1969 Chris Craft Roamer 46 Refit: New Exhaust Risers

The original Cummins exhaust risers that came with my Cummins 6CTAs take a 90° turn after exiting the turbo, but there is no way to safely point the exhaust toward toward my waterlift mufflers. The turbo would end up being on the low side of the system, and that’s bad news for wet marine exhaust–you don’t want water flowing back into the turbo and engine. So two years ago, I checked with several marine exhaust manufacturers about having custom exhaust risers made, with the dry section going as high in the engine room as possible before turning down with the showerhead pointing toward the waterlift mufflers. The estimates that came back were quite high–they averaged $5300 for both sides. So I held off on ordering a set until they were absolutely necessary. Well…we’ve reached that point now.

Oh, and those original, low hour Cummins risers are listed on my  For Sale page.

While I was waiting, I found some brand new DeAngelo hard shell marine exhausts on ebay. They were advertized as being made for Cummins 8.3 engines, which is what I’ve got, and the price was right. So for the last two years I’ve toyed around with the idea of buying those and modifying them to fit my application. Over the holidays, a friendly commenter mentioned the ones on ebay, so I pulled the trigger and bought them. The thing is, while I’d been aware of them for two years and thought about how I’d modify them, I never really investigated the parts themselves. Turns out that was an expensive lesson in why it’s important not to make rash decisions just because somebody double-dog dares you.

The box took a bit of a beating

The box took a bit of a beating

Hard shell insulation coating is cracked

Hard shell insulation coating is cracked and the turbo flange is bent

While it was unfortunate that one of the risers arrived damaged, I wasn’t really concerned about the hard shell being damaged since I would have to cut it off to modify the riser anyway. The flange was a bigger concern. I contacted the ebay seller and let him know about the damage on the one riser.

But the biggest problem of all was that on both risers the showerhead and raw water inlets were too big. The original Cummins risers for these engines have a 6″ diameter outlet, and the raw water inlet is 1-1/2″. These risers had an 8″ outlet and a 2″ water inlet. Without thinking it through very much, I figured I could just use a reducer between the showerhead and the muffler. So while the return was being processed on the damaged riser, I got to work dismantling the good one.

Good looking riser, but it's too big

Good looking riser, but it’s too big

Like removing a cast

Like removing a cast after a broken bone has mended

Hard shell insulation removed

Hard shell insulation removed

Intuitively, I like the fact that these risers quickly increase in diameter after leaving the turbo. It steps from 3-1/2″ at the flange to 4″, then immediately up to 4-1/2″ where it enters the insulated zone and then up to 5″. After the 5″ 45° turn, it goes up to 6″ the rest of the way. It makes a very complicated part, but that big pipe must be good for lowering back pressure. Since my port riser will be longer and have more bends than the starboard, I was thinking that maybe the fat pipe will compensate for the additional back pressure.

Cut-off wheel sliced through a welded joint

Cut-off wheel sliced through a welded joint where the 6″ pipe begins

Inside the pipe looks good

Inside the pipe looks good

The whole time I was dismantling the riser, I was focused on the task and not really thinking about next steps. After I had it cut up, I started looking into the parts it would take to make this riser work. It turns out there are reducers, but they’re somewhat expensive. Then I remembered I’d have to buy two stainless anti-crush rings that go inside the fiberglass reducers. Then I’d still have to buy the materials to modify the riser and pay somebody to weld them up. I’ve had bad luck with fabricators in this area, so there’s that, too. I was looking at no less than $600 more to make each riser which, when added to the purchase price, is getting close to the cost of just having a set of risers custom made.

It was around this time that I really started kicking myself for buying these risers. The ebay ad was misleading, but I should have contacted the seller to verify the dimensions. When I realized they were too big, I should have just sent them both back. But then, with my new-found understanding of how these risers are made, I started thinking about just buying all of the materials and making a set myself…

Next up in our 1969 Chris Craft Roamer 46 Refit: New Riser Materials


8 comments on “1969 Chris Craft Roamer 46 Refit: New Exhaust Risers

  1. victor says:

    I just finished replacing a yanmar mixing elbow that is v clamped to the turbo, I assume the water dump so close to turbo keeps it running cooler, my problem was the raw water was reduced underway and the heat buildup in the rubber 4″ id exhaust downstream actually melted not the hose but the loom that held wire that was draped over the hose !!!!

    • 1969roamer46 says:

      Holy smokes! I don’t think the proximity of the wet section of the exhaust helps cool the turbo, since all of the the marine ones are water-cooled already. I’ve never seen a hose get hot when things are working right!

      • victor says:

        when the turbo housing downstream of the exhaust vanes is continuously building heat it could get cherry red if it was a dry elbow , so the water dump along side keeps it all cool, of course I.m looking at 10-12 hours of full power to get thru open ocean and safely to the otherside before the weather changes

        • 1969roamer46 says:

          Hi Victor.
          A diesel exhaust running cherry red dry exhaust (~1600°F) would not happen for long…that’s a grenade with the pin pulled and the handle released. The spec for Cummins C with wet (water cooled) turbo is 840°F at the turbo outlet at 80% loaded cruise. Real world reports indicate 750-800°F is normal. Running WOT for extended time isn’t recommended by Cummins (or any other engine manufacturer I’ve seen), but even then you’re not going to see EGTs that high for long. You might want to check how hot your turbo can withstand. Also, not that I’d ever run at WOT for 12hrs, but the insulation I’m using is rated for 2,000°F continuous. You could put your hand on the hard shell and it’d be warm. And with the hi-lift dry-to-wet design, there is absolutely no way the water can flow back into the turbo and engine, which happens all too often with the wet elbow design.
          Check out this article for more info on this type of design: http://www.sbmar.com/articles/everything-you-need-to-know-about-marine-exhaust-systems/

          • victor says:

            Thanks for the westcoast link, I guess everyone is trying to put a land based engine into a boat and making it work, here in South FLorida there are plenty of horror stories of engine fires blamed on the turbo, I,m spoiled with my engines, they were designed from a blank page to be marine installed, failsafe, I believe FL started putting turbo,s on the marinized diesels first and we went thru the trial and errors, hence the horror stories

            • 1969roamer46 says:

              You’re welcome, Victor. But Cummins marine engines aren’t land based engines. They’re the same block as Cummins vehicle and stationary power generation engines, but everything from the crankshaft to the exhaust is marine specific. The exhaust manifolds and turbos are watercooled, for example, unlike the vehicle and power generation versions. The original Cummins risers are wet within 2″ of the turbo flange. When they fail, gravity causes the water in them to flow back into the turbo and engine. That design has been around (and destroying marine engines) since the 1960s, at least. I’ve got a 1968 Chris Craft Commander 42 with 427 Ford engines with risers that are a different shape than the original Cummins risers, but the concept is the same–when they fail, the water in them flows into the engine and destroys it. The inherently safe, insulated dry/wet hybrid design has been used by Hatteras, Chris Craft, and many other manufacturers since the mid-1970s or so. But the hardshell, modern version is far better. The modern insulation materials are capable of withstanding 2,000°F sustained, for example.

  2. Scott says:

    Damn!! Yet another obstacle!

    • 1969roamer46 says:

      Yeah, it might have been best if I just ponied up for the custom ones. On the flip-side, now that I’ve done the reverse engineering, I know how to make them myself. this will hopefully work out to be one of those things where an expensive tool purchase pays for itself. Stay tuned… 😉

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