Replacing mufflers with straight pipes.
2001 Moomba Mobius. All measurements, parts, and pictures are based on this boat. Your boat may vary. I had to split this up into several post to get all the picture in. You can see my pics at:
I noticed my muffler was leaking, and after looking at how poorly they are constructed, I decided to simplify the exhaust by replacing the muffler with a straight pipe. There should not be any issues with the performance of the motor. In fact, I might gain a couple of HP as the exhaust is now more free flowing. The only thing to consider is an increase in exhaust noise. For me, I like my carbon footprint to be loud! Plus, with all the music coming out of the boat, motor noise is my last concern.
My exhaust exits the manifold and enters the muffler at 3 1/2" I.D., then exits the muffler at 3" I.D. This 3" line then continues to the exhaust exit at the back of the boat.
Through eBay, I found Ashland Muffler House, Inc.
For about $60: They sold me a 5' piece of 409 stainless 3" O.D. tubing. Cut it in half. Put a 30 degree bend at the end of each 30" pipe, leaving 4" of straight pipe. Here, you can see the pipes, placed next to the mufflers:
HINDSIGHT: If I'd known that I was reducing from 3 1/2" to 3", I would have asked them if they could flange the short end to 3 1/2", saving me the headache of sleaving the small pipe to fit into the large one. I'd also have them bend the tube to 45 degrees.
This is a picture of the pipes inserted into the large rubber exhaust:
In this configuration, the tail of the SS pipe, overlapped the 3" rubber by 7":
Since I'm not going back to the mufflers, I cut off 3 1/2" of this rubber exhaust to use as a sleave for fitting into the larger rubber pipe at the manifold. I used the clamps as a guide to cut. I then inserted the short end of the pipe into the sleave, giving me a perfect 3 1/2" O.D.