Silencer for Series C Comet

[davidd]

Harley uses a crossover on some engines to increase the muffler volume for each cylinder. When a restrictive (quiet) muffler is used it will increase power and it is also known to quiet the exhaust note without a muffler. This seems to comport with Tim's comments, including the cork. A single would need a larger diameter silencer because it does not get to use the the siamesed pipe that a twin can take advantage of.

Exhaust and intake tract tuning is a black art because it takes so long and is so expensive. The math gets you in the ball park, but from there it is all dyno tuning. On a stock street bike you just do not have the flexibilty of changing the size and length of both the intake and exhaust before you start running into problems. The acoustic wave tuning is the same on both sides of the cylinder. The intake benefits from a positive wave arriving at the intake valve just before the valve closes. There are so many variables it is not worth making a complete mathematical model. It is better to go to the dyno and adjust the cork.

David

 

[John Emanuel]

Thanks Neil

Is the absorbtion Armours silencer you are using the very noisy chromed steel one I bought about a couple of years ago, because when I complained about the noise they said it did not contain absorbtion material - wire wool or whatever, because this would be destroyed in the plating process. So which silencers do you have, Armours only offered me the stainless one, which presumably has spiral baffles as an alternative to their chromed steel one with no absorbtion materials.

No doubt the one John Emanuel "fettled" would make the bike go very well - it would be nice to know what he found out and what he did.

Knowing John however I should not think quietness would be the main design driver !!!

Matty

Hi Matty,
You said that you didn't think that quietness was a consideration when I set out to make a silencer. It was! Four years ago I went to the Classic Bike track day at Cadwell and failed the noise test. That concentrated the mind!! Anyway, I've never liked niosy bikes.
I came accross a sectioned drawing of the Shadow from the Motor Cycle magazine in 1949 which showed a sectioned Carbjector silencer. The spiral baffle was the full diameter of the fat part. This is what I aimed to copy. I cheated, in that I wanted to get as much volume as possible whilst still keeping a silencer which looked fairly standard. It's a little longer than most and 3 1/2 in diameter where most are 3 1/4in. (The early VOSC one was only 3in and hopelessly restrictive). Armours sold me the end caps and I found a firm who made segments for the spiral baffle. The resulting silencer passed the following year's track day noise test with 10db to spare and the power output is only a couple of bhp. down at the top end compaired with a straight through silencer.
Unfortunately, when it was fitted to Mark Goodson's 600cc Comet it was not much quieter than the new absorbsion silencer which he was running.

Best wishes
John

 

[Matty]

Thanks John

Your reply seems to verify that what is quiet on a Twin may not be on a single. However I'm not looking for maximum performance - just a pleasant and not too gutless means of doing some gentle Club runs and the annual ride to Heysham (300ish miles) to go to the Manx rally. If I wanted more real poke, I would have to use hotter cams and more than the "cooking" 7.3 compression ratio, plus a my noisy silencer with no baffles which years ago were blown out onto the TTcourse.

May well have to buy the VOCSC spiral baffle silencer in the end and live with the compromises and cost.

Hope to see you in the IOMan, but feel that the event is not as good as it used to be when we had three pretty free runs on parts of the circuit - the present arrangement means that the organisers do not have to organise anything on two of the days now and in any case I thought the speed controlled, £100 lap, a waste of time and money.

All The Best

Matty