View Full Version : Replacing mufflers with straight pipes.

06-18-2011, 09:33 PM
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.


06-18-2011, 09:35 PM
I lubed everything up with a lot of WD40, inserted the 3" end into the 3" rubber, and then muscled the sleaved 3 1/2" end into the 3 1/2" rubber:



06-18-2011, 09:35 PM
Clamped everything down:




I went to the boat ramp and checked for leaks. At the lower RPMs, I did not notice much more noise. The installation was clean and simple.

Afterwards, I used the old muffler clamps to secure the exhaust. I was also careful to route and zip tie all electrical wires, hoses, or anything else that could melt well away from the SS pipe. It will be hotter than the mufflers.

06-18-2011, 09:36 PM
Addtionally, I replaced the the shaft log and repacked the gland nut. To do this, I had to remove the prop shaft flange. I simply put a socket between the flange and the transmission, tightened the bolts, and it popped loose very easily:



06-18-2011, 09:37 PM
Here's a picture of all the parts cleaned up and ready for assemble. NOTE: I was only able to use two of the packings.




I'll let you know what the noise level is like once the winds calm down and I get on the lake. I'm really not worried about it.


KISS: Keep It Simple Stupid

06-19-2011, 09:23 PM
definitely post up some video with the sound of that when you are done if you can :)

06-19-2011, 09:38 PM
This is rad.

I can't wait to hear the sound.

Good job sir!

06-22-2011, 12:41 AM
Excellent post and pics.......curious ...why did you replace the whole packing gland and not just the packing material, you would not have to remove the coupler and everything?

oh and I would put some anti rust stuff on your motor mounts , they are pretty rusted.....and maybe check your alignment if you didn't>.......jmo

06-22-2011, 10:40 AM
oh and I would put some anti rust stuff on your motor mounts , they are pretty rusted.....and maybe check your alignment if you didn't>.......jmo

i talked to the owner of a local boat repair shop today known as water junkies about the alignment. he acted like i was stupid for worrying about it. basically said "did you hit anything" and i said no, and then he said "well then why are you worried about it?"

i could not get my engine to move to the right to close the gap so it will stay at .017.

06-23-2011, 09:03 PM
Excellent post and pics.......curious ...why did you replace the whole packing gland and not just the packing material, you would not have to remove the coupler and everything?

I didn't replace the hardware, just cleaned it up.

06-23-2011, 09:06 PM
The winds finally died down yesterday, so we hit the lake with the new exhaust configuration.

I could barely tell a difference inside the boat. The tone was a little deeper, but it was not appreciably louder.

My riders noticed a difference in the sound from behind the boat, and commented that it was louder.

I think overall, the sound is not an issue, imho.


06-24-2011, 10:46 AM
So any chance of hearing it on a vid? I know we won't get the before and after but still always love that sound.

Glad to hear it's not too crazy loud though!

06-24-2011, 11:40 AM
I'd have to record it someplace behind the boat, and I don't think I'm good enough to keep my recorder dry:oops:

06-24-2011, 01:49 PM
have someone on shore record as you drive away

07-04-2011, 10:59 PM
I am in the same situation as you were. My exhaust is leaking and I have to do something.
I was wondering if you tried it out yet and found out how the sound is?
Also, does the pipe get to hot?

07-04-2011, 11:02 PM
Sorry, I just saw your last posts about the sound, that's great, but still wondering if the new exhaust pipe gets to hot?

07-05-2011, 01:02 PM
Sorry, I just saw your last posts about the sound, that's great, but still wondering if the new exhaust pipe gets to hot?

I haven't actually done any testing to see how hot the pipe gets, but I'm sure it's warm. I've been careful to route everything clear of the pipes. I don't think it'd get too hot, as the mufflers look to be made of PVC and or plastic. I think I've even read where some people have used PVC in this area?

Once the motor is off, they get water in the pipes to cool them down, as well.

Good luck.


07-05-2011, 02:31 PM
Thanks for the info.
I'm going to go the same route as you because I just priced new mufflers and they are $325 each and mine are leaking enough that I have to do something.
So I will get the short end flanged out to 3 1/2 inches and the angle to 45 deg. like you said.
I will let you know how it goes.

07-14-2011, 08:41 PM
No video, No sound! Never happend :D

07-14-2011, 10:32 PM
An easier solution might be to just run pvc or the exhaust rubber tubing jmo. you could use a 3 or 4 inch copper 45 to make the bend as well

11-08-2011, 02:54 PM
Our 2006 LS has leaking 'Muffler's..due to a very quick overheating once. No engine damage but mufflers have small water leaks at forward location & a lousy sounding drone at low to mid revs.

Our Chris craft 1955 Woody has a 283 w/ 2-1/2" ID flexible rubber w/ steel coil exhaust hose purchased from Jamestown Distributors in RI. They have all the sizes, multitude of boat stuff, & great prices. The CC is loudbut not a harsh bark as the rubber attenuates the exhaust note a bit.

I believe the Moomba exhaust is also deeper/closer to transom bottom and below the swim pad to perhaps assist in quieting the exhaust. If I cant fix the mufflers w/ epoxy resin repair, Im going the exhaust hose route next spring.

11-09-2011, 12:51 PM
i find it very interesting that there isnt' much difference in sound. Good know for sure!

Nobody ever said if they could notice any difference in power? I'm guessing not, since it would be such a small gain and it is on a boat.

KG's Supra24
11-09-2011, 01:09 PM
From my experience ... When I went with Dusty to buy his boat that had been straight piped. It was noticeably louder at idle and underway.

They had used PVC and we requested it be switched back to stock before we left. It wasn't a small difference in sound in this case.

11-09-2011, 01:20 PM
I'll be the bad guy here. When I showed up to pick up mine, the P/O has straight piped it, but brought the factory muffler with him to send with me. We took the boat out, and at initial start up, it sounded kinda cool. Reminded me of our old Malibu, but a bit louder. The second some throttle was given it sounded like a WFO Camaro running down a drag strip. It was awful. We couldn't hear a damn thing but exhaust noise multiplied by 50.

IMO, there is a such thing as to much noise. The boat sounded like my Dad's baja going across the water. For a Baja, GREAT, for a wakeboat.....no thanks.