Adventures in Tuning - Part 1

[sansnombre]

I purchased a Yosh pipe for my FZ, but after riding for a bit, I thought it was too loud, and a number of dyno runs I saw showed that it wasn't much of a performer anyway. So off it went to a new owner.
The Arrow looked great and the dyno runs (and word of mouth from tuners) was very, very good, esp in the midrange that I was looking for, so after some interaction with the Italian company, the beautiful header arrived. I didn't like the steel look and painted it black and it turned out great - see the header thread for pictures.
I installed it without the db killer, and it appeared to be much quieter than the Yosh, but after riding for a while, I realized it was just as loud, just not as harsh - it was fat in the low freq range and this came right through the helmet. So I popped in the db killer and thought that sounded just right, but I'm unhappy thinking that the flow is too throttled by the six 1/2" diameter holes in the killer. So I put some more holes in it, lots more, 15 more, same size, similar setup but just all the way down the db killer. With this, it indeed sounded much louder than before but after 3 months of riding, I'm thinking I need ear plugs or I f'd up and now it's too loud again. But it's hard to believe that just a few more holes changed it that much, so I'm thinking I want to play with this and see what's going on. (Yeah, I'm a little anal about it, I guess.)
So I decided to get some aluminum tape, the kind they use on dryer ducting, and taped up some of the holes to do a sound test. Wow, I wasn't wrong; it made a huge difference in sound. With all the new holes taped up it was so quiet, I couldn't hear the exhaust over the wind noise. That's pretty quiet, so I'm thinking somewhere in between is Goldilocks . . . .
So I break out the sound measuring device I have for setting up the surround and do some measurements. These measurements are not absolute and are only for referencing the other setups as the device is relatively basic and you dial in the range you want to measure. In some of the settings, it's redlined and over the top, in others, it's barely picking anything up. The overall range is about 100Hz to 20kHz, but any given test range is across about 100db. So I set it to the mode that had the needle in the middle of it's range and measured it at four different setups: all extra holes open; stock db killer; with two extra sets of holes (furthest away from the exit); and with three extra sets of holes. The difference was much higher than I anticipated. I normalized the "all open" to be at 0, so all the other numbers are relative to this baseline of 0.
Remember it's only looking at a narrow range of frequency, but within that freq, this is what the differences were:
At idle:    0, -1, -0.5, and -0.25At 3-4k:  0, -2, -1, and -1.5
Also remember a db is in a log scale, so a db change of 1.0 is actually a very big change.
It goes exactly as you would expect, it's quieter as the holes are closed, but the change in loudness is actually quite large just by opening up a new set of holes. And I could tell by additional popping and cracking, I was affecting the jetting as well. Interesting. I wouldn't have thought that relatively small changes would have such a large affect in sound or jetting. I can't explain the odd jump getting quieter at 3-4k when the extra set of holes were opened. It should have been a bit louder, just it was at idle, but some complex things going on in there, so who knows why?
A side note, the pressure at the closed end of the db killer is very high because there's no flow there so the pressure increases to "push back" on the pressure in the pipe. What's interesting is that when I went for a ride with all the new holes closed up, I blew through two of the aluminum patches, both at the very end nearest the exit, where the pressure is highest. So the pressure in there is pretty high, not surprisingly.
I ended up with leaving three of the new sets of holes, closing off the two sets closest to the exit of the pipe. Sound is very good, but not too loud (Goldilocks ), and I'm assuming I'm getting a wee bit more flow as well.
Why post this? I dunno, maybe some of you are interested in this sort of triviata.
This is tuning adventures part 1, as I will have some other posts on jetting (EJK and 2WDW), and I've tried 3 seat thus far. So I'll post that bit of info as well as time allows.
Until then, keep the rubber side down.
Cheers