4.3 To 5.7 Swap

[medalist1901]

hello, I have a OMC 4.3l engine with a cobra outdrive, I was thinking about swaping out the 4.3 for a 5.7. The 4.3 is junk and I think the manifolds are too. the v-8 manifolds are about half the price as the 4.3 manifolds (I think). What else needs to be changed to do the swap??

 

[Bondo]

Re: 4.3 To 5.7 Swap

Ayuh,...

The Outdrive,.... Your's has the Wrong gear ratio....

 

[medalist1901]

Re: 4.3 To 5.7 Swap

i remember someone saying that, what does that mean?? how does the gear ratio have to do with a bigger engine?? does it have to do with the rpm of the engine??

 

[mkast]

Re: 4.3 To 5.7 Swap

Sterndrives have different gear ratios depending on the engine used.
4, 6 and 8 cylinder engines all have different RPM operating ranges.

 

[chiefalen]

Re: 4.3 To 5.7 Swap

Your letting yourself in for a world of hurt when you go to change out the 4.3 to a 5.7 even if it will fit in your engine bay.

What size boat cause that 4.3 power to weight ratio is a great motor, wish i had one.

Whats junk in the boat now?

And the gears have to be changed out if you can find someone to work on it is 350 for just the gears for the lower.

And no a prop pitch difference won't do it.

The 5.7 will eat your gears that you have now in 2 minutes believe me.

 

[chiefalen]

Re: 4.3 To 5.7 Swap

My main man Bubba ^5.

 

[Gary H NC]

Re: 4.3 To 5.7 Swap

I used to own the exact same boat,same motor and drive.
I think a 350 would be too much weight in the back of that boat.
Save yourself the headache and just buy the replacement exhaust manifolds.

The 4.3 is a great motor to weight ratio.

 

[Bondo]

Re: 4.3 To 5.7 Swap

Lets use an example. If I have a engine (or gerbal) that can produce 200 lbft of torque at 2000 RPMs I would use one set of gears. If I had an engine (or a bigger gerbal) that can produce 300 lbft at 2000 RPMs I would use a different set of gears to spin th prop faster.

In this posters case, if he ups the output of the existing 4.3 liter (deliver more torque) he would need to change gearing just as if he installed a monster big block. Its ALL about the torque...

Ayuh,... I Agree,.. I think...

While what you're saying about it being All about Torque,.. I totally Agree with....
And,..
While your summarizations of the of the need to Regear, whether it's a slightly Hopped Up 4.3l, or a BBC, is in the least somewhat misleading,...
In that some Regearing is most always done with the Prop....

The Idea, of gearing in our outdrives is to absorb All of the available Torque from our Motors, at the most reasonably,+ applicable RPMs....

Props are only normally available in the 15" to maybe 23" or 25" Tops range....

Too much Torque from the motor,+ you need taller gears in the drive....
'cause you Can't buy a prop Big enough...
Not enough Torque from the motor,+ it'll Never make any RPMs,+ No Torque, or Horsepower,....
Probably Grenading from detonation...
'cause you Can't buy a prop Small enough....

The Whole Idea is to Stall the motor at it's RPM where it's making it's Greatest Torque.....
That's where the motor is gving All there is to give,+ just Can't produce anymore Torque to raise the RPMs,+ Horsepower.....

 

[medalist1901]

Re: 4.3 To 5.7 Swap

4.3 it is then... so, can I take the manifold to a boat store that does repairs and see if they can pressure check them?? How much does that cost around??

 

[Don S]

Re: 4.3 To 5.7 Swap

If you are planning on putting in a new 4.3, don't even think about putting old manifolds and risers on it. If they are plugged up or mostly plugged up, and there were rust streaks and puddle of water in the manifolds. Don't even think about trusting them on a new engine. False economy.

 

[medalist1901]

Re: 4.3 To 5.7 Swap

when I took off the manifolds off the old engine I looked in the exhaust tubes in the manifold, I did not see any signs of any water getting in.. I took out the fitting on ther sides of them. Looks empty, i can see in them with a scope. They dont look bad at all in them. I just want to see if I can get them pressure check to make sure. The only bad thing is there is a crack on the bottom on the outside... I guy that welds for a living drilled the ends of the cracks then welded them shut, said that water will not leak from the crack, but he didnt know if one of the runners inside are cracked?

 

[180shabah]

Re: 4.3 To 5.7 Swap

I think you are missing the point here. Your manifolds no longer leak externally - who cares? at some points internally the wall thickness is pretty thin, yeas of use slowly erodes then making them thinner. Yes it can be pressure tested, BUT even if it hold pressure, nobody can tell you for how much longer. When they leak externally things get wet, when they leak internally you rebuild heads if your lucky, but most often they are not caught before the leak is bad enough to cause a hydrolock. Water intrusion/injestion is the number one killer of marine engines.

 

[Gary H NC]

Re: 4.3 To 5.7 Swap

If they are the original one piece style bat wing looking manifolds its time to replace them anyway.They are at least 15 years or more old if not the originals..

 

[180shabah]

Re: 4.3 To 5.7 Swap

The batwings have a bit of a reputation as hungery littel buggers. They eat engines for lunch.

 

[medalist1901]

Re: 4.3 To 5.7 Swap

well, that sucks!! lol does anybody know the cheapest place to get them from?? would you trust getting them off ebay, or something??

 

[Gary H NC]

Re: 4.3 To 5.7 Swap

Right here...best price.They used to be about 500 more but the price has come down.
http://www.ebasicpower.com/mm5/merc...e_Code=eBasicPower&Product_Code=BPIOMC643-KIT

 

[mylesm260]

Re: 4.3 To 5.7 Swap

I am going to go ahead and go against the crowd here....

First of all, the weight difference between a 4.3 and a 5.7 is negligable. They are both cast iron block cast iron heads, by the time you factor in the fact that they both have the same accessory weight, the only real difference is the extra metal for the 2 cylinders. Maybe like 10-15% heavier overall tops.

The engines share the same bolt pattern, the same flywheel (if they are the later neutral balanced version) the same accessories, etc

Everybody in this forum is against any kind of change or inginuity, it would seem.

Yes, gearing makes a difference, but ballpark the motors put out simlar numbers. At the end of the day, you are only talking about going for a 4.3 to a 5.7. If you were starting with a 4cyl, it would be a different storry.


At the end of the day, you want to make sure you can properly load your engine and not over-rev. Asuming you don't want to change your gear ratio, ask yourself this: What size prop are you running, and what rpms do you run at WOT?

If you're running like a 17P prop at 4400 rpms, then you can solve the gearing issue by going to a bigger prop.

Just rough rough numbers here (I'm sure someone will jump in and correct me) but if your running a 17P prop with a 4.3, I bet you would end up with the same final RPM with a 21 or 23P prop with a 5.7.


Now if you are already running a 21 or 23P prop, then you might be out of luck.

IS your current setup "vortec"?

Going from a non vortec 4.3 to a vortec 4.3 is the same as going from a non vortec 4.3 to a non vortec 5.7.

A Vortec 4.3 and a non vortec 5.7 have roughly the same power output and torque numbers.


I went from a non vortec 4.3 with a 19P prop to a vortec 4.3 with a 23P prop, and I'm running give or take the same RPM at WOT that I was before. (but 11 MPH faster)

 

[Bondo]

Re: 4.3 To 5.7 Swap

First of all, the weight difference between a 4.3 and a 5.7 is negligable.

Ayuh,... About 100lbs...

the same flywheel (if they are the later neutral balanced version)

I believe the 4.3l Flywheels are beasts of their own, depending on vintage,+ None match a SBC neutral balance....

Everybody in this forum is against any kind of change or inginuity, it would seem.

Really,... My barge is a 1974 starcraft that was a 140hp/ Mod.1 when I got it...
It's now sporting a 20 year newer 4.3LX,+ a 10 year newer transom assembley,+ outdrive....

Yes, gearing makes a difference, if you are already running a 21 or 23P prop, then you might be out of luck.

The Whole Point is,....
It Can be done,.....
But there are Many Expen$ive Pitfall$.....

 

[180shabah]

Re: 4.3 To 5.7 Swap

Not against ingenuity or even projects in general, but for the $6-7 grand he will be spending before it is all done he could just go buy a running boat.

There are ALOT good used boats out there that DON'T need all of that effort just to be worth 1/2 of what was put into them.

 

[mylesm260]

Re: 4.3 To 5.7 Swap

Ayuh,... About 100lbs...


I believe the 4.3l Flywheels are beasts of their own, depending on vintage,+ None match a SBC neutral balance....

The Vortec 5700, 5000 and 4300 flywheels are 100% interchangeable, and are both neutral balanced. I'm running a used automotive 5.7 flywheel on my 4.3 in my boat right now. Works perfectly fine with the marine coupler, starter, and "rear engine mount/bellhousing".

The 5.7's are actually listed as being Lighter than the 4.3's on GM's marine website.

GM Marine Vortec 4300 Shipping Weight (from GM's Website): 449 lbs
GM Marine Vortec 5700 Shipping Weight (from GM's Website): 432 lbs

http://www.gm.com/experience/technology/gmpowertrain/engines/specialized/marine/2009_4300_Marine.pdf

http://www.gm.com/experience/technology/gmpowertrain/engines/specialized/marine/2009_5700_Marine.pdf




Honestly, if you think you can get a 5.7 setup for cheaper than a 4.3, and you have some wiggle room left on your prop (IE you're not already running a 21P or 23P, I would say go for it. The engines are very very simliar, and a lot of the parts switch over.


I would be more worried about sinking money into an OMC setup myself.... Mercruiser parts are soooo much easier to get, plus you simply cannot beat the 1300-1400 dollar aftermarket drives with warrenty.