inserted 3.5cm extra header, and I could run the engine reasonably smooth at 16000rpm.
3.5cm is 10% increase with respect to the original 35cm length of the pipe, and this corresponds
good to 90% of 18000. But note that 3.5cm extra header is quite a lot, and the corresponding 1850rpm
headspeed still is pretty high. Furthermore the pickup point will now be at 90% of 15000 or 13500,
which corresponds to 1500rpm headspeed which is very low for hovering.
Hovering at 1850 rpm seems not recommendable either...
Maybe it could just be made to work when aimed for 1600rpm hovering and 1950rpm 3D, but
my conclusion is that this pipe does not combine properly with the 8.5 gear ratio on the raptor.
© W.Pasman, 10/2/2