In communiqué <i6qvko$gjc$(E-Mail Removed)>, Krusty
<(E-Mail Removed)> cast forth these pearls of wisdom
>SIRPip wrote:
>
>> Krusty wrote:
>>
>> > Your twisty metal baffle thingies, do they need to be a snug fit, or
>> > would say a 30mm wide metal strip work in a 47mm hole?
>>
>> I've always used a strip cut to be a snug fit: a 47mm pipe would, IWT,
>> require a ~45mm strip. If you have only have 30mm strip it's worth a
>> go, though I would expect a reduction in benefit as there's going to
>> be a straight shot bypassing the effect of the twister.
>
>Well the biggest I could fit in, given the angle, is 40mm, so that's
>what I did. They're the full length of the can & straight bit of the
>link pipe. The results are:
>
>Race cans with just the db killers at 5k RPM: 99.6db.
>As above but with twisty metal inserts: 99.6db.
>
>So I'd say that's a resounding fail! Maybe a 500cc cylinder's worth of
>gas is too much for the twister to deal with.
>
I'm not totally surprised by the result. Presumably these race cans are
simple absorption type 'silencers' with a perforated tube surrounded by
packing. I'm sure Pip has had success with this strategy, but I wouldn't
have thought just a 'twirl' would achieve a great deal as it's not
actually increasing the area for absorption IYSWIM. TBC may be able to
confirm whether it would significantly slow the speed of passage of the
exhaust gas slug or not. Too much like proper fizziks for me.
It would certainly have passed the old police noise test at Mallory in
the old days. A copper on duty at the car park exit would pull over
particularly noisy bikes and stick something akin to an army officer's
swagger stick [1] up your reverse cone mega. If it hit something you
passed, if not you were in trouble.
The extra bit on the Nordwest's FPM race can also significantly reduced
the outlet diameter and had a short length of pipe with a blanked off
end and several holes drilled in it. Probably a bit like the db killers
you already have I suspect.
It made a huge difference to the noise level. It also significantly
****ed up the gas flow unfortunately. I never bothered to get the
jetting right for the road. although it was OK if you didn't give it a
big handful at low revs in a high gear. I found that, as Frigerio had
predicted it went so much better without it.
The silencer on my old SWM achieved extra efficiency by having two
perforated pipes in the muffler (and two outlets). Which reminds me that
I recently found it in the shed and must list it on Ebay.
[1] No connection with the 'cock' joke on 'First Light' last night.
Looks not that unlike a wooden drumstick. I still have my father's in
the loft.
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