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Discussion Starter #1
I have a 81 Spectra 20 With a 260 Mercruiser. I am going to convert the exhaust, from the log style to center rise and thru hull exhaust. I will be getting rid of the Y pipe and installing a block off plate.
Do I have to buy a different Tstat housing, or can I use my existing housing?
Is there any thing else that won't work with the conversion, like accessory brackets and such? thanks........Hal
 

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I've been checking into this also. I have a 78 centurion 21ft mercruiser 260. From what I've found so far is the complete conversion kit is the best route. New tstat housing new hoses bolts gaskets. I believe it comes with new brackets for alt. I looked it up on eBay. Hopefully somebody answers with more help
 

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exhaust questions

Are you using a performance style manifold with a single water inlet per side or a stock cast iron manifold with 2 water inlets per side?
 

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Discussion Starter #4
I will be using a stock style , like the GLM or an OEM manifold.
 

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You should be able to re use your existing t-stat housing. You will have to relocate the fuel filter from the front of the starboard side exh. manifold or make a bracket that allows it to bolt to the new manifolds attaching bolts. On the port side you will have to relocate the alternator as it attaches to the front of the log style exhaust manifold. I found that Moore Performance out of California (they advertise on EBAY) has an aluminum alt. bracket that attaches to the cyl. head and keeps the alternator in the stock location, it sells for $100.00.
This should be all the issues you will encounter in doing this swap. I just did this swap to my boat, the swap is worth the effort it takes to do it.
 

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My 260's had 3 water inlets per side. 2 on the manifold and 1 on the riser. I converted to center style.

This is the only thing on google that I could find on the difference between warm and cold cooling. If anyone knows more about the pro's and con's, please weigh in.
- Warm Vs Cold Manifolds - Performance Product Technologies

It sounds like the warm cooling is better, so I choose to do the Barr marine manifolds and keep my thermostat housing. Yet, all the newer stuff is cold cooling. Mine are working great, but if I had to do it again I would get the kit that includes the manifolds, riser, brackets, and thermostat housing.
 

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Discussion Starter #7
Ok so I got a set of EMI Thunder exhaust and I have isues and questions.
I got the engine back in the boat, and where it sits the risers come inin a position that will not work. The riser and powersteering unit are interfearing with each other.
What I need to do is have the tips come thru below the power steering unit and in order to do that i will have to use an S pipe and hose, and inorder to do that the risers need to be shorter. My question, is there any reason that I could not cut the risers shorter? I really hate to cut them, but its what I have and I dont want to spend the money to just buy shorter ones.
Also I had to switch T-stat housings and the original water inlet hose came in on the starboard side, but the new housing has the inlet on the port side. I need to rout the hose from the tramsomplate on the starboard to the port side. Can I route the hose and cross under the oilpan to the port and use the molded hose for the port side conection?
Hereare a couple pics of the issues.







047.jpg
 

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Eddie sells an "S" pipe elbow. Take your standard elbow back and swap them out.
 

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Discussion Starter #9 (Edited)
Eddie sells an "S" pipe elbow. Take your standard elbow back and swap them out.
Well the problem i have there is that i baught the set up used. They only have a couple hours run time on them, but i dont know id they will exchange used.
 

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Well the peoblem i have there is that i baught the set up used. They only have a couple hours run time on them, but i dont know id they will exchange used.
Oh, that does change everything.
 

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I did a kit for this when I was working at Dana Marine. We made a couple of brackets for the Solenoid on the back of the motor, the fuel sperator, and the alternator. It was a nice kit. We used Indmar exhaust in the kit. Here's a link to the kit, Value Performance Exhaust Kits - Dana Marine Products

Here's a link to the instruction sheet,
http://www.danamarineproducts.com/installation/260_Instructions_web.pdf

I did this once on one of my own boats, then when it came time to do it on my dads boat I knew there had to be alot of other people like us. On my boat, I did a mild cam, a GM aluminum intake, Holley 750 carb, and this exhaust, the boat (21 foot Omega) picked up 700 RPMs and roughly 7-8 MPH. Huge difference. It was slinging a 19 pitch Hill SS prop.

I still have the templates for the brackets. I can assemble these kits for anyone interested.
 

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Discussion Starter #12 (Edited)
I did a kit for this when I was working at Dana Marine. We made a couple of brackets for the Solenoid on the back of the motor, the fuel sperator, and the alternator. It was a nice kit. We used Indmar exhaust in the kit. Here's a link to the kit, Value Performance Exhaust Kits - Dana Marine Products

Here's a link to the instruction sheet,
http://www.danamarineproducts.com/installation/260_Instructions_web.pdf

I did this once on one of my own boats, then when it came time to do it on my dads boat I knew there had to be alot of other people like us. On my boat, I did a mild cam, a GM aluminum intake, Holley 750 carb, and this exhaust, the boat (21 foot Omega) picked up 700 RPMs and roughly 7-8 MPH. Huge difference. It was slinging a 19 pitch Hill SS prop.

I still have the templates for the brackets. I can assemble these kits for anyone interested.
Thanks for the link to the instructions. I see now what I can do about my water inlet hose routing.
I see on that application it was a non power steering model. If i could do away with the powersteering unit , i wouldnt have this issue with the risers.

Ok here is another picture of the situation. I used a digital angle finder to try and see what degree elbow I might need, to get down below the steering arm. ( :)bulb ijust realized that even if I did not have the power steering, that I would still have to deal with the steering arm:duh:) Tell me if I am going about this the wrong way, but it looks like about a 70*-71* elbow off the riser and a 90* at the bottom? Only thing is that I have only seen 90*, 85* and 45* elbows.

016.jpg

019.jpg

019.jpg

If you look close, in this pic, you can see three black marks, that is where I need to get to to clear the steering arm.

021.jpg

And of course that puts my tips right on top of my swim steps.

Here is kinda what I need to make happen, but with elbows and hese couplers.

 

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That unit is a power steering model. You just don't see the shuttle valve because it's tucked under the gunnel. In that case, we stayed with thru-hub exhaust, not thru transom. The power steering pump is mounted on the lower right hand side.

The Imco picture you posted is called a down pipe. Thats exactly what you need. I see you have EMI manifolds. EMI builds down pipes just as like IMCO or anyone else. They're pretty common. I would call them to see if you can pay the difference and exchange your risers, or simply have them modify your risers.
 

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Discussion Starter #14
That unit is a power steering model. You just don't see the suttle valve because it's tucked under the gunnel. In that case, we stayed with thru-hub exhaust, not thru transom. The power steering pump is mounted on the lowerr right hand side.

The Imco picture you posted is called a down pipe. thats exactly what you need. I see you have EMI manifolds. EMI builds down pipes just as like IMCO or anyone else. They're pretty common. I would call them to see if you can pay the difference and exchange your risers, or simply have them modify your risers.
Yes, Here is the Eddie Marine "S" riser, That would solve My problem, But they are $1400 for the pair.

 

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From all my testing and experience, running thru hub VS. thru transom in this horsepower range will not add any power benefit. It'll add alot of noise, but thats about it. Most people don't realize the vacuum that is created when exhaust is ran thru the outdrive. When underway the exhaust is pulled out pretty well. The last test we did was an exhaust upgrade on a 242 Baja Islander with a 496HO. The boat was equipped with silent choice. We ran the boat and documented the RPM's and speed with the stock exhaust and aftermarket exhaust system with the silent choice opened and closed. Adding the performance manifolds gave it 2 MPH, but it ran 2 MPH better with the exhaust thru the drive or thru the transom. With the stock manifolds the boat also ran the same whether thru hub or thru transom.

You can have your existing tails modified for less than $1400.00 and they can be turned into down pipes like the EMI's you're looking at. You have about $240.00 in bends to double wall them around the corner. My guess would be around $900.00 to modify your risers by the time they're re-polished.
 

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Discussion Starter #16
My risers are essentially the same as this short riser, only with a longer tail pipe.





See the inner pipe with the flare. It is the same on mine, only longer. it is loose, not welded, from the outer weld to the exit and only forms a sleeve. the only place it is sealed is back close to the bend.
If I were to cut mine, I would have the exit same thing as the pic above, only longer and without the flare on the end.
Cut shorter, I could add hose and 90s to go down and out.
Would it work?
 

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Your risers can be modified. It's not as easy is just cutting it off. The outer wall and inner wall must be cut properly and the risers need to be dammed when re-fabricated. The cores you have are usable, just need the bends and a good fabricator who knows what they're doing.
 
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