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Exhaust problems and/or questions.

T

the sMac

Member
I have a 1984 25 foot Convinser (that's the way it's spelled on the name plate.. funny) that had an original 454 in it.

I just dropped in a 454MPI, full dress, all stock. Here's one of my many problems that occured and I don't know the right answers.

1- the original thru hull ports, which are located AT the waterline, are 3 inch. The engine risers have a 4 inch diameter. Is there a problem going from 4 inch to 3 inch? I think I will suffer some HP loss, but will this harm the engine?

2- Here's the tricky part of this issue. The thru hull tips are AT the waterline... AT THE WATERLINE. Very odd. Plus, there were formed areas up higher on the transom to put the exhaust straight out the back. I don't understand why the factory wouldn't do it that way. Anyway, I need to have the exhaust go down about 1 foot and back about 2.5 feet. I just want to get this boat running this season so I can do a sea trial before the ice forms (Ohio) Is there a quick, cheap way of getting the motors started and is there a way to fix this quickly?

Thanks in advance.
 
I have a 1984 25 foot Convinser (that's the way it's spelled on the name plate.. funny) that had an original 454 in it.

I just dropped in a 454MPI, full dress, all stock. Here's one of my many problems that occured and I don't know the right answers.

1- the original thru hull ports, which are located AT the waterline, are 3 inch. The engine risers have a 4 inch diameter. Is there a problem going from 4 inch to 3 inch? I think I will suffer some HP loss, but will this harm the engine?

2- Here's the tricky part of this issue. The thru hull tips are AT the waterline... AT THE WATERLINE. Very odd. Plus, there were formed areas up higher on the transom to put the exhaust straight out the back. I don't understand why the factory wouldn't do it that way. Anyway, I need to have the exhaust go down about 1 foot and back about 2.5 feet. I just want to get this boat running this season so I can do a sea trial before the ice forms (Ohio) Is there a quick, cheap way of getting the motors started and is there a way to fix this quickly?

Thanks in advance.
 
1. The motor is an MPI, which means it has a pretty powerful computer that can make up for any exhaust irregularities (to a point). 3" is more than enough for a big block with such a short exhaust run safely. As far as which is better, 3" or 4", that all depends on what the engine was designed for. That I do not know. Take a look at new boats with your engine, do they have 3" or 4" outlets.........

2. You want your exhaust to exit below the risers, not straight back. The risers must always be the highest point in your exhaust stream. You don't want to risk any water backing into the cylinders!
 
Thanks. I understand the risers need to be higher then the exhaust. But, why would Checkmate mold the boat with 2 exhaust ports built into the transom for the exhaust and not use it. They make a mistake and not realize it until the engines were dropped in? What about all the other boats with a straight exhaust? It just seems odd.

Besides, this isn't just a little low... it's about 1 foot low.

Back to my problem though. Is there a quick and easy way to get this exhaust hooked up in a cheap way for sea trial?????

Thanks again,
 
I don't know why your holes were cut so low. Maybe this will help. Imco make a 12" smooth "S" bend pipe that you can use to drop down to the tips. It is the last itme on the bottom of the page.

http://www.imcomarine.com/pages/exhuast.html

To do the job right, you need to open those 3" holes up to 4". This is pretty simple to do. All you have to do is get a 3" hole saw and put it inside a 4" hole saw. The 3" will guide the 4" through the transom. You will have to stop often to clean out the wood that gets trapped up in between the 2 hole saws.
 
The low holes could be for an s pipe. They are used for reducing noise at idle. You will need 4" holes to take advantage of your new engine. Otherwise you will lose a good bit of HP. Start sawing and don't forget to put resin on the new cutouts.
 
Thanks for the link viny p.

That's not the pretiest or cheapest solution, but it looks like it will work.

Does that "S" restrict the flow and reduce power?

I talked to Corsa Exhaust, they can make me a custom Stainless system, including 4inch tips for about $500. That may be my better option.


But fro now, I just want to get the boat running and take it out for a sea trial. Is there a simple AND cheap way to do this? PVC pipe? heheehee
 
Anything other than a strait 4" pipes out the back will reduce HP, but in your case it will be negligible. Now if you were running a big blower motor, I would not go with S pipes. You're not going to find anything much cheaper than $500 for the Corsa setup. You could buy two stainless elbows and try it, but that money would be lost later. Do it righ the first time, this sport is way too expensive to play around.
 
Are the thru hulls at the waterline metal?

I knew a guy who hated the thru hull noise and went to through prop. He welded a plate over the pipes and left them there. Now his was a straight thru hull set up. That might work for you but you better find a good welder - no leaks please!!!! In the long run fix it properly, fill and gel the holes and install the new thru hulls where you want.
 
I love the sound of transom exhaust when it's right at the water line. Even better, below the water line at rest, and above the water line when on plane............
 
Yes the thru hulls are metal, stainless I think. They are pretty heavy.

I love the sound of them there also. When the boat is sitting at idle, they are about 1/2 half in the water. Cool sound. But on plane, they are out of the water like a normal thru hull. Being at the waterline muffles the sound also.

I think I may have Corsa make me those custom Stainless exhaust, but he wants to add a muffler to the system. I don't want a muffler. He says it's the law. This stock 454/MPI isn't that loud.

Thanks again.
 
Yep, every state now has a noise law in effect. If you get checked, they'll make you pull the boat and write you a fix-it ticket. Even the switchable exhaust is illegal without mufflers. If you run a semi-private lake, and the lake dwellers are cool, you'll get away with open exhaust. IF you run a public 'much used' lake, you'll eventually get nailed..........
 
JW,

Isn't the law for LOUD exhaust... not open exhaust?

I thought they gauged it by DB levels. My old stock 454 wasn't bad at all (in my opinion) But if the law says you need a muffler, then I better get one. I'm on Lake Erie and I do a lot of "wrong time" boating. Meaning, late at night, leaving the island bars. Once you pull out of the marina, the police boats are waiting.

So, the law makes everyone buy a muffler for their boats, no matter the age?
 
From what I've read, it's all I/O boats. And they take a decible reading from a pretty close distance from the transom, which basically makes all boats without mufflers fail (unless the exhaust is through the prop). I saw a link to the states decible laws a few months ago, too bad I can't remember where I saw it.
 
I was told to try and use a 22.5* pvc "STREET" elbow for the exhaust bend... does this make sense??? A guy from the boat store says "Your wet exhaust temp. is under 200*F. the pvc will handle that no problem."

I made a joke about using PVC here, and this guy is really suggesting it... Funny, but it sounds like the perfect temp. solution.
 
DO NOT TRY IT!
Very dangerous

It will burn a hole right through the elbow and possibly cause a fire. Even a piece of 4" hose bent a little to far can burn right through.

Be safe and get metal elbows as a minimum.
 
Nice job to all the guys on the answers... I thought I'd throw in my 2-cents on the issue of the factory molded in thru hull locations being higher than the risers.

In several applications, manufactures use taller risers and mount the
thru-hulls higher to keep them out of the sea-water when these boats sit so low in the a$$ end... You don't want a wave rushing up the pipe and over the riser hump to flood the motor!!!

Your factory molded in locations were probably designed to be used with RISER EXTENSIONS... they install between the manifold and riser elbow... usually available in 4" and 8" heights. If you have the room under the hatch, taller risers are a safer bet on ANY application...

And as someone else stated...the riser elbow MUST BE the TALLEST location in the system!!!

P.S. Don't even think of PVC..anything exhaust must be high-temp including the special exhaust hose! There are some manufactures of high-temp fiberglass exhaust components out there, but more expensive than metal choices... if your looking for cheap to try-out... hit your local auto or truck parts store... they can get you 3" or 4" aluminized mandrel-bent steel elbows fairly cheap...just hook them up with the correctly rated exhaust hose (keep in mind that steel will rust eventually)... you could even weld two together for a cheap "s" pipe test. If you like it, order the correct marine parts for the system to make it right!
 
Yes, thank you everyone for helping me through some of these last-minute finishing touches on my repower.

As far as my exhaust goes... I just got off the phone with Jim Downing at Gill Marine. He's going to fabricate an exhaust pipe from my riser to the exhaust port as a one piece stainless pipe.

Both he and Leon Derebery agree that I do not need to change my exhaust port to a 4 inch. They said the restriction and loss of HP is not worth the hassle, maybe a couple of HP loss. Not a big deal to me since this boat isn't really a speed demon. I rarely get to blast across the lake at anything over 45-50mph anyway since it's too rough.

They both made some great points; the actual exhaust size on the riser is 2 5/8 inches; boats have the same engine and run the exhaust thru the prop (2 1/4"),etc.

So, I'll be keeping the 3" exahust tips for now.

Thanks again for all the replies to both my questions here... I hope to take the boat out next week for it's "return" to the lake sea trial.
 
sMac... If your spending the money on a set of stainlees pipes...why not fly first class amd use 4" stuff...YES... 3" duals is better than ANY thru-prop set-up...but NO-ONE uses 3" thru-hulls anymore for a reason!!! Also note that your stock exhaust manifolds & risers use 4" hose that would go directly onto the 4" stainless "S" pipe... The $" to 3" rubber exhaust hose reducters are spendy compared to straight hose! You could always change to 4" thru-hulls later.

P.S. Check your pricing...
Those nice cast aluminum anodized 4" S-pipes from Imco are about $140 each... $280 pair...
Imoc 4" thru-hulls start at about $65 each (cast brass/chromed) up to $100 for straight stainless...a bit more if you want baffles or mufflers...
 
sMac... If your spending the money on a set of stainlees pipes...why not fly first class amd use 4" stuff...YES... 3" duals is better than ANY thru-prop set-up...but NO-ONE uses 3" thru-hulls anymore for a reason!!! Also note that your stock exhaust manifolds & risers use 4" hose that would go directly onto the 4" stainless "S" pipe... The 4" to 3" rubber exhaust hose reducters are spendy compared to straight hose! You could always change to 4" thru-hulls later.

P.S. Check your pricing...
Those nice cast aluminum anodized 4" S-pipes from Imco are about $140 each... $280 pair...
Imoc 4" thru-hulls start at about $65 each (cast brass/chromed) up to $100 for straight stainless...a bit more if you want baffles or mufflers...
 
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