Saturday, July 2, 2011

Exhaust Modifications

     In the March 23, 2011 post, I talked about refurbishing the water lift mufflers. At the time, I had not carefully examined the location the factory chose for them. After a bit of research, and some careful measuring, I realized that the stock location was only an inch or two from the front of the engines. Note that when I say "front" of the engine, I mean the rear facing end, since these are V-drives. So, if any maintenance has to be performed, such as a belt change, water pump impeller service, water circulator pump service, etc., the mufflers are in the way.

     Now, there is plenty of room behind the mufflers, so it would seem to be an easy task to simply move them back 6 or 8 inches. But, an interference is then created between the 4-inch exhaust inlets, on the outboard side, and the larger underwater exhaust discharge piping. To solve this problem, I cut the existing exhaust inlets off the mufflers, and reshaped them to allow for the repositioning.

     To make the new bends I needed, I ordered a few lengths of 4 inch diameter, marine, fiberglass exhaust tubing. Then, using a fixture for my band saw, I cut a supply of angled wedges, as seen below:
     I cut these at either 10 or 15 degree angles, and then just stacked them until the desired bend was achieved. As I fitted them in the boat, I used a bit of Crazy Glue to hold them together. If an angle needed adjusting, I would simply break the wedge off, reposition it, and re-glue. Then, once I had the desired bend shape mocked up, I fiberglassed the mufflers back together.

    A few words of caution on a job like this. First, you cannot use polyester resin from the local hardware store!! Marine exhaust systems should be fabricated from Class 1, Isophthalic, fire retardant, high temperature resins. These are typically tooling resins, that exhibit good strength and thermal stability. I used Reichold DION ISO 6631T resin, available readily at These guys provide great service, fast shipping, and reasonable prices for small quantities. For cloth, I made it easy with a 100' roll of 1 1/2" wide, 8 oz. fabric.

     Once the angled sections were glued in place, I coated the inside and outside with a layer of resin, then wrapped the fiberglass tape around the first bend. It was a simple matter to then apply a bit of resin, wrap the tape tightly, apply more resin, wrap the tape, etc., etc. The finished layup is strong, tight, continuous, completely waterproof and leak free, and fire resistant. The laminate thickness now exceeds 1/4 inch where the new bends are, lending greater strength than the original tubing. The bends are nice and free flowing, so there will be no horsepower penalty or loss of efficiency. In fact, the new bends are actually superior to the factory setup, with the fiberglass elbows leading into the mufflers. I won't be using any of them for the exhaust on this boat. And best of all, I now have almost 12 inches in front of each engine!!! I can change belts and water pumps without removing the mufflers, and there is room to stand right in front of the engines. A huge improvement.....

     I'll post pics of the finished exhaust, in the boat, soon...

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