"Excessive exhaust noise" - MoT failure point

We've been told that bikes which get much noisier during the race will
not be tolerated, tho I've no idea how they will police this.

 

I like that - much simpler than plan B. I will endeavour to try tonight
and report back

Cheers

Chris D

 

Well, if it works that's a Good Thing. If it don't, then all it will
have cost you is a strip of steel and about 20 minutes.

May the Farce be with you and let's know how it goes ;-)

 

hmmm. considerable fiddling later....


I've not been on the road, just been revving it in the yard. tried
short-ish ones, (about 7") and longer ones (about 11"), and let me tell you,
's bloody difficult putting the twist in 'em without it going a funny shape.
I was using some galved steel about 22G.

Tried 'em flat and twisted. The longer ones go in far enough that they're
into the bit inside the can with the holey metal. None of 'em made a huge
difference - the longer twisted ones made a different note, slightly
quieter, at low revs but not much notable difference at high revs.

I also tried a baffle in the middle of the can, first using a large penny
washer with an 8mm hole, which wasn't as wide as the bore, and then a solid
one, roughly octagonal, which almost-fits the bore. These last blew out of
the end... but before blowing out, they didn't make that much difference to
the noise level or quality.

 

IMO the exhausts on the Guzzi are a bit too loud at full chat and really
want to quieten them a bit.

As the 'silencers' are essentially absorption lined open megaphones
there's not a lot that can be done (or I'd like to do) in the baffle
department.

It seems the splitters haven't helped Austin. Although it may be worth
trying to insert one up the straight section of the header.
http://www.tallbloke.net/img/BoSM05/album/images/DSCF0033_jpg.jpg

I was considering having a balance pipe made up between the two headers.
I'm led to believe that would have an effect, would it?

 

Hmm. The longer, the better. Yes, it is difficult getting a smooth
twist. Gradually does it, glassoppa.

Longer twisted ones should make a difference. Flat won't, I've tried
it (due to the difficulty in twisting the strips ;-)) Bear in mind
that the smallest difference the human ear can detect is several
decibels.

When Lozzo got his XR400 it was restricted. We removed the
restriction from the exhaust which made a huge difference to
performance, but also made the bloody thing unbearably bloody loud.
We tried washers and things (didn't have any steel handy to make a
splitter) without much success - then came up with the solution.

There was a coffee percolator filter in the garage. Shaped like a
little top hat, made of aluminium and pierced with many tiny holes, it
was the same diameter as the ID of the pipe. We wedged it in and the
thing quieted - but it was strangled again. So we took it out again
and ran an 1/8" drill through all the holes. Marvellous compromise -
much quieter, acceptably small loss of power.

Baffling, innit?

 

you should note that I've not tried it at full chat on the road. It's as
noisey as a noisey thing anyway when revved up to about 5500 on the yard;
has a viscious crackle to the exhaust.

--
Austin Shackles. www.ddol-las.fsnet.co.uk my opinions are just that
Too Busy: Your mind is like a motorway. Sometimes it can be jammed by
too much traffic. Avoid the jams by never using your mind on a
Bank Holiday weekend.
from the Little Book of Complete B***ocks by Alistair Beaton.

 

Not really, no. A balance box (CX500 or Superdream stylee) might, as it
acts like a sort of pre-silencer chamber, I think, but just linking the
headers won't quieten anything.

 

Why do you think that?

The theory I was given (FWIW) is it allows the gasses to clear through
both silencers and the sound waves, being out of sync, would have the
effect of cancelling each other out.

 

Hm. I was told that it evens out the exhaust pulses and (to a tiny
degree) improves power.

I can't see how the sound from one pot could possibly cancel out the
sound from another. Active noise suppression is something that's been
worked on by many automotive companies, and they ain't got there yet. If
it was as easy as adding a balance pipe.....

I *think* military aircraft have some form of active noise suppression
because it's relatively easy to plot a jet turbine noise signature,
whereas a reciprocating piston engine is another kettle of fish -
especially something whose revs vary as much as a bike engine.

Bloody interesting point though, and I don't know much - anything -
about the science of it.

Paging SWK!!!

 

The SOBoxer has one, but then again, I've not tried it without.

 

Guzzis' also have them as standard. The Le Mans type (the header pipes
joined just under the cylinder heads) is generally viewed as ineffective
and later bikes all had theirs as a H piece behind the engine and just
before the silencers.

 

on the boxer it's down at the bottom in front of the engine.

 

The Older Gentleman says...

Well I'm not really SWK but it was covered when I was doing an
engineering course[1]. IIRC The best way to silence an engine is to make
the gasses change direction. On each change they lose pressure and hence
sound. Some sort of removable labyrinth type arrangement would probably
work, at the expense of increased back pressure of course.

[1] I left before I finished it.

 

mups wrote

Is that change direction as in Reverse Flow or change direction as in go
round a corner?

 

IRound a corner I believe - at least explains how the 'mangled coke tin'
approach might work

Chris D

 

"Me too"

"Me too" again unfortunately

This approximates to my earlier 'plan B' which is now under way.
Will report back soon...

Chris D

 

steve auvache says...

Either as long as its a sharp change of direction. I think car
exhausts[1] force the gasses to do a couple of 180s as they pass though
the collector boxes.

[1] and probably bike ones as well.

 

Just as matter of interest has anybody tried Brooklands' cans? They
were designed to reduce the noise output of racing engines with
minimal effect on power.

Exhaust systems are a black art anyway, especially on racing two
strokes. With engines with a large valve overlap restrictions in the
exhaust can actually improve performance. It's years since I read this
so I am not sure if I have got it correct. However the idea is to
arrange things so that at the start of the inlet stroke the pressure
in the (still open) exhaust port is atmospheric or below with exhaust
pressure rising above inlet pressure just before the exhaust valve
closes to prevent charge loss. With a single cylinder engine there is
not much you can do apart from varying pipe length, megaphone angle
and megaphone length. With multiple cylinders you can do clever
tricks.

By the way the DKW split single supercharged two strokes were so
noisy that they could be heard on the main land going down Bray Hill
in the TT.



Ken Young

Maternity is a matter of fact
Paternity is a matter of opinion

 

Two things: let the exhaust note beat against itself or let it expand.

Silencer technology is actually pretty simple.

All my best,
do9c