I'll start with the punch line first. During the first test ride with new mufflers welded in, my first impression was WOW...OMG...WOW. If I had know this one single modification would result in so much more power, I'd have done it 20 years ago. The external look of the CR1 sidepipe gave no indication about how restrictive and poor that piece really was.
After cutting the pipe open, here's the source of the rattle:
The baffle plate ruptured and the resulting loose chunk of metal was floating around in the muffler until it lodged under the internal piping. The result was a straight through muffler that wasn't stopping much exhaust noise.
I did go with the Flowmaster Slimline sidepipe mufflers. Here's pics showing the installation process.
Hint: make sure the bandsaw is cutting straight before doing this. Also, measure carefully as there's no good way to hold the resulting pieces for secondary cuts.
Original CR1 muffler compared to the 30" Flowmaster after tacking in pipe size reducers. With reducers the 30" muffler grows to about 33.5" in length. The Flowmaster is also 4.5" diameter versus 4". This didn't turn out to be an issue at all.
After lots' of cutting, welding, and metal finishing, here's the resulting raw sidepipe after the first test drive.
With the old mufflers, the power dropped off considerably at about 5,500 RPM. With the new mufflers, power between 5,000 and 6,500 RPM was pucker worthy.
I shifted at 6,500 even though the RPM was still climbing fast and there was no fall off in power.
After the test drive, I discovered this:
The blue/purple areas are the uncoated reducers that are mild steel. The mufflers are SS and didn't show any discoloration. The existing ceramic coat didn't show any damage either. The ceramic coat is rated at 1,500 degrees, so the pipes didn't get that hot. I don't know what temp leaves mild steel purple though.
My gut tells me the new amount of muffler flow is resulting in a lean condition at WOT. Especially given the discoloration present after the muffler. I only wound it up 3 times on the test drive so it got hot fast.
Question for you guys who know about the signs of a lean condition. Do you think this evidence is showing it? My conservative side says I should get the computer re-tuned. It would also be nice to understand the HP gain purely from muffler replacement.