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Old 08-24-2013, 09:10 PM
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boat engine build

I have a 1984 Chris Craft Scorpion that the oil pump went out of this boating season. It really bugs me that it happened but my wife is keeping me optimistic about it and telling me "now we can just build something to make it go faster". We really enjoy boating and this was a major buzz kill this year. It happened about 3 weeks ago and haven't really done anything with it except for pull the engine out. My buddy tells me I should go to a salvage yard and find a vortec 350 and start my build with that. The original motor is a 305. He is suggesting i build a 383 but not really sure. The way he explains it to me is it goes from a 3.48 stroke to a 3.75 stroke. He tells me to think of it as a ratchet loosening a bolt. The longer the ratchet the more torque which is what a boat wants. If I look at two different ratchets and one is only a 1/4" longer I don't think there is going to be much of an advantage in the longer ratchet. Kinda would like some insight on this and any other suggestions you have. Please bare with me if some of my responses are inadequate but I will try my best. If I just went to a salvage yard and picked up let's say a vortec truck engine and put in the boat wouldI it be adequate for a boat engine? camshaft, cylinder heads, etc....

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Old 08-24-2013, 09:40 PM
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adequate and severe duty boat use is opposite,,,,
The 383 will make decent power at a lower RPM setting than a 305.
What do you "need" for power?
Build the engine to live in a harsh environment(boat)
Pick a power level and buy the best quality parts that give the engine the ability to perform to your needs
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Old 08-24-2013, 09:56 PM
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Is buying a vortec truck engine from a salvage yard a good way to start even if it needed rebuilt?
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Old 08-24-2013, 11:42 PM
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The biggest advantage of the Vortec block is that it is provisioned for a factory roller cam (as opposed to your flat tappet cam block now). The roller cam is better for power production (valves open and close faster adding power in at all RPM ranges), and it is not susceptible to wiping a cam lobe like a flat tappet cam does with modern oils.

The 383 makes more power due to more cubic inches. The ratchet analogy is not correct. The only real reason to upgrade to a 383 is if the original crank is shot and needs to be replaced. The other reason is because you want a 383 and have the money to build it. Now days 383's are common and you can get a complete rotating assembly for cheap (Eagle and Scat offer these, but again, they are cheap and you get what you pay for).
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Old 08-25-2013, 12:26 AM
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My friend here had the exact same boat. We did up a low cr 355 with a Weiand 142 blower ( mild boost)
with a hyd cam. It was assume. Way more real deal boat motor power than any 383.
Its not hard to do. The results are more than worth the effort.
Starting wiht a good L-31 350 vortec motor as a core is a good way to go for this.
(vortec heads/factory OEM roller cam provision.)
You can do it easily with a good flat tappet cam too. its a matter of using the right high quality flat tappet camshaft. The flat tappet cam is not a deal breaker at all.
But the vortec roller motors core works too.
Just make sure the heads are not cracked. Most of these motors re sold cause they overheated.
Consider just starting with brand new GM vortec heads .
A vortec cylinder head is only one good way to skin this cat. There are many good high perf cylinder heads to choose for this
The mild roots supercharged low cr 355 will leave any 383 in the wake.
The Merc Alpha Drive outdrive will take the power if it is in good condition.
If it is all used up a healthy 355 or 383 will eat it. The Outdrive must be in good shape.
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Old 08-25-2013, 12:38 AM
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The 350 block does not need to be a roller block. its not a deal killer.
You can do it with a retrofit Hyd roller cam too.
The supercharger cams are "off the shelf" items. (Either hyd flat tappet or Hyd roller.)
This supercharged 355 low compression ratio marine engine is not difficult to build.
All the parts of this combo are "of the shelf" and goof proof.

Runs on 92 octane unleaded all day long and blows the doors off any N/A 383.
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Old 08-25-2013, 05:50 AM
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Not being much of a boat person, had to look up that Scorpion model and found that it came in a variety of lengths and model types - mostly dang big. From what I could see, a 305 in something that big, and I have to assume it is a pretty big boat, would make it a real slug. OK and moving along, what do you really want to do with this boat? Just make it go faster or add reliability? Then you get into how it's propelled. Installing a mega sized motor with 50-200 or more % more horsepower and torque in front of an out drive or even a shaft drive that was built for something around what a 305 puts out is a quick way for a boating disaster and from my little bit of experience with boats, very expensive. Then there is the added feature of higher gas consumption. Again drawing on my limited boating experience, have found that marinas tend to be legally stealing you blind for the 'convenience' of having a fuel dock and extra long hose just to help your wallet get lighter. There are several pluses for a later car based engine - you can use unleaded gas, add fuel injection and be able to buy most parts at a NAPA store. Oh and one more consideration - direction of engine rotation. Some marine engines rotate opposite a normal car. This means at least a different camshaft, distributor and starter and maybe a few more parts.

OK - all the negatives are over, and if you have a fat enough wallet, everything in the boat is or can be reasonably fixed, modified or replaced - go for the biggest that will fit in that hole and have a great time!!!
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Old 08-25-2013, 09:17 AM
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thanks for the replies guys. I talked to the guy atmy local salvage yard and he doesn't have a complete vortec engine. I then went to my local machine shop and talked to him and he has a block that is a 2 bolt 1 piece rear main and a4 bolt 2 piece rear main. he said the four bolt block would work with all the stuff from my 305. oil pan, flywheel, etc. he said the crank out of my 305 won't work on a350 due to balancing issues, Is this correct? He also said if I am going to buy heads he would suggest a set of gm vortec heads and a edelbrock performer intake but wasn't really sure on the cam. am I headed in the right direction?
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Old 08-25-2013, 11:44 AM
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You ARE headed in the right direction!! Engine rotation: make sure you are building a motor with same rotation as whats coming out. Some transmissions will change rotaion of the engine. Reverse rotation requires different crankshaft, camshaft, and rear seal. Distributer and oil pump should stay the same as the cam still goes clockwise due to geared timing set.
Check your oiling system, many Chris Crafts used a flexible oil pickup hose that was prone to collapse with age. This may have been your reason for failure in the first place. SBC pump failure is uncommon. I HAVE see the housing break off the cap and Pickups fall off, drives break, and relief valves hang up.
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Old 08-25-2013, 06:27 PM
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so how do I determine engine rotation? how about camshaft selection? I read a few articles online about cam selection and I think I know less about camshafts after almost two hours of reading than I did before I started. the guy at the machine shop was also kinda lost on cam choices. He said he knows how to select camshaft just doesn't really know if a boat engine needs to be all torque or not.
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Old 08-25-2013, 06:47 PM
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reread my post
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Old 08-25-2013, 07:00 PM
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Quote:
Originally Posted by BobBoat View Post
so how do I determine engine rotation? how about camshaft selection? I read a few articles online about cam selection and I think I know less about camshafts after almost two hours of reading than I did before I started. the guy at the machine shop was also kinda lost on cam choices. He said he knows how to select camshaft just doesn't really know if a boat engine needs to be all torque or not.
You should spend a couple minutes or hours and google "how to build a sbc boat motor".
A bunch of interesting topics surface.
Way different than building your typical street motor.
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Old 08-25-2013, 07:05 PM
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talking about the drive: if a new larger/ more efficient engine was installed and the prop remained the same it wouldn't be any harder on the drive than the old engine, would it?

I don't plan on racing the boat anytime soon. The majority of what we do is float in the lake and get sunburnt. It gets to expensive to fly around the lake all day. Maybe twice a year we get the two person tube out and tow it around the lake.

This is the best picture of the boat I could find.
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Old 08-26-2013, 12:26 AM
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If all you do is sunbath in it, don;t need to go faster which will burn more fuel then don;t replace the 305 with a engine that is bigger or more powerfull.
Just replace the 305 with another 305. Get a used 305 and install it. use the accessories from your old motor on the new Used one.
If you do replace the cam in the new 305 get a mild short duration cam.
not more than 204deg in 214 ex on a wide 112+ LSA.
It will do everything this boat needs to do for you.
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Old 08-26-2013, 07:13 AM
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if you repower with another engine your drive gear ratio needs to change. personally I would recommend the upgrade to a 260 hp 350 and matching drive. however I don't thinktheres anything wrong with the 305 you have. I would rebuild the engine you have and add a few upgrades. 305s run good in boats other than the plastic timing chain gear they used on your engine. those always fail. if your 305 is the 200 hp version, find a 4 barrel intake and carb and you now have a 230hp 305 and you don't have to change your drive gear ratio. if you have the 230 already then rebuild that engine the way it is. keeping the engine you have will put you back on the water much cheaper than a repower. whatever you do, don't buy a used running engine match because your timing gear will fail and you will be back at square 1 almost guaranteed. fix the issues that your engine has and it will live a long long time. plus a 311 CI runs a good bit harder than a 305 would. plus a .040 overbore 305 can use l31 vortec 350 heads and not too much valve shrouding and you will love it. roller timing chain new distributor gear and oil pump and some +.040 flat top pistons and that cam f-bird suggested is a reall good runner...I built one and its a great engine very fuel efficient.
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