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  • Mandrel Bent Exhaust and Muffler Bypass

    • Year: 2010 Brand: Malibu Model / Trim: VLX Cost: $250-$499 DIY or Professional Install: I installed it / created it Platform Type: Fiberglass Engine Type: Monsoon 350

    I have been looking at the AFE exhaust you can purchase and wondered why they pie cut their bends. I’m assuming that it must be the overall cost that keeps them from buying a mandrel bender. I started estimating the cost of this project, if I purchased pre-bent stainless tubing. In the beginning I did not know how I was going to make the down/outlet tube.  So with tubing cost, flappers, Misc. stainless, hose and hose clamps, I figured materials would cost approx. $250. Part of the reason for the higher cost, was that my boat did not have the plastic standard exhaust flappers, it had the 4” stainless steel turndown tips. As I progressed with the project, I added the muffler delete, upgraded the clamps and changed to a 2 hose connection instead of a 4 hose connection and this made me buy extra tubing. Because of these changes I have $410 in materials in it. 
    My muffler connections are 3 1/2”, so all I’d did here was purchase (2) 45 degree 3 1/2 mandrel bent SS tubes and cut one end off to make it fit.  I just had to rotate my SS exhaust outs on the manifolds a little and then rotated my rubber transom connect hoses a little also. 
    I then made my thru-transom connect pipes. 4”  long by 3 1/2” tube welded to a 6” od x 3” id outer plate. I then welded my  8” long by 3” tube to the outer face, orienting them to point toward each outer exhaust. This outer tube is where I installed the flappers.
    I then welded the (2) 3” 90 degree tubes (with longer leg on each). After this, is when I decided to make a 2 hose connection instead of the 4 hose connection. so I had to purchase (2) 3” 15 degree tubes and weld them to the ends pointing toward the thru-transom connectors. The down/outlet tube was originally going to be a 3” oval tube with no modification. Then my buddy said he could make some molds from wood that we could use in the hydraulic press and form the oval tube to make it round at the weld connection at the top and slightly v-shaped at the outlet end. It worked awesome. Then I studied some of the. AFE photos longer and decided to buy the 3 1/2” oval tube, made more molds and   pressed it, then welded it to my upper tube. I then ground all my TIG welds smooth. 
    I spent a fair amount of time sanding and polishing (more than I wanted). I wasn’t going to install the flappers, but decided too, after deleting the muffler. On all the tubes that have a hose connection with clamp, I welded a small 1/4” round spot of weld that sticks up an 1/8” in 4 equal spots around the tube. This is to allow the hose to “grip” without pulling off. I actually got this idea from looking at my 4” turndown tips that I removed.  I’m amazed how ridged this setup is, I can stand on the the outer tube and it barely moves. I have yet to test it in the water, so I might still have to modify it some. 
    All parts were sourced from Amazon, EBay, Stainless Headers & local hardware store. 




































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    Awesome!  Nice work @Catman   Totally agree on mandrels --- I bought premade when I did mine as well!  Larry at FAE will do them custom, but his challenges are relying on customers for perfect measurements and requires some pretty tight turns, etc.  And I think it's easier for him not to deal with it.

    Man those blended welds and polish turned out great.  You'll be glad you took the time... looks amazing.


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    great work!  did you make that hydraulic press shaper to narrow the down tube just for this project?  if they're easy enough to reproduce just the narrowed downpipe i would gladly pay you for it.  i'm converting my centurion sideswipe exhaust to a single rear outlet (4" downtubes to 3.5" exit) and am not looking forward to having to shop around to my local exhaust shops to have that downpipe done.

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    Fellow Wakegarage guys,  I appreciate the positive comments.


    jtryon,  Yes, I did make them for this project only. My buddy and I actually made 2 sets of wood dies, one set for the 3” oval tube I did not use, and another for the 3 1/2” oval tube I did use. A set consists of: one round and one v-shaped oval (last photo above). I threw both 3” oval dies away already. I gave the 3 1/2” v-shaped oval die to a friend, because he was going to possibly do a similar down pipe for a Moomba. Do you just need the dies? Or a formed down pipe?




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    2 hours ago, jtryon said:

    The 3.5” formed down pipe would be awesome because I’m sure it will be hard to get an exhaust or fab shop around here to make that exact piece.

    What length would you need? I started with a 12” long piece of 3 1/2” oval tube. Would you want the the weld-on end to be 3  1/2” round? And the outlet end the same shape as mine? Can you make the cuts shown in the photo?


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    yes a 3.5" round weld-on end is perfect, because i'll be welding a single 3.5" tube to it rather than duals like you did.  12" long is great and I can trim from the top if needed, and the outlet end the same shape as yours as well, or if it's easier to do a straight angled cut on the outlet that's perfectly fine as well.

    Edited by jtryon
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    Amazing. Curious why you went to the trouble of the muffler delete. Why not just attach your dual custom exhaust tubing? Is it to get more space in the engine compartment or mufflers no longer needed for noise reduction? Curious if your boat got quieter after the install as FAE describes on their site.

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    Sorry, I have been out all summer enjoying my homemade surf tabs and exhaust system.
    Yes, the extra room is great, especially when changing the impeller on the raw water pump. I put 85 hours on it this summer and it is definitely quieter than before these modifications. I debated a lot on wether to put those flappers in my exhaust vs. keep the muffler. I did not want any chance for water to push into the engine.  I finally decided to install them, thus I deleted the muffler. I’m very happy with the noise reduction inside the boat and while surfing. My only downfall of this modification was that I lost 1-2mph on the top end. This is a result of the drag of the exhaust down pipe producing drag, it’s not a back pressure problem. I have modified the down pipe from my first design to reduce the drag.

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    Hey it looks really great! just wanted to ask you some questions. why didn’t you go up and down through the center of the wedge? I’ve been looking at doing it, and i’m wondering if there was a reason you didn’t?


    Edited by Darin Harris
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    I did think about doing it that way, but decided not to because of the swim deck clearance. I did not want to raise my swim deck. It just looked like it would be extremely tight. You would also have to use really tight radius on the tubing. I’m sure someone could make it work.

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    "My muffler connections are 3 1/2”, so all I’d did here was purchase (2) 45 degree 3 1/2 mandrel bent SS tubes and cut one end off to make it fit.  I just had to rotate my SS exhaust outs on the manifolds a little and then rotated my rubber transom connect hoses a little also."


    @CatmanI didn't realize the connections coming off the manifolds are able to rotate. Do you know what the CLR is of the 45degree elbows you used for the muffler delete? 

    This is great as I want to do a muffler delete for my FAE project as well and this makes it a whole lot easier! Great info and great project. Very well done!

    Edited by dcwilson
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