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Old 03-21-2006, 03:01 PM
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Chevy 4.3 to 350 or 454

Hi, Im a Ford V8 expert, but know nothing about chevys. I bought a 21 foot Scarab boat and as fate would have it, it happens to have a GM motor in it, it says on the top 4.3L 205/V6. Not knowing anything about GM Im assuming its a 205hp motor. The boat runs 55mph now with its current V6 and I want to get it up to 70mph. I have been told that any 350 or 454 chevy motor will bolt in other then the exhaust. This would make me think that the V6,350,454 share the same crankshaft bolt pattern??? I dont want a killer race motor and guessing that something in the 300 to 350hp range would be nice. I have found both 454's and 350's at good prices, but which would be better for a boat application. Im not sure what the weight difference is between the two are or what the torque difference is. Can you hold a BB chevy at 5000rpms without tearing it up? Basically I just need a few characteristics of the two motors instead of starting a BB/SB debate

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Old 03-21-2006, 05:06 PM
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In automotive applications anyway, the cranks have different bolt patterns. But, the outdrive manufacturer would have the proper drive flange for whatever one you want.

Be rather cautious though. Your scarab most likely has an Alpha outdrive if it came with the V6. At about 300 hp (about what you'll need for 70mph) is where the Alpha likes to break. Bravos are much stronger (but heavier). They also come in more ratios than the alpha for more fine tuning of the drive gearing.

The SBC will be a closer bolt-in and much lighter. Marinization parts are also a little cheaper and more common just like their automotive cousins. The 4.3 is a small block chevy with two cylinders lopped off, so the SBC is an obvious no-brainer. The big block will get you a bit more HP potential at the expense of much greater fuel consumption and weight and size.

I recommend a 400 SBC. You'll never have overheating issues since you have a non-stop supply of 70-degree water, and the beefy siamesed block and larger displacement will make 300+ hp possible at 4500 rpm instead of 5000 in a 350.

Here, I already built it for you Gear and prop a Bravo for perfect 4500 rpms, 300+ hp, 70 mph. Have fun.

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Old 03-22-2006, 06:38 AM
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Quote:
In automotive applications anyway, the cranks have different bolt patterns.
Huh,.?????????????

There's Only 2 Chevy Crankshaft Bolt Patterns,.............

The 2 pc. rear main seal is the 3 1/4"(I think)
And the 1pc. seal uses a 3" pattern.......

And,... The Bellhousing Bolt Pattern is the Same for All the Marinized Chevies......Matter of Fact,.... Most ALL chevies share the Same bolt pattern.....

As stated above,.... It's Much More Important as to Which,+ Who's Drive is on the boat.......
I hang out at iboats.com forums almost Daily,+ only venture over here from time to time,...... Come on over,+ we can get into the Specfics of What you're Wanting to Do..........

BTW,...... You might get to 60mph,..... But 70mph is going to Co$t Major Dollar$...........
A rule of thumb in Boating is,....... Once you see 50mph or so,... It Co$t$ about $1000.00 Per Mph after that...........
In other words,..... It's Cheaper to just Buy the boat you Want,.... Instead of Building it...........
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Old 03-22-2006, 07:08 AM
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Prop cavitation will be an issue at low speed with a prop that will get you to 70mph at a low rpm.

What rpm is the 4.3 turning at while doing 55mph?

Maybe a bigger cam and a better set of heads on the 4.3 could get you to 65mph by getting more rpms out of the 4.3.

Other than that, a 350 with some higher rpm potential would be a good choice.
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Old 03-22-2006, 08:15 AM
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Just alittle FYI,........

Boats are powered by Torque,.........

Upping the RPMs is Not an Answer.........

Outdrives tend to start Coming Apart at about 5000rpms.......

And,...... A Bigger Cam more often than Not, leads to Reversion,+ Blown Motors........
You need a Low Lift,+ an LSA of between 109*,+ 112*,...........
Ya gotta build Torque,.. Down Low......

With Boats,..... Much More So than Cars,......
There's No Replacement for Displacement.................
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Old 03-22-2006, 01:46 PM
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Right now its running dead on 5000rpms at 55mph. This is with a 22p 4 blade bravo 1 prop.

I kow I could probably build up the 4.3L but running fast with a V6 just isnt the same. I like to cruz around too, and nothing beats the sound of a V8.

Anyhow someone said all mercruisers are the same for crank pattern or at least only 2 were offered, so, If I cant find a mercruiser, Can I build a 350 chevy car motor? Will the drive from the 4.3L bolt to the 350 chevy?
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Old 03-22-2006, 03:14 PM
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Quote:
Anyhow someone said all mercruisers are the same for crank pattern or at least only 2 were offered, so, If I cant find a mercruiser, Can I build a 350 chevy car motor? Will the drive from the 4.3L bolt to the 350 chevy?
Question 1 was Already Answered by me,.. above.......

Question #2 Can't be answered until you state What Drive You Have...........
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Old 03-22-2006, 06:31 PM
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Quote:
Originally Posted by bondo
There's Only 2 Chevy Crankshaft Bolt Patterns,.............
...
You're right... I was thinking of the shape of the flange to identify which crank it is... Sorry. Don't drink and post at the same time

But there are a couple more, like the LS1 has a metric pattern, and the new 8100 has one of its own.

That 400 is a darn close exact match to a vee drive motor that I built for an 18' modified 12* vee race boat. It took that boat to 79 mph at 4550 RPM. And cavitation? Because of RPM? Aside from atmospheric air getting to the prop, Cavitation comes from improper pitch causing pressure delamination on the low pressure side of the prop, not RPM. The way to combat that is with proper gear ratio in the outdrive and a matched pitch.
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Old 03-22-2006, 06:37 PM
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Quote:
Originally Posted by brainsboy
so, If I cant find a mercruiser, Can I build a 350 chevy car motor?
Yes you can build a car 350 without issue. You'll have to pay close attention to using marine parts; brass freeze plugs instead of steel, stainless bolts in certain places instead of steel, marine exhaust manifolds, water pump, carb, alternator, distributor, etc.

Its pretty obvious that we're giving conflicting information (myself especially ) on the "boat" parts of this thread, so here are a couple forums where you'll get more concise answers on the marine part of this installation:

http://www.boatdesign.net/forums/
http://www.glen-l.com/wwwboard/wwwindex.html
http://boards.trailerboats.com/cgi-b...ultimatebb.cgi

Sorry for my misinformation. I'm a dork
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Old 03-23-2006, 07:38 AM
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Well,......... It seems that brainsboy has Left the Building,..........

But,... Anyway,......
While the 3 boating forum sites you listed are OK,..... they Are Very Slow sites,... Not alot of Traffic,.... Or Engine Expertize....

I hang out, Daily at iboats.com
There are Several Full Time Marine Techs that also hang out there,+ they boast the Highest Traffic of All the boating forums,....
And,... The topics are generally about the boats of thses Sizes.....

And,....... There's a couple of "Creative Engineers" such as Myself who will Help with people who want to Go a Different Way............

Good Luck.......
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Old 03-23-2006, 09:57 AM
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So if I want to keep the same drive i have on the V6, I need to know if its the 3" or 3.25" bolt pattern motor and find a V8 that matches? correct?
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Old 03-23-2006, 11:00 AM
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So far that sounds right. The blocks are physically the "same" except the V6 is lacking two cylinders. All of the front accessories and belt drive from the V6 will transfer over to the V8. If what Bondo says is true, then you should have the same crank bolt pattern.
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Old 03-23-2006, 04:01 PM
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Quote:
Originally Posted by brainsboy
So if I want to keep the same drive i have on the V6, I need to know if its the 3" or 3.25" bolt pattern motor and find a V8 that matches? correct?
No,.......
That's Not what I'm saying,.......

The Crankshaft Bolt Pattern, Only determines Which Flywheel you have to Use.................

Aside from That,.......

PLEASE,.... Tell me Which Mercruiser Drive you have bolted to the Back of that Engine..............

WHICH Drive you have will determine What Motors you can Drop in there.........

For Instance,.....
If you have an Alpha series drive,..... You're limited to about 300hp,@5000rpms,..... Much More,+ the Drive will be in Pieces.........No Big Blocks for an Alpha drive.......

If it's a Bravo series,....... You're Somewhat Safe up to the 500hp range,...... But, It's Not Bullet-Proof either......

Reguardless of Which drive you have,....... It'll need to be Regeared to a Taller Ratio...........

And,........ 1 more thing,......The Crankshaft Bolt Pattern Doesn't Matter,.... There are Easy Work-arounds for any appl.....
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Old 03-24-2006, 06:20 AM
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I have the Alpha one outdrive on it now. I dont want to change drives yet so running anywhere from 260 to 300hp would make me happy enough at this point.

By the way If later on down the road I want to change to the bravo unit, can I keep the same gimble?

Thanks

Ben
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Old 03-24-2006, 06:37 AM
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Quote:
By the way If later on down the road I want to change to the bravo unit, can I keep the same gimble?
Nope,....... Totally Different.......

Well,.... With the Exception of the "Hole" that's cut out of the Transom....And that Might need a little Modifying...........

What Year boat are we talking about,..???

You'll Still need to Regear the drive, to change to a V8......
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