Decibels from a hot Comet

Tom Gaynor

Well Known and Active Forum User
VOC Member
Nobby's Comet

No need to take Nobby's word for it. He only lives in Holland. Get him to start it up, and if the wind is in the right direction, you can hear it for yourself. From Britain. After his visit to Applecross, Highland A & E departments were choked with sheep complaining of sudden inexplicable deafness. Honest.

So far as the motor and its gas dynamics are concerned, the start of the megaphone is a change of section. The mega exit is another. At each change of section a reverse pressure wave is sent BACK the way it came. A positive pressure pulse OUT becomes a negative pressure wave IN, timed to suck fresh mixture down the exhaust tract, and the following positive wave crams it back into the pot just as the valves close. Result? Supercharging. This was Walter Kaaden (of MZ)'s great discovery. It's rather more complicated than that, but essentially what Peter V has been doing is getting the timing of the resonant system that is his Rapide, right. That and leaning it over until his ears touch down.
Most of the fastest modern classic Manxes and G50's have a pipe that tapers all the way from the exhaust port to the end of the pipe, from 1 5/8" at the port to about 3" at the exit. I don't know how that works, but judging by the speed at which they clear off, it does.
The best book on the subject is Graham Bell's Four-Stroke Performance Tuning. He also deals with two-into-one (Vincent...), as well as four-into-two-into-one, eight-into... but you get the picture.
TFS is rather outdated.

Tom


Yes you can tune them with plates for sound but more important for power! Peter Volkers is using a normal Vincent diameter Supertrapp and he won several horsepowers on a Dyno. With the 500 I rode with an open cone (like on the picture in this thread), but later I discovered that with a closed endplate the riding on the 600 was nicer and more smooth through all revs. I am not an expert in this, but maybe the same reason that megaphones are not a good solution for Comets?
 

Bazlerker

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Non-VOC Member
There was a book written by Gordon Jennings regarding 2 stroke expansion chambers..2 strokes are of course more sensitive to changes in their exhausts than 4 strokes are...but the basic idea has more similarities than not...
 

Black Flash

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VOC Member
if you are worried about the noisetest, there are two easy things to try out. But only do this temporaryly to pass the test and return to original after !!!

First increase your valve clearance (standard is zero) for the visit to the MOT station. Especially with a hot cam this works wonders, it dramatically reduces the overlap and takes the sharpnes out of the exhaust note (changing the sound from bang bang to almost fluff fluff). I would suggest something like 0.3-0.4 mm, but you can simply try in your yard. WARNING! Do not overdo it, the last thing you want is the pushrod to jump out of the adjuster or cam follower!

secondly put some fine steel wool pads ( ask your wife, she probably has them in the kitchen, but use the ones without soap in them) into the front of your silencer and check they do not fly out the end when reving hard. they are easy to remove afterwards or simply burn away. Drive to the MOT station slowly and it should last the test.

Over here in Germany, once the bike is registered nobody will ever check it again, so this works well.

I did this on my BSA B44 VS which is definately very loud and passed the MOT with ease.
 
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Tom Gaynor

Well Known and Active Forum User
VOC Member
Noise test

Something I was told at race tracks was to avoid standing the bike on tarmac when testing, because it makes it 2dB louder than grass does.

The MV three was tested at mallory park at a classic meeting several years ago. The limit was then 110 dB. The MV was the right side (i.e. the far side) of 135. When the reading was shown to the mechanic warming it up, he flashed a smile and bellowed "Yeah. Itsa loud, eh?" and went back to his business.

No further action was taken. Lynchings are pretty ugly, although I might make an exception for any ACU official who excluded the MV.

(I can vouch for the steel wool dodge, I ran a barely modified Blair (should have been "blare") megaphone on a Gold Star and it started quiet and gradually got louder.)

Tom

if you are worried about the noisetest, there are two easy things to try out. But only do this temporaryly to pass the test and return to original after !!!

First increase your valve clearance (standard is zero) for the visit to the MOT station. Especially with a hot cam this works wonders, it dramatically reduces the overlap and takes the sharpnes out of the exhaust note (changing the sound from bang bang to almost fluff fluff). I would suggest something like 0.3-0.4 mm, but you can simply try in your yard. WARNING! Do not overdo it, the last thing you want is the pushrod to jump out of the adjuster or cam follower!

secondly put some fine steel wool pads ( ask your wife, she probably has them in the kitchen, but use the ones without soap in them) into the front of your silencer and check they do not fly out the end when reving hard. they are easy to remove afterwards or simply burn away. Drive to the MOT station slowly and it should last the test.

Over here in Germany, once the bike is registered nobody will ever check it again, so this works well.

I did this on my BSA B44 VS which is definately very loud and passed the MOT with ease.
 
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