Measured Supertrapps for loudness tonight... (holy crap!)

I've been thinking that the trapps are a bit... excessive. 8 discs per muffler, and while there's packing material in there, I'm pretty sure it's been in there for 20+ years and over 15K miles. Anyway, this isn't about the packing, it's about the discs. I pulled the bike out into the driveway, and assaulted my neighbor's sensitivities and tested the bike per SAE-J2825. Which boils down to using an SPL meter at a 20" distance from the exhaust tip, 45 degrees away from the back of the bike.

I measured at idle, 2000 rpm (just to see), and at 4500 rpm (which I know is a resonant loud spot and happens to be where the engine runs at when given full choke when it's warmed up, as a poor-man's really-fast idle).

With 8 discs in:
84 dBA idle
96 dBA at 2000 rpm
102 dBA at 4500 rpm

102dBA... yep, it's $@#$%@ loud all right. So I pulled out 4 of the baffles in each muffler and re-tested:

With 4 discs in:
81 dBA idle
91 dBA at 2000 rpm
98 dBa at 4500 rpm

4-5 dB quieter... that's pretty significant. But then I halved the size of the exit, so I expected about a halving of volume. More datapoints would be cool, but it was getting late. I'll ride it to work on Tuesday, and see how that goes.

One thing I noticed at idle with the fewer number of discs in is that it sounds mellower, less raspy. I've been listening to clips on her and VFRD of different exhausts (all of which as glasspacks of some kind), and all sound better than my poor 2nd gen does. Wheezy/raspy, not deep and rumbling.

 

Rode to work today with only 4 baffles installed. Much quieter than before, but it bogs down, misses, and backfires at WOT in the mid rpms before the cams open up at 6-7Krpm. I'll put two of the diffusers back in each muffler before riding home (brought them all and an allen wrench along, just in case this happened).

The difference is odd. When it was still fairly cool (engine "warm" needle about 1/3 from C to H, but air still cool at <60*F or so), at WOT it was much happier than it had been, with a nice round, full exhaust note. As it continued to warm up (middle to 2/3 from C to H), it was less happy. Which sounds like a lean condition. But the bogging seems like it went excessively lean, or excessively rich... anyway, I'm going to end up putting all 4 diffusers back in. And I'll pull the carbs to make sure they're clean, and put in new boots and a jet kit. The carbs haven't been off in 20 years, so they're probably pretty gummy.

I'm not actually trying to accomplish anything with this, other than experiment with the loudness and performance changes from using different numbers of discs. Unfortunately, I don't have an AFR meter or a dyno, so I can't really be *sure* what's happening on those fronts, but loudness (at no load), I can easily measure with the SAE method.

 

I am pretty sure you need to run 8-9 discs minimum. My gen 2 is loud as well.......retarded kinda loud.

 

Just for a little sanity check....



Your numbers (with 8 discs) are pretty much right in the ballpark.

 

Are those the WSB numbers? 14000rpm redline sounds high for a stocker.

 

Those are from the latest 2012 street bike comparison test. Those are streetbikes for sure. Online article is here:
2012 Superbike Smackdown IX Street - Motorcycle USA

 

I'm curious what the test method was (they didn't seem to say anywhere). Hopefully it was via the SAE method. If so, I don't feel nearly so bad about how loud my bike is.

 

They use the 'new' standardized SAE test method.

 

Good to know. Then their numbers and mine should be apples to apples.

I put all the discs back in for riding home. A much happier bike.

 

Just a note for my fellow viffers, dB (dB = decibels - a measure of sound pressure levels) is a logarithmic scale, so there is actually a huge difference in intensity between 80 and 90, and an even bigger difference in intensity between 90 and 100. :ear:

 

10dB louder roughly sounds twice as loud. At least that was Bell's original plan. It's 10x the power of the sound waves, but since hearing isn't remotely linear, loudness is always an approximation (and also frequency dependent)