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My neighbour has been working on an old BSA C11 for a couple of months and has slowly turned it into a very nice bobber styled bike. He's done a really good job of it and I was chatting to him over the fence about it. I commented how great it sounded for a 250 and he said that he'd just cut the silencer straight off the pipe, so literally you just had a piece of pipe run out of the engine up to where the silencer would have been and just a straight, clean cut.
It looks good, sounds good but wouldn't the removal of the silencer have an affect of the engine performance in some way or is an engine that small and that old (1947) not going to be impacted by that sort of modification? Just curious as I always thought silencers/pipes were an integral part of an engines performance as well as sound output.
Most cars are bloody gutless with no back pressure, but I would have thought motorcycles would be the same.
Afaik they are indeed an integral part.
Was it a two stroke then you'd be right, but for four stroke - the less restriction the better.
Look at go bikes with megaphone exhausts!
Yes, back pressure will be greatly reduced so he'll almost certainly need to rejet the carb(s) and maybe move the needles to get the mixture right and not run it too lean. Bet it's not quiet either!!
The science of it is kind of mindboggling, but with a modern road bike, just running open pipes (or even a straight-through end can) can do orrible things- lumpy idles, low torque, mixture issues and messy power curves. Sometimes the gains are worth it but often messing with exhausts without knowing what you're doing is a pretty negative thing. (Though it doesn't always feel it, the brain's better at comparisons than absolutes so sometimes a bike with knackered midrange feels fast because of the big pickup to the top end)
But this isn't a modern bike, so all bets are off, it probably wasn't that optimised in the first place even when it was new. If it runs alright, pulls alright and smells alright, it's probably alright. Not the sort of bike you're chasing bhp on, it's for cruising around being all Steve McQueen, going slow and sounding fast.
If it was mine I'd stump up for a single dyno run for the a/f curve and just make sure it's safe and good. Though I'd probably spend a month fannying around with the carbs first, doing it right's no fun.
Yeah, i guess in 1947 optimisation of horsepower on bsa c11 ( which was underpowered and heavy to boot) isn't something they worried about. I wonder if there is anything other than baffles in the silencer?
It's to do with setting up an area of low pressure just next to the exhaust valve so that when it opens there's that little bit extra helping to remove exhaust gases from the cyclinder. In modern cars, the exhaust is tuned to create a standing sound wave (and therefore low pressure) at that exact point, which is why altering the exhaust on a normally-aspirated engine can be detrimental. Whether or not BSA were aware of this low pressure effect back in the 1940s is another matter. As above, it sounds like this bike will be for cruising and making a lot of noise rather than setting any land speed records.
If it was mine I'd stump up for a single dyno run...
This is 1940s technologie here. What's wrong with a good old fashioned plug chop? 😛
[i]or is an engine that small and that old (1947) not going to be impacted by that sort of modification?[/i]
This.
MidlandTrailquestsGraham - MemberThis is 1940s technologie here. What's wrong with a good old fashioned plug chop?
That's where the month of fannying around comes in 🙂