Oddball stroker question

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murfman

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I'm a life long Mopar guy and a life long boat guy. I have been boat less for about 12 years, and that is changing at the end of this month. I am picking up a 1973 27' Magnum Marine sedan that is powered by twin small block Chrysler marine engines. I'm not sure yet if they are 318s or 340s. They were dressed the same from the factory so I wont know until I get the boat and check the block numbers. Anyway my plan is to build a pair of 402s out of 360 magnum blocks to get me the Hydraulic roller cams. I am going to add a closed cooling system so I can run an aluminum intake, and I'm on the fence about RHS Iron or Eddy Aluminum heads. My question is do I NEED to run the Forged Scat crank or is the Cast one going to be enough? I'm keeping the compression low, around 9 to one so I can run the 87 Octane at the marina pumps. The motors will never see over 4600 RPM, but will see 42-4400 RPM WOT for long periods of time.

Engine compartment:
http://s184.photobucket.com/user/murfman_photo/media/image_zps3a3180ba.jpg.html

Thanks to some guys down under the restrictive Log manifods can be replace with these:
http://s184.photobucket.com/user/murfman_photo/media/IMGP1781-croppoed_zps961e1e4f.jpg.html

The boat:
http://s184.photobucket.com/user/mu...b-4cc2-8a28-3d1040082a80_zps3a3db728.jpg.html

For people that don't know the Magnum, it was built by Don Aronow the guy that started Donzi, Formula, Cigarette. He had a huge hand in creating the Offshore racing scene. The US Coast Guard has used the 27 Sedan as a patrol boat and they were in service for over 45 years, they were also the boat of choice for the Kuwaiti Coast Guard. With twin 400 HP motors I should be able to get 70 MPH from her and she is a Wave Crusher.

http://s184.photobucket.com/user/murfman_photo/media/PRESSECA_zps8fc086d8.jpg.html

http://s184.photobucket.com/user/mu...1984-Kuwait-Patrol-Boats_zps7896f9f4.jpg.html
 
From what I've gathered the 340 Marine engines came with 318 heads and were rated around 250 prop shaft HP and the 318s were 225HP



 
So is the Scat cast crank going to be up to the task? 360s hold up well and obviously they are all cast cranks
 
I am not a boat guy at all, but I would think that boats have much less shock loading than a car. I would assume power applications would be gentle and smooth.

When I built my 408 I was told the cast crank was good for 500 horse. I broke it on a 1-2 powershift. Ask your engine builder or machine shop, but I would think you are OK with cast. Does one of those engine counter rotate? Are the rod journals clocked different on the backwards turning one?
 
The nice thing about the Volvo drives is they can be setup to run counter clockwise by reversing the shift linkage and both motors can turn the same direction.
 
The only shock loading is when the boat gets aired out and the props leave the water and freewheel when the hit the water again it can cause problems. Just have to be vigilant on the throttles as this also breaks outdrives, drive plates, not to mention over revving the motors.
 
You can always super charge or turbo charge them for more power.
 
d55dave said:
I am not a boat guy at all, but I would think that boats have much less shock loading than a car. I would assume power applications would be gentle and smooth.

When I built my 408 I was told the cast crank was good for 500 horse. I broke it on a 1-2 powershift. Ask your engine builder or machine shop, but I would think you are OK with cast. Does one of those engine counter rotate? Are the rod journals clocked different on the backwards turning one?
Click to expand...

Was the crank and Eagle or a Scat?
 
what is the process to change to a closed cooling system?

You could just run a braided hose across from head to head and block the manifolds water cross over in the front.

A good iron head like IMM Machine makes more HP than aluminum edelbrock and little cheaper with better machine work. It's an IIRC, it's an RHS iron derivative .

Then you can avoid the cooling changeover.
 
Eagle. It fractured between the forward main and the first set of rod journals. I rebuilt it with an Eagle forged, it is a much nicer piece.
 

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An acquaintance has a Chrysler early 70s boat. Says it still runs perfectly. Original and in good condition and from what I understand it is a single 318. You have a really cool project!
 
shop i use to work on big trucks at owners friend had a boat that we did a hot rod 350 chevy motor for,...cast crank,pink rods dubble hump heads,big roller cam...first time out bottom end cleaned house,..wont nuttun left to rebuild,...so went back with a big block poncho motor stock but high performance,..it lasted 2 trips to lake for bottom end went down,..ended up with forged crank an rods for it would live,..water harder on a motor than you think spicaly the bottom end...good luck
 
I've searched around and all the performace marine crate motors for brand X use the Scat 9000 series so I'm thinking cast will be fine. I'm not looking to cut corners per se, but crap adds up when you have to buy 2 of everything lol. The damn exhaust manifods are 2200$ a set so that is 4400$ alone. I think I'm gonna run the Chineese air gap manifolds for both price, and the fact that the bypass hose and thermostat housing are in a better location.

As to the closed cooling there is a 5" round x24" or so long heat exchanger, basically a water to water radiator so the sea or lake water gets pumped up from the outdrive through the heat exchanger and out the manifolds. The engine is filled with antifreeze which keeps the inside of the block cleaner and when I head to the coast or Florida it keeps the salt water out of my motors. Plus it makes for easier winterization, just drain the exchanger and manifolds.
 
I'm really hoping they are 340 blocks so I can sell them and gather money for the stuff I want
 
if it was me i'd run stock 318 cranks

they are forged

i.e. rebuild your current bottom ends (318 or 340) and just upgrade the top ends

i'd stay away from the cast cranks because although you say "everything costs twice as much" you are better off not having to experience crank failure(s)
 
I've decided on forged cranks, is there any advantage to the Scat or K1 cranks? I see the K1 Forged cranks are a little cheaper. now that that is sorted out, I want to run the Magnum blocks so I can use the Hyd Roller cam setup. That eing said which heads should I run, RHS, Iron Ram, or those E-bay cast Iron ones? Or should I go with the Eddys? Either way I think I'm going to go with the Crosswind Air gap intakes mainly because they are set up for the angled bypass hose and old style thermostats which allows me to use the stock marine thermostat housing. I'm planning on a cam around 264-270 degrees advertised duration and 110 deg separation and keeping the compression around 9 to 1 Hoping to get 400 or so HP out of each, any holes in my plans? What would it take to get to 450? I want them to idle decently around 750 RPM or so. Thanks Murf
 
undo to popular belief the cast steel scat crank is way more than able to do the task you ask .
 
Do you plan on running one engine backwards? or does the outdrive take care of that?
 
Back at this! I finally picked up the boat, and have been working on it, here is what has gone down, I completely de-rigged the boat, pulled the motors, out drives, and transom assemblies. I found some moisture in the Transom so I replaced it with new Okoume Marine plywood, bonded it to the hull, and glassed it in. I wanted to get the glass work done before the weather went to **** and mostly got it done. Now I am tearing apart the engines, they had both been replaced with 360s sometime in the early 80s. As I suspected both engines were junk. Seawater cooled for 30 years with saltwater... Anyhow, while I have 2 5.9 Magnum cores to build I cant help but wonder if I would be better off finding a couple circa 1990 LA 360 blocks with the Hyd Roller cams and build those instead. What is the consensus? Magnum or LA? If I go Magnum, Stock, EQ heads, The RHS/Indy heads, or Edelbrock? Which heads if I go LA? They will be ported regardless. As for an intake I'm thinking Performer RPM air gap, or a crosswind one. My goal is a 9 to 1 Compression motor that will sing at 5000 RPM for decent stretches of time. I'm thinking a cam with these specs: 264 274 210 220 .512 .512 112°
Seeing as how the 360s lasted for 600 Hours with cast cranks if I go strokers I may just go with the Scat cast ones. I'm hoping for somewhere between 400 and 450 HP. Any thoughts?
 
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