View Full Version : Putting Chevy 350 where 4.3 was, same drive

07-26-2012, 04:02 PM
Ok so what would happen, I hear so many people talking about the gearing being radically different, what would happen?

Say you have a stock 4.3 motor, drop in say a 300hp 350 in the same boat, put SBC exhaust manifolds in etc. how would it run with the 'wrong' drive, could it be propped? have a prop cut to get it going or would it be a total disaster? Will the boat not rev right, especially where the motors are very similar...just thinking could drop a small block in, getting running, then eventually swap in gears/drive

Coming from a car background, doesnt make sense to me, that it would be catastrphic, especially if going from a 4.3 to a 5.7.

Please school me on this...Tim

07-26-2012, 05:14 PM
The outdrive gearing is a torque multiplier. Big motors need less, like 1.3,1.5, 1.6 , smaller motors need more, 2 and more to 1.
You could spin a bigger prop slower at the same engine rpm, but maybe lose some efficiency.

07-26-2012, 06:36 PM
Look on the side of the drive, on the side sticker you will find the drives ratio. Let us know what you find!

07-26-2012, 06:45 PM
Just thinking if all you want id 300hp that very easy on these late vortec V6's, Jim has been waiting for someone to pull the trigger on a V6. You can same a ton of coin working with what you have! We ave a few sceaming S10's locally that give V8's a fit, one has tubbed wells with 10" erasers and believe me it lights them up!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

07-26-2012, 07:34 PM
This isnt for my current boat, my current boat has a 5.0 Ford and OMC...which I am pulling the motor on now for crank pulley repairs, eventually I may either do a SBC swap in this boat, but am thinking about finding a 4.3 Sweet 16 and build that, leaving my current 16 as a driver, normal type boat

thanks, Tim

07-26-2012, 11:56 PM
Not sure what way you would want to go with that. Like said you can make a 4.3 a pretty stout motor but dual exhaust on a 4.3 sounds like an 80's camaro.
Not sure what drive we are talking about but if its a Volvo 270, 80, 90 you will spend more money on a prop (because there is none available in a pitch over a 23) then you would on a drive trying to get the pitch of the prop to compensate for a V6 drive.

07-27-2012, 07:47 AM
it would be easier to build the 302/351 to 300-330 hp and have the same boat that is a daily driver. Bill Nagy built a screaming ford/omc 16 ( ran mid 70's all day long not sure on exact hp) that was turn key and a very nice boat. I have pics some where in my database I'll see if I can find them.

07-27-2012, 08:35 AM
from '84-'90 the ratio of a 4.3/alpha should be 1.84; 1990-1996 1.81

The bigger issue of the ratio isn't how the boat will run... you should still be well within the ranges of available props even with the higher ratio... The issue will be with how strong the drive is. With the higher ratio drive the gears are weaker. for the 3 years they put the 454 in front of the Alpha they used the 1.32? ratio. Basically, the lower numerical number == stronger drive. So I'd be a bit concerned that a 1.8 something ratio wouldn't hold up against 300+ HP. One thing you would have going for you is the lightness of the boat.

07-27-2012, 08:48 AM
here is a pic of Bill's SBF/omc 16 the boat is named SUMMER FUN, I am still searching for pics of the business end of the boat


07-27-2012, 10:18 AM
I know I could build the 302 in my boat...but I am a diehard Chevy guy, we are sick about having the Ford in there but it is fine for now....

We may find a second boat, say something that has a 4.3 and a Mercruiser, Alpha 1, then build that boat with a pretty decent small block Chevy.

thanks for the info!

07-27-2012, 11:22 AM
I guess I am missing something you don't like the ford but you'll keep the boat and drive it and buy another boat and make it they way you want??
converting a 6 to an 8 is going to need some glass work most 4.3 are not thru hull most would say why built a warmed up v 8 and not let it breathe or be heard?? so drilling holes in the transom will be needed that can be a real can of worms.
you'll have to address the motor mounts as well so now you are drilling into the stringers another crap shoot. then the outdrive is going to need a new gear set.
My point would be you know what you have now, removing the ford an going to a chevy is not really a big deal omc used ford and chevy power so did volvo on their setups so finding a chevy coupler should not be a big deal . this is just me but i would rather have a omc/volvo sx type drive or bravo on a mill that was north of 300 hp than an alpha any day.

also isn't the cut out for an omc or volvo sx the same as a merc cut out?? my point is it is easier to build what you have , also not saying having two boats in the stable is a bad thing but having basically the same two 16s might be a little redundant, variety is the spice of life, so an 18,19,20,21 or 22 foot classic might be an good 2nd boat

07-27-2012, 11:37 AM
that may be another option, build a small block, get the right bellhousing etc and convert our current boat to a small block chevy

not sure which way we will go....also am open to getting a classic 18, so at this point it is more of bench racing...

I hate Fords...we have raced against them for years, we have a strong brand loyalty thing that is being challenged right now...so maybe we will change it to a Chevy...point is, if we build a motor for our current 16..it wont be a Ford

we are limited by a canal width of 84 inches, so if we got an 18 I would have to play wiht the hull some, perhaps just have to fab up a nice razor thin aluminum rub rail...to get through

a larger boat than that for where we are in Maine, is out of the question

I am not familiar with the cut out..I was assuming that if I changed my current boat from OMC to MERC that we would have to do transom work, that is something I wouldnt want to do

also thanks for the info...I heard Alphas are the way to go, but perhaps a stout OMC / Volvo or whatever is better? maybe would build a healthy 350 is all around 350hp

07-27-2012, 11:45 AM
also are the SBC and SBF motor mounts located in the same general location?

so if I pulled the Ford 302 out, will Chevy mounts bolt right into the stringer?

07-27-2012, 12:22 PM
i know they are further fwd on a 8 cyl sbc than a 6 cyl if they are in the exact spot for a 8cyl sbf and an 8 cyl sbc not sure. I have seen a few different mounts on the sbf.

07-27-2012, 12:35 PM
not really sure why you bought the ford in the first place if you have such feelings.?

i am not positive that the corba has the same cutout but the newer volvo stuff does share the same cut out as merc I do know that volvo makes a cobra/sx replacement drive for the omc stuff. that replacement drive will bolt to a sbc
so you could buy the coupler and replacement drive then built the mill and then deal with the motor mounts.

07-27-2012, 01:56 PM
also are the SBC and SBF motor mounts located in the same general location?

so if I pulled the Ford 302 out, will Chevy mounts bolt right into the stringer?

I don't know if all Ford mounts are the same, but in these pics you can see how far the center of the mounts moved forward when I switched from a 351C to SBC.

07-27-2012, 02:22 PM
Bought the boat as it was a clean, solid boat, great deal, our short season was upon us, not many around in general in my area, also knowing one day we may swap a Chevy in....yeah the Ford motor is depressing, my dad was prepping the new oil pan and pulley for paint today when I stopped by at lunch, he looked very down in the dumps :( haha kididng, we just prefer Chevies, have tons of Chevy motors, early Camaros, Corvette and so that is where our allegiance lies...

thanks for the pics, yeah so maybe will do a swap in my current boat, going to start slowly gathering parts as well as keep an eye out for a 4.3 that needs a motor, as far as I have every seen on the earlier 4.3s say 1990s, 1980s the mounts should be in the same exact spot from say where the bellhousing bolts up, as it is for a 350..a 350 should bolt up in the same place as a 4.3 of what I have seen, though I could be wrong - even if not the exact same should be same ballpark and easy to fab up some adapter plates as needed

thanks for the responses...

07-30-2012, 07:43 PM
going from 4.3 to a 5.7 will move the front feet slightly more than 4" forward

no biggie, been there, done that

07-30-2012, 11:34 PM
buizilla, did you drop a SBC in? did you leave your drive alone, original gears and just reprop it? let me know, thanks

07-31-2012, 08:10 AM
it was in a 205 Four Winns, wayyyyy underpowered with the v6-2bbl, yes, put in a new 350-310 .... few hours later took out the upper gears.... had the Cobra rebuilt by a great guy and never looked back, 1.65 gears, went from a 17" to a 23" Turbo, been together since 1999 and still running every summer, hauled ass too...

08-01-2012, 07:35 PM
plus 2 on what Jim said . I used the forward hole as the rear hole when I moved my mounts forward . It was around 4 inches forward . you have to take off the exhaust but thats all . The engine has to face starboard and then spun around after its in . I used a davit and a few come alongs . the alignment took a while but it is real straight forward . I had never done anything like that before and I did everything by myself .. You might have a 1.81 or 1.65 gear set . Going from each gear set is about 4 inches of prop pitch I found . good luck and get the hd gearset !!! trust me ...

Northern Sweet
08-02-2012, 09:26 PM
I have a sweet 16 that was refitted with a 350/300hp replacing the original 4.3L. It's an Alpha drive which waze s regeared to 1.47. Ran really good but would still overrun the rpm a bit. I have been playing with props lately to maximize performance. The 4.3 had a 23 vengence. I went up to a 25, but settled back to a 23 Mirage plus. Seemed to be the best compromise for acceleration, lower RPM cruising and good top end.

Good luck.