David Davis Posted January 10, 2017 Share Posted January 10, 2017 It's amazing what you can think of when you're retired and enjoying your mid morning coffee. Last summer I bought a number of old but functional two-strokes at a club sale. Few of these had effective silencers so I bought a BCM 20-40/50 silencer from Just Engines. Here it is in action strapped to a Merco 35 but I have also used it with an OS 40 and an Enya 45 and it seems to be pretty effective. It is 155mm long by 40mm in diameter. I also re-acquired a Junior 60 from my cousin. It was powered by an Irvine 20 which had gummed up by through lack of use and a castor based fuel. The silencer was one of those chrome-plated car racing dustbin silencers with two exhaust pipes which doesn't silence anything. As the BCM is supposed to be suitable for engines ranging from a 20 to a 50, I've bolted it to the Irvine and it fits. So my question to all of you knowledgeable gentlemen is as follows: it is in two parts. Will there be more noise coming out of the silencer if it is fitted to the Irvine 20 because the exhaust gases will bounce around inside of a pretty large silencer, or would it sound noisier strapped on to an Enya 45 because the engine is over twice the size, and all other things being equal the explosions should be louder? Quote Link to comment Share on other sites More sharing options...
Denis Watkins Posted January 10, 2017 Share Posted January 10, 2017 Noise can be a bit subjective David in that it can be an irritating noise or just loud etc etc. I bet it is quieter on the 20 as those Irvines were noisy. Thick aluminium silencers do fair better than thin ones. The noise is generated by the exhaust gas expanding as it leaves the port, and this expansion will be adjusted by the box it is allowed into. Now look at a 50cc moped with the exhaust a yard long, yet the irritating hair dryer profound din they make is very annoying. Now our tiny lightweight box is a compromise, it is too lightweight, it is too small, and it is too thin. The correct silencer would be 2 - 3 feet long. To get into the air, we have to fit these silencers. A properly made silencer will modify the frequency and sound waves bouncing around, but for the time being strap on what you have. Quote Link to comment Share on other sites More sharing options...
David Davis Posted January 10, 2017 Author Share Posted January 10, 2017 Thank you Denis, but to put the question another way, "Will the noise be greater or less when the BCM silencer is fitted to the smaller engine?" Quote Link to comment Share on other sites More sharing options...
onetenor Posted January 10, 2017 Share Posted January 10, 2017 Less Quote Link to comment Share on other sites More sharing options...
Don Fry Posted January 10, 2017 Share Posted January 10, 2017 Usually, a larger silence is more efficient, but the big motor has more noise energy to destroy. I would guess the smaller moter will now be the quieter of the two. Old trick, with oversize silencers is to put some stainless wool, i.e. Washing up scrubbing pad inside, gets the gases swirling and banging into each other, and slowing and quieting. Don't overdo it, the idea is to disrupt gas flow, not block it. Years ago Irvine were one of the first makers to get serious with noise surpression, and one of the ways they achieved it was to put a sleeve in the silencer, to make it an inch longer. The called it a Q40, for quiet, as I recall. Edited By Donald Fry on 10/01/2017 11:49:59 Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted January 10, 2017 Share Posted January 10, 2017 I managed to damage the silencer on my Super-Tigre .45 a couple of months back. It was basically an Irvine Q40 silencer on an adapter to mate it up to the Super-Tigre. Very quiet it was, too! When I broke it, I discovered that the standard Super-Tigre silencer adapters are no longer available, so I got one of those BCM silencers as an alternative, and very impressed I am, too! Its just as quiet as the Q40 silencer, but seems less restrictive. It is a little bulkier, but still quite acceptable. But to answer the question, I would expect it to be quieter on the 20. And as Donald has said, since you have an over-sized silencer for a 20, a bit of steel wool inside will quieten it even further. -- Pete Quote Link to comment Share on other sites More sharing options...
Derek Stevenson Posted January 10, 2017 Share Posted January 10, 2017 Peter, I wouldn't put steel wool in the silencer it may go on fire (don't ask me why I know that)? I would put a piece of metal scourer for pots into it. I think that's what Donald was suggesting? Quote Link to comment Share on other sites More sharing options...
David Davis Posted January 10, 2017 Author Share Posted January 10, 2017 Funnily enough, I had some wire wool inside the old car-racing dustbin silencer but I don't think it made much difference. Those silencers are alright for cars where you only use full throttle for a short time, but no good for model aeroplanes. I now have the Irvine cooking nicely in an bath of anti freeze on top of the cooker in order to get rid of baked-on castor oil. Quote Link to comment Share on other sites More sharing options...
Peter Christy Posted January 10, 2017 Share Posted January 10, 2017 Posted by Derek Stevenson on 10/01/2017 12:07:30: Peter, I wouldn't put steel wool in the silencer it may go on fire (don't ask me why I know that)? I would put a piece of metal scourer for pots into it. I think that's what Donald was suggesting? I thought those were steel wool, with a bit of detergent added, but I am happy to be corrected! I think I used a time expired Brillo pad last time....! -- Pete Quote Link to comment Share on other sites More sharing options...
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