The Continuing Quest for the Perfect Sound
The stock Aprilia Tuono sounds pretty choked up, as delivered. This project was an attempt to try and get some more "basso profundo" from the exhaust note. The problem is, every aftermarket pipe I have heard for the Tuono/Mille is loud. Unbelievably loud. Loud enough to set off car alarms. Loud enough to cause mothers to grab their children off the street. Loud enough to attract cops from the next county.
The trick is not to mistake sound quality for quantity, which takes us to this particular random exploit.
OK, it's not so random: when you have a plasma cutter, a good project starts off by looking for things to cut in half. What could be finer than slicing a muffler apart to see what is inside?
Nothing, obviously.
So it was off to eBay, where I bought an extra pair of Aprilia mufflers. If you get into this project, note that there is a vast supply of great condition stock Aprilia mufflers available on eBay, probably because they sound so flatulent. If you pay more than $30, you paid too much. In fact, expect to pay about as much for shipping as for the pipe, mainly because they are stainless (read: heavy), and rather ungainly to box up.
Step one was to "derestrict" two of the stock pipes. The baffle removal process is covered here.
Step two was to use the plasma cutter to slice up one of the derestricted pipes. Finding the proper place to cut involved measuring and a bit of luck. The stock pipe has three chambers in it. If you shove a length of welding filler rod down the outlets, you will find that there are two interior chamber walls inside the pipe that form the three chambers. The lucky part is that the front chamber wall is pretty much right under the mounting strap. This means that if you slice the pipe open at that point, the mounting strap will cover up the weld when it is time to put things back together. For me, that is an important consideration since I could politely be described as a self-taught welder. More accurately, a "crap" welder.
Actually, I would have preferred to use the bandsaw to cut the pipe apart, but it was too big for my cheesy saw to handle.
Once the can was severed with the plasma cutter, the interior transfer pipes were hacksawed to finish separating the can. This step might not be necessary if your transfer pipes are a loose enough fit at the back end (see below for more on that).
Looking inside the front section, it is easy to see the catalytic converter. For whatever reason, notice that the exhaust gasses are not required to flow through the converter. There is a secondary path through the perforated walls surrounding the converter:
A close-up of the converter appears as follows:
The Aprilia mufflers use an extremely common design for sound reduction. There are three chambers: front, middle, and rear. In this case, the gasses are dumped into the front chamber first. From there, a pair of transfer pipes take the sound all the way to the back of the pipe. The gasses get turned around, and dumped into the middle chamber. In the middle chamber, the gasses get turned around again, finally exiting out the back of the pipe through another pair of transfer pipes. This next shot shows the middle chamber with the main transfer pipes in place:
The holes at the bottom are the exit pipes that pass through the rear chamber and go out the back of the muffler. Note the difference in size between the transfer pipes (big) and the exit pipes (small).
It turns out that the main transfer pipes are a rather loose press fit in the chamber wall. Yanking on them with some pliers pulled them right out:
As you can see, the rear chamber is pretty small compared to the middle chamber. The rear chamber also has a perforated back that covers some sound-absorbing steel wool material.
At this point, I got to thinking that it was going to be a pain in the neck to try an experiment, weld the pipe up, be unsatisfied with the result, cut it apart again, and so on. And the answer came to me, and the answer was "duct tape":
So with duct tape and bungee cords securely holding everything in place, I got to try it out.
How did it sound? Judge for yourself. The following .wav files have four sequential recordings on each. I found that it was nearly impossible to discern a difference unless the sounds were played back-to-back, so I stitched pieces of the individual recordings together to form these .wav files. All of the sounds were recorded using a decent microphone and a Boss BR864 digital multi-track recorder. All were recorded at the same levels with the same microphone position, so the sound level differences should be quite accurate. In each recording, the sequence sequence is in the order:
- Totally stock muffler
- Derestricted, but otherwise stock
- Gutted (transfer pipes removed, pipe duct taped back together)
- The "stubby" (see below)
Here are some MP3 files of the results:
I was surprised at how much difference removing the front baffle made. I was not expecting that much, especially since I had removed the baffle months before, and was unable to tell if the change had made it any louder than stock. I guess that being able to do an A-B comparison makes things like that clear though.
Honestly, I have not found the perfect sound yet. I was hoping for something a bit less raspy. More bass, less treble. I kind of like the looks of the "stubby", though. It would look even better if it had the last couple of inches from the other half grafted back on. Maybe some day, I'll do that.
It will be worth it in the end.
Wright Cyclones in full song.