View Full Version : Inboard motor exhaust questions?
06-07-2004, 10:56 PM
I recently purchased a new Crusader 5.7 engine/velvet drive set up to install in my 1962 Higgins. After getting the motor , Im concerned about a couple of things, in regard to the exhaust system..
First the boat currently has 2 1/2" dual copper exhaust . Crusader recommends 3 1/2 to 4" duals.(it has water-cooled exhaust). There is no more room to cut larger holes thru the support stringers without jeapordizing the strength of them.
Second, they want a continuous 4"of fall from the manifolds to exhaust ports in transom, with no low spots. My exhuast goes beneath the floor boards and under the rear gas tank, before rising aprox 4-6" to exit just above the water line.
Im sure someone has run across these situations before , and am wondering how they were addressed?
06-08-2004, 02:46 AM
Sounds like it's time to compromise. ;) While the four inch exhaust would be best, I wouldn't feel too bad about going with a 3", and wouldn't consider 3.5" a compromise at all. You'll lose some performance (hp) on the top end, but the exhaust run in a Higgins is pretty short, which helps, and you won't be running wide open for long periods of time, and that's where the loss will occur.
I've run 3" on Chrysler 440's with a 12' exhaust run, and didn't feel like I was giving up that much performance, maybe 10% in terms of top horsepower. Your fuel economy might drop a bit too, but do you really care? It won't hurt the motor.
I WOULD feel badly about sticking with the 2.5" you've got- that's a pretty significant reduction from the optimum 4"- it's only about 40% of the capacity. But remember, much of the time you'll be running along at about 45-50% of max hp, so the smaller hose will be more than adequate. As for the stringers, I like to weld up mild steel reinforcements that can be bolted on to beef things up where larger holes need to go!
As for the fall/rise, the "plug" of water that accumulates in the low spot won't have any effect once the engine is "off idle", but may make it difficult to start, and can lead to fouled plugs if you idle around a lot. Try a hotter plug if you need to.
Re-powers always seem to come with a set of challenges. But remember, there's often a lot of distance between optimum and satisfactory, and don't beat yourself up over a compromise here and there. Good luck! smile.gif
[ 06-08-2004, 02:52 AM: Message edited by: Conrad ]
Something that you can do to also help the situation is to add a cooling water bypass line. Typically all the cooling water from the engine is dumped into the exhaust (for two reasons, 1) to cool the exhaust, & 2) to quite it.) It doesn't require all the cooling water to accomplish those two things, the rest of the water just adds back pressure to the exhaust. You can add a "T" with an adjustment valve in it. As the cooling water comes out of the engine, run it through hte "T", with one side then going to the exhaust and the other side running through a valve & back to the transom (exiting through a small trough hull fitting). when you take her out for the first time start with the valve closed (so all the water goes through the exhaust), then start opening the valve as much as you can until either the exhaust noise starts to increase or the exhaust pipes start to warm up. I bet you'll be suprised how much water you are able to divert out the transom (which reduces the back pressure - & will allow you to use the smaller pipes).
We've done that on two boats in our family & I bet 50-60% of the water is divered.
06-08-2004, 09:07 AM
pretty much what conrad said ...
Really, you could get away with changing NOTHING, if you are just running around doing normal things ...
With a low spot, it may go dry for a momemt at idle, the glob out a glob of water, go dry, glob out, etc. I kinda like that sound anyway!
Plugs (spark, that is) are cheap, and easier to replace than rebuilding structure ...
Plus, you'll notice no difference in performance, as it will all (the 350, that is) be new to you!
06-08-2004, 11:56 PM
At first I was kinda "sick" when I saw the factory recommendations. This motor is rated at 300 hp ... a far cry from the original 135hp 'gray. I wouldnt probably have ordered the 300 horse , but found it on ebay...still in the crate, complete with new velvet drive :D !
One thing , I dont think I'll ever be able to take advantage of its peak output because of prop limitations.
Im going to look into a couple of your ideas when I get the boat right-side-up again. (just stripped paint of bottom). If I can get larger dia exhaust, what is best material to use? Is copper still available?$$$?
I will look into the bypass water line also!!
Being new to this "boat stuff" , I sometimes feel overwhelmed, But,Im amazed at the experience and knowledge here!!! THANKS AGAIN!! Steve :cool:
06-09-2004, 12:48 AM
Copper was the "cat's meow" when your boat was built. But heavy rubber hose has pretty much replaced it. Many newer exhausts also feature fiberglass tubing, often made with high-temp tolerant epoxy resins.
If it were mine, I'd go with the rubber exhaust line- it's easy to use and form, makes nice smooth bends, is very quiet, and will last a decade in your application. The 3" runs about $12/foot around here, and it pays to shop around. ;)
If you know your boat's weight and the performance/power curve of the engine you can get a lot of info on prop sizes and expected performance by joining www.boatdiesel.com (http://www.boatdiesel.com) for $25/yr., and using their on-line prop calculator. The similar boats I've had have run 12/13" dia. by 14/16" pitch with a good cup, and do quite well.
Unfortunately, with the heavier center engine many old style inboards, and even the newer ski boats don't go as fast as you might expect. All the weight forward causes too much of the hull to remain immersed at speed, and the friction drag of the water is significant. For example, a 19' Ski Nautique will do about 42mph with a 220 h.p., and only 46mph with 300 h.p. The acceleration sure improves though! :D
[ 06-09-2004, 12:59 AM: Message edited by: Conrad ]
06-10-2004, 11:16 AM
Rubber works just fine ...
Now, I say this somewhat tongue in cheek ... but what about closed-cooling ... ??
I've often said my next boat will have it ... so on those nice January days, I can go for a ride!!!
06-10-2004, 08:04 PM
THe recomendation of 4" Dia is fine but way over rated for YOUR use. Many engines of the size you are using are run just fine on the 2 1/2" Dia DUAL pipes.. IF and I mean IF IT's EASY to change size then do so, but look at the Dia of the pipe that comes out of the riser, that is most definatly a clue. I would stick with copper as it will last forever. As others have pointed out the only thing you are giving up is full HP at MAX RPM which I am going to guess you are not going use and will not miss.
One thing I would try to change is that rise to the transom. If at all possible eliminate as much of that as possible.
[ 06-10-2004, 08:54 PM: Message edited by: Gary E ]
06-11-2004, 12:20 AM
Ive talked with another guy from Crusader and he suggested ("off the record")using 4" from the riser down to the existing 2 1/2" pipe under the floorboards.
I should end up with a vertical rise of at least 16" in that 4" pc, and any water in the low spot of the existing 2 1/2" exhaust shouldnt be able to "siphon" up to the manifold because of the extra volume the 4" pipe will hold.
As Gary E pointed out , I dont think I will be able to take advantage of the max rpm/hp anyway.
I also like the idea of a bypass line to divert part of the water from the exhaust.
brad, Im about 5 hrs north of you and most of the water in the area has big chunks of ice in it in Jan :D
THANKS TO EVERYONE AGAIN FOR ALL YOUR HELP!!!!!!!
06-12-2004, 08:27 AM
keep in mind cc, owens, pace maker, island seas
all these boat builders used 3inch single exhuast
in there twin engine set up boats with some exhuast runs as long as fifteen ft.
your low spot in the exhuast will not hurt anything install and run it and enjoy the new life your installing in her.
06-12-2004, 03:22 PM
" My exhuast goes beneath the floor boards and under the rear gas tank, before rising aprox 4-6" to exit just above the water line."
Let me explain why this is a bad idea and should be changed if at all possible.
I dont know where you do your boating but lets for an example assume it's on a calm lake or river and there are other boats in the area to create waves. As long as you are running along the water is pushed out by the running engine, so far your ok. Now lets say you stop and have the engine off or are just idling while picking up a waterskier, and since your not the only boat in the area pulling people on ski's, a wave comes at you and since your exhaust pipe allready has a load of water in it, it does not take much to push a litle up into the manifold and into the engine. If that happens, you gona wish you had corrected the path of the pipes. The same thing can happen if you just stop and drift for a while as your eating lunch.
Should you chose to leave it this way, the use of a good set of flapers that fall down and block reverse flow might help save you... and I wish I could say MIGHT is a way to say that they also might NOT help.
But az they say... tiz your money..
[ 06-12-2004, 03:36 PM: Message edited by: Gary E ]
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