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Decibels from a hot Comet

ET43

Guest
Hello Folks,
Has any one got any idea as to how many decibels a 600cc Comet on Mk11 cams is likely to register. Does any one know if the motor will be allowed a higher rating in decibels for an SVA test, due to it's age.
Many Thanks,
ET43
 

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
Noisy Exhausts

Phil
The first answer is 'no' you don't get any allowance on noise levels for an old engine, although you do get an exemption on exhaust emissions.

I am currently building a 600 with Gary Robinson '105' cams that will also need to meet SVA. With a modern silencer from a 500cc Honda the exhaust noise is well below the limits, although I can't remember exactly what the reading was.

My only worry at the moment is the mechanical noise from the engine. With a proper, modern, silencer the exhaust note is very muted but if you put the dB meter alongside the engine you get a spectacular reading. Fortunately for the test the dB meter is placed 1m behind and 1m to one side of the tailpipe. I am waiting to see what happens to the engine noise once the bodywork and panniers are fitted. It is possible that some form of sound deadening may be needed around the engine sides.

Clearly, once the test is over the sound deadening goes in the bin and a far more interesting silencer goes on.

The other thing to bear in mind is that on the SVA application form you are asked to specify the maximum revs for the engine. The sound test is conducted at 3/4 of the maximum speed! If you specify a very low maximum revs and put a suitable red line on the tacho then that may help.
 

nobby

Well Known and Active Forum Website User
VOC Member
thundersound

On my 600 I use a 2" pipe, also longer then a standard pipe and a stainless 2" Supertrapp silencer. It makes more noise (especially in low revs) then when it was a 500 and used for racing. The last time at the track that the 500 engine was not admitted it made 112 dB without pulling the throttle. These noise "challenges" are one of the reasons to quit racing for a lot of people. On the track (or on the road ofcourse) the 600 (and the 500) make less noise. But idling and so is terrible for some bystanders...
If anyone knows of a silencer a bit similar to a Vincent one, but 1 3/4" or even 2" then I would like to know. I also have a pipe with a measure between standard and 2" (must be 1 3/4"?).
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
Why do you need to pass an SVA test? I thought this test was for kit cars and low volume manufacturers currently in production.

Have they changed the rules on registering old vehicles and getting an age related registration if you cannot prove the original registration?

Just curious as my dad has just about finished a Black Shadow and will require an age related registration as no evidence of the original exists other than factory records which only show manufacturing dates not registration info.

Cheers,
Simon.
 

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
Sva

Why do you need to pass an SVA test? I thought this test was for kit cars and low volume manufacturers currently in production.
Cheers,
Simon.

You are quite right, If you are putting an old bike back on the road then all that you need is proof of age. The club machine registrar can help you with this. You only need an SVA if you are foolish enough to build a bike from scratch. I don't know what Phil(ET43) is playing with but an early picture of my effort is attached.

View attachment 397
 

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  • Comet.jpg
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ET43

Guest
Prim's new plaything

Hello Pete and Simon,
There's no fool like an old fool, and I'm both! If Graham is kind to me an explanation will be in MPH at some time, but to answer the question as to what I am up to, is, that because of a wrecked lower back which now gives me discomfort when sitting on a bike saddle, I am about to build a reverse trike powered by a large-ish Comet engine. It will have an open chassis, possibly two seats, and large fairly skinny wheels. Think of the Mog-Vin or Peter Brookes-Tee's Gran Vin, and you will have some idea of what I'm at.
Now back to the drawing board.
Cheers, ET43
 

Bill Thomas

Well Known and Active Forum Website User
VOC Member
Big Comet !

Hello Pete,
Your big comet, Have you stroked it ?.
I have got a short con rod from U.S.A. and am looking for a good bloke to
Grind out an 1, 1/4'' hole in the flywheels.
I am sticking with a standard piston and am thinking a 1/4'' will give me
about 560cc ?
Good Luck Bill.
 

Simon Dinsdale

VOC Machine Registrar
VOC Member
VOC Forum Website Moderator
I understand now. Looks like both projects would require SVA test.

I seem to remember reading 10+ years ago the Dutch or Scandanavian section had a foam engine silencing kit and quiet exhaust system which they fitted to Vincents when importing to the country to get around the import requirement of a max db sound reading. They found most of the noise generated came from the engine and not the exhaust. After passing the test, all the extras just fell off just around the corner to be retrieved for the next import!!!!

Not that i'm suggesting somebody would enter an SVA test with the same intentions.

Cheers,
Simon.
 

Pete Appleton

VOC Hon. Editor
VOC Member
VOC Forum Website Administrator
Hello Pete,
Your big comet, Have you stroked it ?.
Good Luck Bill.

Hello Bill
No I haven't stroked it. I have used one of Mr Kemp's big bore kits and a standard length Carillo Rod. This also gives somewhere in the region of 560cc. You do, of course, have to machine the crankcase mouths and the cylinder head, but as all of the castings that I started with were pretty much scrap then I am not wrecking anything valuable.

The whole concept of my machine is a long distance tourer with mild off-road capabilities so I was reluctant to go for the increased stroke with the resultant higher piston speeds. I have no knowledge of reliability problems on high mileage, stroked Comets but didn't want to risk it. Has anyone done high mileages on a bored and stroked machine?

Well, it beats watching the telly

Pete
 
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