4.3 to sbc question...

[wca_tim]

I kinda of thinking about removing / selling the 4.3 I have in my boat and putting a 350, 355 or even 383 back in it. I'm curious, how far will the engine mounts have to move?

I'd be making up a good portion of the cost by selling the good running 4.3 that's in it - it's only got 40 or 50 hours on it... it runs great, but I think i want to see about making it go just a bit... (ok, a lot) faster. I've thought about building the bottom end I have sitting on the porch and going with a blower on the 4.3, but just can't beat horsepower per dollar with a small block. note that I do know it's probably smarter from a purely financial standpoint to look for a different boat, but if I'm honest, I'd be doing it more to enjoy playing with it than anything else. I presume i could make up a significant portion of the weight difference by going with aluminum manifolds and risers? Know would still be a little heavier overall, but then again, so far I've always had a 50.+ pound tool box back in the engine compartment anyhow... and I know... that if I too far with engine or heavy handed at the same time, I'll probably lunch the alpha 1 drive... but could deal with that if / when the time comes...

anybody know for certain on the motor mounts?

Thanks!

 

[flabum]

Re: 4.3 to sbc question...

The front mounts will wind up about 3 inches forward of where they are now.

You also need to think of the gear ratio in the drive. The V-6's used a 1.84 ration, you will need to rebuild and install a 1.5 gearset.

 

[wca_tim]

Re: 4.3 to sbc question...

I realize the difference in gear ratios... I would actually toy with the idea of leaving the 1.84 gear ratio I have for a bit and runnin the 29 pitch 4 bladed prop that's sitting on the fireplace at first... Just out of curiosity. I understand the fact that the gear ratios come from the factory as 1.5 or 1.65 for v-8's . I also gather that the upper gear case is the weak spot on the alpha drive. I wonder whether that would be harder or easier on the gear case. I can imagine that would make it more susepctible to trashing the outdrive out of the hole with the higher gear ratio and higher pitch prop, but by the same token, may get a little higher top end with the extra torque from the higher gear ratio at speed as long as wasn't silly about romping on before planed out. In addition, from what I read the Alpha's don't like higher rpm's. The lower gear would turn the outdrive slower except for the input drive shaft and that would be about the same as it would with a 1.5 gear ratio and a lower pitch prop.

btw, I put the prop above on for a little while a few weeks ago just out of curiosity knowing it was way too much prop for the boat. With a fairly light load, it turned right at 50 mph at 3600 rpm. With enough power to turn the engine to 5000+ rpms, that would put speed right about 70... I could live with that as a starting point (grinning)

 

[whywhyzed]

Re: 4.3 to sbc question...

If the boat is shorter than 18 feet, I would stay V6

can't beat horsepower per dollar with a small block.

you are right there

With a fairly light load, it turned right at 50 mph at 3600 rpm. With enough power to turn the engine to 5000+ rpms, that would put speed right about 70... I could live with that as a starting point (grinning)

not sure the speed to pitch ratio is going to that linear.... but you'd be in the 60's
50mph at 3600?... musta just about taken a tail wind to get on plane...

 

[wca_tim]

Re: 4.3 to sbc question...

boat is 18 feet...

I'm currently at mid 50's to lower 60's (couple runs at 62) depending on which prop, chop, temp, loading, moon phases, etc... with the current engine. I would think that a healthy smallblock (tight 355, or 383) would get it well into the 70's.

and on the tailwind to plane...not quite, but I would drowned a skiier before he got out of the water....

 

[oops!]

Re: 4.3 to sbc question...

im doin the same swap on a 19.
havent decided on a motor yet.either a built 350 at 325-350 hp
or a stock 383.
but im finding this is a difficult thing to do.
the alpha is rated at max 300 hp, appearntly you can go higher but reliability becomes a factor. over 300 hp they tend to go blewey!
so that means a bravo...
the bravo needs a whole lotta new parts... different tran plates. not to mention the suck raw water from thru the hull not the leg.
i bought a new b3 for cheap after i found that out!

my guys (the boat shops around here) tell me to buy a used boat with the engine/ drive pkg i want and do a straight swap!
then ill have another boat to sell. that will offset the cost and if i buy right i may even come out ahead.

it seems we bolth are right on the threshold of a hi per boat and a regular cruiser. that magical 300 hp is the money line!
keep me posted!
cheers
oops

 

[Bondo]

Re: 4.3 to sbc question...

I also gather that the upper gear case is the weak spot on the alpha drive. I wonder whether that would be harder or easier on the gear case.

Ayuh,........ You're going the Wrong Way........

Back before the Bravos,+ Merc was putting the BBCs ahead of the Alphas,......
Merc's idea for keeping them together was to use the 1.32:1 gearset........
I haven't seen them for awhile, but the 1.32:1 gear set would be the right direction to go.......

 

[John_S]

Re: 4.3 to sbc question...

70mph, reliable, and an alpha drive? That might be a tall order.

To have a shot at 70, you are going to want about 350hp at crank (about 320-330 @ prop) at 5K rpms. To get that hp reliably, a mild 383 would be the best choice. It brings with it, enough torque to eat an alpha. You will want to control your launches to keep it alive as long as possible. Merc proved torque kills the alpha when they mated to a BB for a couple of years. I'd have to check, but it was only about 300-320 prop hp. I think the standard drive ratio was 1.32 for those engines.

Even though alpha is rated for 300hp (@ prop), many boat builders op to the Bravo with the 350/300hp mpi. If you go with 350, try to keep the crank shaft hp to 320hp or less. You may still have to be careful on launches, for it is the torque you are really woried about. Definitly change the ratio to 1.5.


"The lower gear would turn the outdrive slower except for the input drive shaft and that would be about the same as it would with a 1.5 gear ratio and a lower pitch prop."

You have this reversed. A 1.84 ratio drive takes 1.84 revolutions to turn the prop once. A 1.5 ratio, onlly takes a turn in a half.

 

[wca_tim]

Re: 4.3 to sbc question...

"The lower gear would turn the outdrive slower except for the input drive shaft and that would be about the same as it would with a 1.5 gear ratio and a lower pitch prop."


You have this reversed. A 1.84 ratio drive takes 1.84 revolutions to turn the prop once. A 1.5 ratio, onlly takes a turn in a half.

How so? (politely of course...). If the engine has to turn at higher rpms to net the same prop rpm, then everything in the outdrive turns slower per unit speed with the higher gear ratio... for 5000 rpm engine speed: prop shaft rpm would be: 2720 rpm for 1.84:1 and 3330 for 1.5:1. since the gear reduction is in the upper gear case, at the higher gear ratio everything past the upper gear case turns slower with the lower gear ratio. That's why higher pitch prop would be needed to get to a similar speed and why higher gear ratios are needed to get more torque out of smaller engines...

Everything else folks posted makes sense, and is quite frankly beyond my personal experience.

when merc had problems with alpha's behind big blocks, was it the upper or lower gearcase that came unglued? Presumably the two main factors in either one holding together are torque and rpm (as in at higher rpm's more heat, more oscillation, etc...). Both are inherent, but for example if the lowers on the older ones came unglued more easily because of the higher prop shaft rpm, then a lower gear ratio at in the upper gear set combined with a higher pitch prop would shift the rpm's down but increase the toque the lower had to deal with. I can appreciate that there is a sweet spot / design use range for each for both torque and horsepower, and hear everyone loud and clear that in both cases anything over the 300 hp mark - especially something approaching 400+ could easily reduce the alpha internals to a worthless pile of broken metal...

How much of it has to do with the weight of the boat / resistance to forward motion? it seems that should be important - maybe only with regard to which part of the outdrive breaks first though, eh? If the outdrive comes apart from the torque of getting a 4500 pound deep v hull on plane with a torquey big bock that's different than if it comes unglued behind a 350 turning 5500 rpms pushing a 2300 pound shallow v with a pad at 70 mph. (the smart *** in me says, yea it's different, when it comes apart at high rpm there will be more smaller pieces...).

Bond-o, thanks for the info. Were the bb alpha combos coming apart in the upper gear case or the lower? If it were lower, then would you infer that the change to higher gear ratio was to try to lower the amount of torque at the lower gear case?

I'm going to look around at other boats, etc... maybe just make some more incremental changes with this one and find something with a little faster hull to really get silly with... that way if I blow it all to pieces, we've stil got something to play with on the river, pull skiers and so forth... it's kinda nice to be able to run from dawn to dusk, a lot of it at 30+ mph without having to worry about spending the grocery money on gas...

Thanks very much to everyone for patience with my questions! I do very much appreciate the patience some of you show... especially after reading the same answers to the same queswtions people post... over and over... I've learned an awful lot searching through the old posts. I'm going to go work for a while and then think about going to the river... and freezing my proverbial *** off...

 

[wca_tim]

Re: 4.3 to sbc question...

"....since the gear reduction is in the upper gear case, at the higher gear ratio everything past the upper gear case turns slower with the lower gear ratio....

should say something like... "since the gear reduction is in the upper gear case, everything past the upper gear case turns slower with more gear reduction (ie 1.84) than with less gear reduction (ie 1.50)..."

so much for being clear...

 

[Bondo]

Re: 4.3 to sbc question...

Bond-o, thanks for the info. Were the bb alpha combos coming apart in the upper gear case or the lower?

Ayuh,.......

I my experince,......
The Uppers Grenaded,+ took out the lower with it........

 

[John_S]

Re: 4.3 to sbc question...

Yes, that is my understanding, as well. But they typically grenade when you put the hammer down, and significantly increase the torque. They typically didn't fail from gradual speed increase and running at high 5K rpm. In other words, you are probably not going to solve this by keeping some internal parts rpm lower.

I have also heard that they do not like running above 5.5K rpms, either. I am not sure what the weak link is there. You will find allot more info on Offshore Only and Speedwake forums. There are plenty of guys there that abuse their drives over on those sites. Some with success, some not.

Yes, a lighter boat is better than a heavy boat, when it comes to the torque. Is that enough to call it reliable??? The bottom line is you will be running the drive well out of where the mfg does. To me, that breaks the reliability requirement.

Over the years, I have heard rumers that the 350 Mag (300hp vortec) alpha had a special set of hardened gears and/or shafts. I thought I found a service bulletin that elluded to this, but now can't seem to find it. It was some info buried within a different topic. Looking in online parts manuals, could not confirm its existance. I do know that in 1997 when they offered both 350 mag (250HP) and 350 mag gen + (300hp), the latter was only offered with Bravo drives. The alphas were used on gen+ engines up to 280hp.

Here is the SB I mentioned: http://www-alt.mercurymarine.com/mnetdata/Service/Cruiser/Bullet/89/89_17.pdf
See section B. Now, this may not apply to gen II alphas, since it was released in 1989. But, as far as I know, the "M" stamped on the yoke is still valid for gen ii alpha mags.

 

[flabum]

Re: 4.3 to sbc question...

If you do wipe out the drive, consider looking for a used Volvo SX or SXDP drive and transom as a replacement... they can handle lots of power. The transom cutout is nearly identical to the Alpha cutout.

 

[TilliamWe]

Re: 4.3 to sbc question...

wca, my dad changed his 4.3 to a 350 in a 19' Sea Ray. I don't remember him having much trouble at all with the front motor mounts. I'll ask him what he did exactly.
He also had a 1.84 drive. He was able to put a high pitch prop on it and it ran fine. I think he was at a 29 or something like that.
Unless you build a real torque monster sbc, you won't explode an Alpha. And even if you do put in a 383, if you don't launch it hard, don't pull a tube, or get it airborn, you won't break it. My dad also had one of those 330hp 454Mags in front of an Alpha (in a ridiculously heavy Sea Ray 268 Sundancer!) and it didn't break in 4 years or the 400 hours he put on it. And I don't think it had ever broken in it's 10 years and 700 hours of it's life. So an Alpha can handle some power.

 

[achris]

Re: 4.3 to sbc question...

boat is 18 feet...

I'm currently at mid 50's to lower 60's (couple runs at 62) depending on which chop, temp, loading, moon phases, etc... with the current engine. I would think that a healthy smallblock (tight 355, or 383) would get it well into the 70's.....

Another one with a death-wish!!!

 

[wca_tim]

Re: 4.3 to sbc question...

Thanks for the input... both here and the many posts from some of the more experienced members that emerged in searching.

Here's what I'm considering...

Option 1: go with a 383 (or 355 if can't find good deal on 383) crate motor designed at 350+ hp, aluminim manifolds and risers, etc... and current outdrive after having upper checked over and tightened up if needed, perhaps heavy duty gears and adding cooling. Set aside money in "play" budget to replace outdrive if I scatter it, or pick up a replacement at higher gear ratio along the way somewhere. extend and reenforce stringers as needed, move engine mounts clean up engine compartment better, etc... while the engine's is out.

Option 2. Build the second 4.3 block I have adding 4-bolt mains, etc... and 8:1 compression pistons,appropriate cam. Have current heads and manifold cleaner up, and pick up a whipple, procharger or vortech set-up with enough boost to net somewhere between 300 and 400 hp... more homework needed to know exactly which components, etc... would have same considerations for outdrive as above...

option 3: leave it the way it is, except for minor and cosmetic improvements, etc... and start to look for a little larger project with a bravo 1 and big block in a light fast hull.

I can imagine that 1 and 2 are going to be in the same generall range of cost overall, but if I go with a new v-8 crate motor, I can make up some ground by selling the 4.3 (once I have the new set-up in and am sure I don't want to put it back in). I Haven't been able to actually weigh my current hull, but based on manufacturer specs I estimate that it's at about 2100 -2300 pounds wet in the current configuration and trim, etc...

Either way, it's beautiful out today - sunny and already 62!!! Headed for water... have a great one!

 

[TilliamWe]

Re: 4.3 to sbc question...

Tim, this is what my dad wrote when I asked him about his 4.3 to 5.7 conversion of a 1989 Sea Ray Seville 19' bowrider:

"I constructed motor mounts by attaching a flat plate of steel across the top of the mounts already there. (The one that are lag screwed to the stringers.) Then I drilled holes to match the mounts that are on the engine. The mounts on a V 8 are approximately 4" further forward(1 more cyl). Not too difficult."

He had more trouble with making the engine cover fit and the gas tank was in the way. He ended up having to have a custom tank made ($300).

 

[wca_tim]

Re: 4.3 to sbc question...

Thanks Tilliamwe! I;ve already measured and have plenty of room in the engine compartment without having to do anything drastic. And I think with the layout I've got back there I am gonna need to redo / extend the stringers where the mounts screw into. One side looks a little damp, but not soft... can't hurt to redo and make stronger... knowing about how far mounts will have to be forward will help...

ps. do you boat there on the Illinois river??

 

[TilliamWe]

Re: 4.3 to sbc question...

I boat around Peoria, mostly. But have been as far north as through the Starved Rock Lock and Dam, and as far south as Havana, to Tall Timbers Marina. I love it.

You would be very wise to make sure your hull and stringers are Solid, before you go trying to make the boat "fast". But specifically, what my dad was trying to say was that he didn't move the mounts in the stringers. He basically made adapter plates that extended forward to the mounts bolted to the motor (which are the same on a 4.3 as a 5.7). But if you have stringer issues, you would be just as well off to fix them and glass in or remount your front motor mounts where you really need them.

 

[oops!]

Re: 4.3 to sbc question...

tim
it is going to be hard to get 350 ponies outta a 4.3.
so hard in fact i think your relyability factor would go down.

if you sell the pkg you got and (4.3/alpha) go 350/383/b1.
you can recoup the cost of the b1. that way, the built 350. $3500.raw block
add another 500 at least for mani's dist start/alt. you could get your 70 mph rig for around $4000. and the reyliability factor is way up!
no point spending all that cash if your dead in the water waving your arms sos style at every boater that passes.

disclaimer... im still investigating too... exactally the same swap....

cheers
oops