Decibels from a hot Comet

[ET43]

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]

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.

 

[Tnecniv Edipar]

4000 rpm it is then !!

 

[nobby]

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"?).

 

[vin998]

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]

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

 

[ET43]

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]

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.

 

[vin998]

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]

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

 

[Howard]

Has anyone tried a Harley silencer? I don't know anything about them, but I've noticed a few with very small after market silencers that seem to run very quiet (for a lumpy vee twin).

In answer to the original question ... "a lot". My racing comet with Mk IIs and a short tuned 1 5/8" pipe (or was it 2"?) was off the scale on the noise meter in the 80s (and I didn't actually take it up to the right revs). Luckily it was VOC Cadwell and Jaqueline Bickerstaff had the meter, so I sneaked out. After that it was hard work keeping the power and losing the noise. I could never understand why some simillar sized bikes on open pipes were so much quieter than the Comet.

H

 

[nobby]

size is not important?

Has anyone tried a Harley silencer? I don't know anything about them, but I've noticed a few with very small after market silencers that seem to run very quiet (for a lumpy vee twin).
H

My 2" Supertrapp is the same as another Vincent owner in my hometown used for his Harley Lowrider, only he had two of them.

By the way: my wife is looking here for a mirror, I changed it from left to right when we visited Scotland... She was already confused because whe had a weekend without breakdowns!

 

[Howard]

My 2" Supertrapp is the same as another Vincent owner in my hometown used for his Harley Lowrider, only he had two of them.

By the way: my wife is looking here for a mirror, I changed it from left to right when we visited Scotland... She was already confused because whe had a weekend without breakdowns!

That looks very tidy, is it quiet? ............................ The silencer, not your wife, that is.

H

 

[nobby]

quiet?

It is NOT quiet. I can compete with those ' persons' on japanese and americain apehangers who leave their silencers at home. You know what a terrible noise some of those guys can make.
So I am looking for a more silent replacement.
I am still talking about the Supertrapp!

 

[Bill Thomas]

Supertrapp

Hello howard,
I think you can tune the Supertrapp to make it quiet by taking plates out,
Or putting them in, Is that right Nobby ?.
I saw one in a shop the other day, Trouble is you will loose power ?.
If it was me I would leave the end off, where I live even the cars have
6'' end exhausts, I spoke to the local car MOT man and he says he has to pass them !!.
They are very expensive, But I like Nobby's idea of a 2" pipe + Supertrapp,
I have a 2" front pipe off the Lightning I could use !!.
If I ever get a 600cc built, That is the way I would like to go !.
All the best Bill.

 

[bmetcalf]

One of the Vancouver lot has a Supertrapp, maybe he will chime in.

 

[nobby]

Hello howard,
I think you can tune the Supertrapp to make it quiet by taking plates out,
Or putting them in, Is that right Nobby ?.

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?

 

[Howard]

Hello howard,
I think you can tune the Supertrapp to make it quiet by taking plates out,
Or putting them in, Is that right Nobby ?.
I saw one in a shop the other day, Trouble is you will loose power ?.
If it was me I would leave the end off, where I live even the cars have
6'' end exhausts, I spoke to the local car MOT man and he says he has to pass them !!.
They are very expensive, But I like Nobby's idea of a 2" pipe + Supertrapp,
I have a 2" front pipe off the Lightning I could use !!.
If I ever get a 600cc built, That is the way I would like to go !.
All the best Bill.

Hi Bill

This is just curiosity for me. I've used a Dunstall Decibel (copy now) on the Egli since the 70s, it seems to suit the engine and it gives a nice bark that other road users notice. No one's ever said "Sorry mate, I didn't hear you."

H

ps One club member says it should be called an amplifier not a silencer.

 

[Howard]

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?

Hi Nobby

I don't know the theory, but it was "known from experience" (I don't know who's experience or when) that Vincents didn't run best on megaphones. I tried one for a very short time, and, if I remember right, it caused flat spots all over the rev range.

On the track, I used a straight 2" pipe that reached the RFM pivot (not the wheel spindle) I was laughed at when I arrived at the VOC Cadwell meet, but after the first practice, several Comet riders were seen with hacksaws. Everyone knows a racing Vin exhaust should reach the rear wheel spindle, but what was forgotten was that the rules of the day (50s) said that the exhaust had to be at least that long. My memory fades a bit at this point, but I think the standard length pipe is tuned for about 4000 rpm, which would mean it was also good at 2000 and 8000 rpm, mine was calculated for 6000 rpm (ish) which was closer to where I was revving. All incidental now, as it would not be allowed on the track with that setup.

H

 

[bmetcalf]

Hi Nobby

I don't know the theory, but it was "known from experience" (I don't know who's experience or when) that Vincents didn't run best on megaphones.

See Tuning for Speed, "Improved Cylinder Filling", by PEI.