Son has just joined a new club and the noise level has to be below
82db. His magnum, fitted with the west 51/genesis pipe is running at
around 87db. Prop is a 9x7 at the moment. How can he get it down to
82db? Anyone done this and if so what prop did you end up using?
Ted shuffled out of his cave and grunted these great (and sometimes not
so great) words of knowledge:
If you are not using an APC prop, try one. It is usually several db
quieter than a MAS prop. Another thing is to loose the pipe and go to a
I presume you mean 82 dBA at 7 meters. That's very close to 90 dBA at
9 feet, the standard distance we use in the colonies (dunno why Brits
use a metric measure and we're stuck with a count of some bloody Brit
A 9X7 prop will generate 87 dBA @ 7M while turning at about 15 K rpm,
sans any contribution from engine intake and exhaust. A tuned pipe
has a large (capacitive) reactance, and does a good job of quieting,
so I doubt you are turning the 9X7 at less than 15 K, and possibly a
good bit more. At 15K, an APC prop is absorbing about 1.1 bhp, most
others somewhat more.
You would have to reduce the rpm to less than 13 K with that prop to
meet the club standards.
A 10X6 absorbs the same power while turning 11.9 K rpm. Sound level
generated by the prop would be a fraction of a decibel below you 82 dB
The pipe header would have to be lengthened to function properly at
the lower rpm.
The power level with either prop would be only 0.72 bhp, which is
rather milquetoast for an engine with a TP in the .40 to .50 cu in
class. The reduced sound limit is going cost you big in the power
department,. The best solution is is to trade in that high rpm
screamer for an engine that gets it's power from high torque vs high
rpm. That's a big plus for the four-stroke engines, and little doubt
a major factor in their popularity..
FWIW, my experience with engines of this class and a decent stock
muffler (OS is an example of one that works decently) need to be
fitted with an 11X6 or 11X7 to get below the sound level your club
requires. That much prop load is horrifying to some modelers that
have not been subject to noise contraints, but there are many engines
that can handle it nicely. I used an APC 11X8 on a plain-bearing Fox
40, with a custom muffler that did produce a fair increase in power
output as well as being far quieter than the stock Fox muffler, which
is not notworthy for it's quieting performance. It turned that prop
at 10.5 to 11 K and served reliably for over ten years before the
crank bushing was worn to the point that excessive fuel was leaking
out the front of the crankcase. Overheating was never a problem.