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BMW tried it for awhile, looks like they used a heat riser tho':

mattcfish wrote:Any buddy try any of "Flat Racer's" mufflers. It's a good selection of after market megaphones.
http://flatracer.com/#/exhausts/4537795367

I have a replica Dunstall, a Decibel, which I bought from Trojan in Oz, but the detailing looks the same as Flatracers stuff, although he doesn't list the Decibel.

I don't have much hearing left but it is doesn't seem any louder low down, and it seems to combine well with the 2 in 1 higher up to give a delightful howl.

They are indeed good quality, as claimed in the advert , probably better than OEM, on Brit Iron at least , and vastly superior to other pattern stuff I have seen.

The Decibel is a long mega, 28" long and similar in appearance to a Hoske. It has the U channel fixing rather than the strap.
It fits straight to the OEM brackets on a twin shock, but only just gets past the saddle on the rear brake pedal , without indents the like the OEM muffler.

I have it fitted to a 900 cc 75/7 with a 2 in 1 , and it performs better than the Staintune it replaced, but it is pretty well documented that OEM pattern silencers struggle with most 2 in 1s, and mine is just about as low tech as I have seen.

I tried fitting a balance tube between the vacuum ports, couldn't detect any difference in the way it ran but it seemed to affect starting , with it fitted the motor never burst into life the first crank, it needed to be churned over a couple of seconds before it would fire.

Tim Shepherd wrote:BMW tried it for awhile, looks like they used a heat riser tho':

As did VW.

vanzen@rockerboxer.com wrote:

Tim Shepherd wrote:BMW tried it for awhile, looks like they used a heat riser tho':

As did VW.

Yep. I had a glass-body "dune buggy" a hundred years ago with a single 2-barrel Holley. That manifold did away with the heat riser. In any weather below about 45 degrees that carb would ice up and freeze solid.

On second glance it started to look like a breather tube, but I found another photo on Phil Hawksleys excellent website that seems to confirm it is indeed a heat riser. R11 from the other side, note how it attaches to the exhaust port:

I've been following this thread with interest. I don't know all that much about headers and back pressure but want to know more. What could I expect from using 40mm headers on a 38mm exhaust head? Running Mikuni TM38 flat slide carburetors in conjunction with perforated air box and K&N air filter. From what I've read and understood, smaller pipes will be an advantage at low range but not so much at high RPMs. A larger header will reduce low range but mid range and high RPMs. Of course it does depend on the type of muffler restricting the flow? The system I have is TM38 flat slides, stock intake tubes,(40mm) and perforated air box. Heads with 40mm exhaust valves. What could I expect from installing 40mm headers to 38mm port, (using adapter to fit head) with whisper tone mufflers?

You might get a couple of extra hp.. My engine has been fitted with proper 40mm exhaust headers with the heads bored so that they they will fit, I can't really remember what positive effect that this modification had in isolation as they were fitted in conjunction with a change to Dell carbs. Most of the stainless steel 40 mm header pipes are 38 mm pipes swaged out to 40 mm at the ends, so be careful what you are buying

chasbmw wrote:Most of the stainless steel 40 mm header pipes are 38 mm pipes swaged out to 40 mm at the ends, so be careful what you are buying

What I am thinking of is 40mm headers swaged to fit 38mm port. I'm not too sure that is something that will be beneficial performance wise.

Neck down just before is never a good idea..........expansion is a good idea, if the amont don't over load the package and can exhaust freely part of the reason for the BMW system to work as well as it does

When I was looking, the only 40mm pipe I could find were the original BMW chromed ones