So Ryan was looking to get a little more tone out of his daily driven WRX wagon while avoiding the drone of an aftermarket muffler. While there are some solid aftermarket options, they are all $$$. Being broke and bored, we decided to cut open a stock STi muffler I had laying around to see if there is anything we could do to alter the tone. Here is our process including our findings and conclusions. Tools needed: 1) cutting tools to cut open the muffler and it’s contents (drill helps) 2) pry bar (hammer helps) 3) vice grips/pliers 4) metal sheers (90 degree angled ones help) 5) protective gear (gloves, eye protection, respirator for fiberglass) 6) A way to button it back up Step one: cut an access hole Here is the stock axleback: Cut along the lines, and drill out the rivets outlined in red here. You can follow the general outline I’ve posted, but I’d suggest making longer cuts for better access. I also left an inch or so of room in case of error… which turned out to be a very good idea lol: The inside of the STi muffler looks like this: Exhaust flows from the inlet side (chamber 1) to the outlet side (chamber 3), back to the center section (chamber 2) through a bunch of small baffles in the chamber wall, then into chamber 1 via 3 trumpets and a number of small baffles in the chamber and finally out the 2.5” exhaust outlet that goes from chamber 1 to the exhaust tip. Here’s a quick diagram: [Step 2: cut away the center pipe] We decided to cut the tips off of the 3 pipes leading to the inlet-side chamber (what I called trumpets from earlier) so that we could access more of the center pipe. The idea is to allow some of the exhaust gasses move straight from the inlet to the outlet. Given the high and low pressure areas that still exist in the gutted version, we thought some of the gasses would still bounce around from chamber to chamber, and ultimately (hopefully) produce a slightly lower tone with slightly higher noise output and slightly better flow. The center pipe is actually 2 pipes with fiberglass packed in between. The inner pipe is baffled. This can be seen here: We decided to cut the entire center pipe out, rather than trying to cut the outer center pipe away from the inner one. It’s strongly advised to wear a respirator, as inhaling fiberglass is baaaaad juju. Step 3: Clean up and weld-up the muffler Take a grinding wheel and smooth out the jagged edges of the trumpets and center pipe. We also took some JBWeld around the leading edges of the center pipe that we cut away, as we didn’t want the inner and outer pipes vibrating against on another. Re-weld the axleback, or find a way to stitch it back together. In our case, it was more like find someone to make a mess out of the muffler, then have Boomer cut out the original welds and weld in a new plate. Thanks Dan! Very nice work, sir! Re-welding proved to be the most difficult part of the process. Partially due to the thin steel, and partially due to the debacle described above. I'm exploring other options (not involving welding) for the future. If you have any suggestions, lmk! Results The car sounds very slightly louder at all rpms with a distinctive lower tone. I would put the tone and volume somewhere in between a stock sti catback and an spt catback (leaning slightly closer to the sti in terms of output, and slightly closer to the spt in terms of tone). So we were able to accomplish at least 2/3 of our goals. Slightly louder output and slightly lower tone. I think it's safe to assume that the flow has freed up as well, but we don't have any data to back this up. The data that we do have is with a sound meter and in cabin as well as out of car impressions from 3 different people. Sound metering: These numbers were collected on the same car with no other changes in between testing except for the axleback. The test car is Ryan’s blob-eye WRX wagon running a stromung shorty (divorced wastegate) downpipe, stock last cat, and STi resonator pipe. The test equipment is a hobbyist digital sound meter (+- 2db at 114db. In my use, I've found it very precise and have not found anything to suggest it's not accurate as well). All data was collected from a distance of 6ft with the meter set to a-weighting and slow response. Sound metering on Ryan's wagon STi axleback 76db at cold start 67db at warm idle 74db at 3500rpm JDM STi Genome 70db at warm idle 78db at 3500rpm Gutted STi muffler: 72db at warm idle 77db at 3500rpm Sound metering on other cars Cobb catback on an 07 STI (Maddad header/uppipe, stromung shorty, MD test pipe, Cobb catback with 2.5" flat flange on the DP side) 92db at cold start 81db at warm idle 89db at 3500rpm Eli’s exhaust 72db at warm idle 94db at 3500rpm Stay tuned for driving impressions from Ryan and Eli.