4.3 to 5.7 Swap

[LiquidC]

I'm swapping out my Mercruiser 4.3 V6 for a 5.7 V8 (350), I'm thinking it should be a straightforward swap. I have an Alpha 1 Gen 2 outdrive, and I know that I need to modify the upper gear from 1:81 to 1:47, and the only other item is the exhaust elbows are different and I have them already. Any mods needed in the gimbal or bell housing?

Can anyone tell me if there is anything else I need to do or get? The 5.7 I'm installing is a complete setup, I have the motor running great outside of the boat. I have a 22' Bayliner Cuddy, 8' beam. Any hints on prop changes?

 

[Chris1956]

Re: 4.3 to 5.7 Swap

What about the motor mounts? Are they in the correct place?

Is the coupler the same?

 

[180shabah]

Re: 4.3 to 5.7 Swap

No idea on props yet, but once you get it in the water and test it, start a thread in the prop forum and all will be solved....

 

[LiquidC]

Re: 4.3 to 5.7 Swap

My mechanic told me the motor mounts should be the same, I'll measure them to verify. The coupler on my new motor is for an Alpha 1 Gen 2, and that's what I have. Just need the upper unit changed out.

 

[wca_tim]

Re: 4.3 to 5.7 Swap

um... I think you need to move the motor mounts a few inches forward (to account for the difference in length added by the extra cylinder. Also check forward clearance to be sure you have room for it.

Do you have the exhaust elbows for the v-8 as well?

Another thing to consider is that depending on year, some of the non-balance shaft 4.3's were mounted on vibration damping motor mounts that have a different height than the most common mounts that come on a 5.7 (and many 4.3's as well). You may have to play with it a bit if you don't have the original mounts for the 5.7... or you may find that you have to mix and match the upper and lower mounts to get the height you need for it to line up. I did the swap you're talking about in a different boat last year and had to shim the mounts up 3/16 or 1/4 of an inch to use the stouter 5.7 mounts I had.

I wouldn't change the upper housing / gear ratio just yet. I would see what you wind up with for a prop with the current gear ratio - you may find you like it better with the higher gear. what are you running now for a prop? something 17" ish maybe???

anyhow, you should be able to find a prop that'll work well for that set up with a mild v-8 and a 1.8 gear... probably in the 21, 23 ish...

Coupler should be fine. Assuming the motors are from the same era (ie single piece rear main seals) everything else should work fine.

 

[LiquidC]

Re: 4.3 to 5.7 Swap

I'm not sure if it'll need to be moved forward that much. The 4.3 was pulled forward a bit and the coupler had a nipple of some sort coming out of the coupler that made up for the few inches. The 5.7 does not have the "nipple" or maybe it's just not pulled out.

I do have the V8 exhaust elbows, so I should be all set there.

Didn't know about the motor mounts ... good info, thanks. I'll let my mechanic know and see if he needs to put a piece of metal under the mounts to bump it up like you say. He will notice this when he goes to use the alignment tool, correct?

Too late on the upper gear, I already ordered a 1:47 gear. I think I'd rather go that route because I'd be afraid that the RPM's on the new motor would be too high @WOT, and I'd be doing 25MPH. I'll find out the hard way though, I can play around with the prop if I don't like it.

Thanks for the help!!

 

[bhammer]

Re: 4.3 to 5.7 Swap

One other thing, you will almost certainly need to run new fuel lines. Most fuel lines in newer boats are stainless steel and forming them takes a very high quality tubing bender. The hand tubing benders won't prevent the tubing from "kinking" in the bends and getting the "double flare" on the ends is a real pain. It will almost certainly have to be done in a shop with a hydraulic bender and flaring tool.

I trick I used in the past... Plug one end with a cap, fill up the stainless tubing with water and freeze it overnight. Then it will bend without kinking. That's an old trick that instrument makers use when making various wind instruments.