Get below 100DB

FWIW, making a muffler that reduces noise to quite low levels without any negative effect on power is actually not difficult at all...technically. The problem is the size/appearance. I suspect you could find mufflers that would do exactly what you need but they wouldn't look like Norton Peashooters.

I realize you are not trying to make it quiet, just reduce the dB a few points. Are there other bikes in the class that meet the noise specs? What sort of mufflers are they running?

Use heavy woven roving fiberglass and it won't blow out. Make your cap removable because you'll have to restuff it. Use metal hardware cloth to roll up the fiberglass and fold the fiberglass between the metal cloth (I use 1/4") at the exit so it won't blow out. I don't know if it will pass the noise regs but its been working for me on the street and it will handle WOT.

Hi Mike
My 850 failed on standard pipes at Brands Hatch (105) I complained (loudly) but they wouldn't let me out. This is about 15 years ago. Thin peashooters, nearly as much noise coming through the wall as out the back.
My Maney 2 into 1 on the race bike sat on the 105 limit with the reverse cone. Steve always said warm the engine up well & open the throttle smoothly to the required revs. I was always plus or minus the 3 allowance. Could not cope Steve supplied me his short extension. The Supertrapp fits on the end. I run it with all the plates. (Loudest) & have always passed but you need to keep an eye on the packing material. Surprisingly it was louder on the 750 than it is on the 960?

I took my 750 to Brands last year, I put in a baffle from ....... something Worxs, I forget, Nigel posted some pictures last year in another thread. It was borderline 102 db, the limit for that track day. If I had said I was in the beginners group they would have let me out. I made up a Supertrapp with some 50 -52mm exhaust pipe and the supertrapp bolted to it. It got through no problem at around 100db. This is with a Many 2 into 1. It was the smaller supertrapp discs I used. It was (and is) a bit Heath Robinson but works.

I had to put some home made mute type baffles in my stock peashooters to pass the noise testing at Cadwell! Can’t remember the db figures though.

I don't think there is anything which can be done to shut my 850 up, short of opening the exhaust valve later. But I think that anything I do which increases back pressure would take the top off the power band. The size of the tail-pipe after the collector on a 2 into 1 pipe, is critical. It has to be the same cross-sectional area as the two pipes entering it, or you lose too much. I think the Exup thing might work, but it is extra complication, so I have not done it. My 850 has excellent power right through the rev range and right through the top. If it lost off the top and got the same midrange or better, it would still be good. A throttle controlled butterfly might defeat the noise meters.