Exhaust manifold swap 5.7 Mercruiser

[J. Mark]

I am restoring a 1988 Marlin Ski Magnum. The boat is an inboard and I have it stripped to the hull and am going through everything as the boat was not running when I got it.

The exhaust manifolds look good, but the risers are looking kind of rough. I have not been able to locate a local shop to pressure test the manifolds.

I have potential access to Indmar exhaust manifolds and though the risers are shaped differently, my exhaust is being replaced and I have plenty of clearance under the doghouse, so that is not an issue.

Is there something I am failing to consider?

If the indmar parts source falls through I had also considered using Volvo exhaust manifolds. Again is there anything I am missing about the parts interchangeability?

Engine rebuild is almost done and I need to line up the next round of parts to keep this restore moving.

 

[mylesm260]

Re: Exhaust manifold swap 5.7 Mercruiser

I am by no means an expert...but....

The manifolds bolt the engine, the risers bolt to the tops of the manifolds....

The Y Pipe bolts to transom plate.

In order for everything to work, the Risers have to match up to the Y Pipe.
If you start mixing and matching exhaust components, you will likely run into a situation where the risers no longer match up to the Y pipe, and you will no longer be able to couple the two together.

The Y-pipe is not flexable or bendable, so if the outlets of your risers don't end up directly infront of the Y-pipes, your screwed.

 

[Buttanic]

Re: Exhaust manifold swap 5.7 Mercruiser

I am by no means an expert...but....

The manifolds bolt the engine, the risers bolt to the tops of the manifolds....

The Y Pipe bolts to transom plate.

In order for everything to work, the Risers have to match up to the Y Pipe.
If you start mixing and matching exhaust components, you will likely run into a situation where the risers no longer match up to the Y pipe, and you will no longer be able to couple the two together.

The Y-pipe is not flexable or bendable, so if the outlets of your risers don't end up directly infront of the Y-pipes, your screwed.

He said it's inboard so it shouldn't have a Y pipe.

 

[Buttanic]

Re: Exhaust manifold swap 5.7 Mercruiser

If he has a V-6 or V-8 he has a "Y" pipe in the exhaust. (Unless its through the hull.) As he says manifolds (plural) I assume its a V engine.

Since it's a ski boat I'm pretty sure he is referring to a straight inboard not an I/O and most ski boats have dual through hull exhaust.

 

[Bondo]

Re: Exhaust manifold swap 5.7 Mercruiser

a 1988 Marlin Ski Magnum. The boat is an inboard

Ayuh,.....

Most of the Inboard Ski boats I've run across run down,+ Out the back,... 2 pipes, 1 each side....
Sometimes they might have a water muffler, but usually just thru-hull at the waterline...

So,.. That Said,... That's what I happen to read out of the OP's question....

I would say you can run Any set of manifolds,+ risers that you want too....
They're ALL just abit Different than each other,+ you'll just have to adapt the rest of the exhaust to match up...
There'll be mounting bolt Holes on some,+ others won't...
Some will dump out at 3",.. Some at 4",.. Some at 3, 1/2",... Some at 2, 1/2"...

 

[J. Mark]

Re: Exhaust manifold swap 5.7 Mercruiser

No y pipe, it is an inboard and runs dual exhaust through silencers and out the hull.

3" set up currently and I am going to stick with that.

Sounds like I have a few options and that other than the 3" softwall hose length and angle being slightly different, a number of manifold kits will work.

Gotta love the versatility of that chevy small block 350 set up. I always liked ford motors in my play cars, but the chevy really is easier to deal with on the parts front!

 

[J. Mark]

Re: Exhaust manifold swap 5.7 Mercruiser

oh and as to the substitute for pressure testing. . . I have a gallon of acetone, waste rubber hose and clamps and I have already had the manifolds dipped in an acid bath and cleaned out thoroughly (even had them freshly painted) My neighbor is a welder and owes me 13 dozen favors so he does a lot of stuff for free! You should see the new pylon brace we designed and he welded up . . . as soon as I can I'll post some pics of it.

Thanks for the link and the tip, it makes perfect sense. Wish I had a borescope, but I have dental mirrors to look inside at least

He makes some beautiful bimini top type frames too. John is an outstanding welder (and a good neighbor too-just don't lend him your best chainsaw!)