Boom Tubes too loud?

[mickb]

I've just fitted these to my M1100S, but they're too loud for urban use here in the UK. I'm measuring around 96db on idle, going to 116db at 5k, from about 3ft away.
They're fitted with short baffles, and Mark @ Motocreations is looking at options, however in the meantime does anyone have any ideas of potential solutions, without impacting power/torque delivery?

[ungeheuer]

Never seen any figures for M1100/Boomtubes, but I'd be surprised if you didnt already impact the power/torque delivery.  They do look cool though .

Sticking a more restrictive baffle up the end of it has got to further impact on power output since Boomtubes have no expansion volume for the slowed (and quietened) exhaust gasses to inhabit whilst waiting to exit that restriction.

But I could be wrong. Anybody with a better understanding care to contradict me?

[mattc7]

with the steel used and the ceramic coat used, there's not a ton of cooling going down that creates much expansion of gas, I would think (compared to others), and with the short length, it's likely that the exhaust gasses flow quick enough to not bog down.

However the only restrictions are the baffles, which mean the quieter it is, the more likely it is to lose alittle power up top.

However, it is also likely that there will no loss of torque from solid backpressure

Power issues aside.  These weigh a noticeable amount less than the other options available.  I haven't use a scale to check, but I'ld say -8 or -10 lbs off the full termi system (for example). They also reposition the weight they do have down lower on the bike.  Both of these are positive adjustments as well that improve acceleration, braking and flciking side to side

[mickb]

thanks guys, useful info. Would you have any ideas on how to make the baffles slightly more restrictive? They appear to be similiar to those used in the termi slip-ons, although maybe an inch shorter. There doesn't appear to be any baffle packing between the baffle & the pipe, so I wasn't sure whether to try this, or reduce the size of the slots in the baffle?

[He Man]

if you understand how baffles work then yes. Its all about gas expansion. most of hte issue is fitment since you have so little to work with.

[ungeheuer]

with the steel used and the ceramic coat used, there's not a ton of cooling going down that creates much expansion of gas, I would think (compared to others), and with the short length, it's likely that the exhaust gasses flow quick enough to not bog down.

Huh .  If you fit a baffle up the end of a tube, the exiting gasses will take longer to exit because the baffle creates a restriction, no?  If those   s l o w e d  gasses have no place into which they can expand whilst they "wait" to exit the system (a conventional exhaust can exists for that purpose) then you lose exhaust efficiency.  Or have I missed something??

[Raux]

actually you can create a reverse wave that can mess up the flow out of the head that's where the inefficiencies come into play.

one way to fix it is with a custom header with an antireversion chamber

another way is to tune the length of the pipes according to the back pressure

since neither is an option with the boomtubes, you'll have to work with Mark to come up with the right baffle that won't create too much backpressure

[The Mad King Pepe']

The stock exhausts should already come with a 'lip' to help prevent exhaust gasses from flowing back into the combustion chamber, not sure about the boom tubes, but I would assume so.

The baffles affect high end power not torque. In general you get the most high-end power from the least restrictive system == larger pipes. But larger pipes mean lower gas speed and that can cause exhaust gasses to reverse into the engine. Thinner pipes (and baffles) actually increase low-end torque by increasing exhaust speed (to a certain point, obviously!)

That's also why race bikes (and cars) have large exhaust pipes for maximum high end power, BUT max torque is also way up there so the engine alwasy hast to stay above a certain RPM to be usable. Street bikes have smaller pipes to get you more torque so you won't stall as often and look like a dork!

In the end what you want is a compromise between torque and power and that means the right size pipes for your engine and your application (street vs race).

[ggemelos]

I have the Boomtubes on my M1100.  I cannot say that I have noticed a performance hit, in fact I think it has smoothed out the power curve around 4k that the bike seemed to have stock.  The pipes are a bit load, quieter than some bikes with Termis I have seen around.  Is the noise issue due to the local laws or are they too loud for you personally? 

[mickb]

thanks to all for the feedback. Ggemmelos - its just too loud for me, and would definately be considered excessive by the authorities...
Just spoken with Mark, and he's going to send me a more restrictive set of baffles to try, fingers xssed!