Alpha one to bravo 1

MCL · Mar 21, 2011

Hi everybody

Thinking about upgrading the 4.3merc to a 300-330hp diesel.The boat has a alpha one gen 1(1989) with a 1.8 ratio which is useless for my application.I want to go bravo 1 for reliability.I know about the 2 holes that needs to be drilled.My largest dilemma is does the bravo prop sitt lower or higher than the alpha if I use the original holes.

Also since that kind of power should push this 20ft daycruiser 50-55 knots,is there any help to lift the drive higher to reduce drag?The transom needs to be rebuilt and strenghtened so might do it aswell at the same time.Is there any guideline to measure how deep the drive should be?

Also.Can I use the original trim pump as long as I make new hoses with correct fittings?I know there is alot of other issues with the conversion.

Bondo · Mar 21, 2011

Re: Alpha one to bravo 1

My largest dilemma is does the bravo prop sitt lower or higher than the alpha if I use the original holes.

Ayuh,... I'm pretty Sure the crankshaft center-line to propshaft center-line is the same....

The Cav-plate should be even with the bottom of the hull,...
Raising it can add efficiency, but it's a Gamble as to what's Too High...

The trim pumps are the Same...

stonyloam · Mar 21, 2011

Re: Alpha one to bravo 1

MCL said:
Hi everybody
...Also since that kind of power should push this 20ft daycruiser 50-55 knots,is there any help to lift the drive higher to reduce drag?

If you reduce drag, you are also reducing lift, and since you will probably be adding several hundred pounds to the stern of the boat (engine and heavier outdrive) make sure that you take into consideration how the powertrain change will affect the overall handling characteristics of the boat, including how it rides at anchor (you could figure out the wt. difference and get some buddies to sit on the engine cover and take her out for a run). Good luck.

TilliamWe · Mar 21, 2011

Re: Alpha one to bravo 1

A Bravo 3 would probably be a better choice.
And I am with Bond-o, I believe the crankshaft to prop shaft length of an Alpha and a Bravo1 (and a Bravo 3) is the same.

HT32BSX115 · Mar 21, 2011

Re: Alpha one to bravo 1

Also since that kind of power should push this 20ft daycruiser 50-55 knots

Howdy,

AT LEAST 400HP.

I have a similar boat. Four Winns 211 Liberator. 300 PROP HP with a Bravo III does 57 MPH lightly loaded.

If you're going to do it with a diesel and you want to go 55 kts, you'll need MORE HP and probably a 1.65:1 or 1.36:1 ratio Bravo IIIx (DIESEL type drive).

Also, under stand that you're probably going to nearly DOUBLE the weight in the back of that boat going from a 4.3/Alpha to something like a Cummins, IHI 7.3 or Duramax + Bravo .......which will reduce your performamce too...........

What diesel are you wanting to use?

MCL · Mar 21, 2011

Re: Alpha one to bravo 1

The engine im going to use is either a MB om603 or om606 td engine.Stock output is 150hp and 215hp at around 4600rpm.They rev very well for a diesel and are very robust.A friend of mine has a 580hp om606 engine with stock internals as his daily driver,its on the edge but holding up for quite some time now.The engine is a straight six and weighs far less than a big block or the diesels you mentioned.300hp is childsplay with pump tweaking,turbo,manifolds and intercooler.

I was thinking about going with the 1.5 ratio bravo one.Its rated at 600hp so its should handle a 300+hp diesel just fine.

Bondo.That liberator is one sweet boat.Always liked four winns boat but hate the omc.Your boat is over 21 feet while mine is a bit shy of 20.So I dont see why I shouldnt see those speeds.Still,im not doing it for the speed.Im still in the process of rebuilding the boat itself,so it will be a while before it will have a engine.

TilliamWe · Mar 21, 2011

Re: Alpha one to bravo 1

The standard Bravo One is rated to 600hp? Why does Mercruiser insist on the Bravo One X above 400hp, then?

MCL · Mar 22, 2011

Re: Alpha one to bravo 1

Yeah must have got some wrong info.Definately need to go with the bravo 1 x.They are hard to come by used though.Can anyone tell me whats the difference between the bravo 1 and the x version?

HT32BSX115 · Mar 23, 2011

Re: Alpha one to bravo 1

The engine im going to use is either a MB om603 or om606 td engine.Stock output is 150hp and 215hp at around 4600rpm.They rev very well for a diesel and are very robust.A friend of mine has a 580hp om606 engine with stock internals as his daily driver,

Well,

That sounds pretty cool.....

Where will you obtain the coupler, flywheel housing, inner transom plate and (wet)exhaust system for this installation?

MCL · Mar 23, 2011

Re: Alpha one to bravo 1

Thats the om603.Im actually going with the om606 engine since its more durable and easier to gain hp on.I will use the om603 mechanical injection pump to keep it simple with no electronics.

I actually found a dutch company who marinize these engines for the merc drives.I think I will just get flywheel cover,flywheel with coupler from them.I suspect they use the oem bravo coupler and modify the flywheel for fit.The flywheel cover is obviously custom made.It will bolt straight up to the bravo transom assembly.

The wet exhaust system will have to be custom made.

Since the bravo x drives are hard to get by and expensive.You think I could get away with a bravo 1 and using a watercooled billet top cover.

HT32BSX115 · Mar 23, 2011

Re: Alpha one to bravo 1

I actually found a dutch company who marinize these engines for the merc drives.I think I will just get flywheel cover,flywheel with coupler from them.I suspect they use the oem bravo coupler and modify the flywheel for fit.The flywheel cover is obviously custom made.It will bolt straight up to the bravo transom assembly.

That's probably 75% of the battle.........a jacketed exhaust is probably the rest of it........

Since the bravo x drives are hard to get by and expensive.You think I could get away with a bravo 1 and using a watercooled billet top cover.

If I'm not mistaken, the "X" drives ARE the diesel drives. ......possibly required because of the "harder" torque pulses of a diesel (at much lower RPM presumably).....Yes, the X drives are much more expensive, but might be worth it....

If that engine produces it's torque in the same range as a gasoline engine, it might not be as much of a problem. You're in experimental territory though using a "standard" Bravo drive behind a diesel.

A "standard " Bravo drive might not last long with 600hp input.....

The OM606 sounds interesting..... I don't know if you're going to get 600hp out of it......it appears that you might have to spend a LOT of money on it to get it to make a LOT of HP..... http://www.superturbodiesel.com/std/thread-om606-tuning

When you get it running, post a LOT of pictures!!!

MCL · Mar 23, 2011

Re: Alpha one to bravo 1

HT32BSX115 said:
That's probably 75% of the battle.........a jacketed exhaust is probably the rest of it........

If I'm not mistaken, the "X" drives ARE the diesel drives. ......possibly required because of the "harder" torque pulses of a diesel (at much lower RPM presumably).....Yes, the X drives are much more expensive, but might be worth it....

If that engine produces it's torque in the same range as a gasoline engine, it might not be as much of a problem. You're in experimental territory though using a "standard" Bravo drive behind a diesel.

A "standard " Bravo drive might not last long with 600hp input.....

The OM606 sounds interesting..... I don't know if you're going to get 600hp out of it......it appears that you might have to spend a LOT of money on it to get it to make a LOT of HP..... http://www.superturbodiesel.com/std/thread-om606-tuning

When you get it running, post a LOT of pictures!!!

Well there is alot more to it than the rear assembly and exhaust.I would like to be as optimistic as you but there will be more problems.Perhaps a blown bravo drive

From what I gathered from the internet the x drive is a strengthened version of the normal bravo drive.And there is no difference between the bravo x drive and the bravo x diesel drive besides the stickers.A weak point on the bravo 1 is the top cap that can twist/warp with high power applications,thats why I was asking about the watercooled billet cap.

Diesels do make alot of torgue and "rattle" gearboxes down.Im not going nowhere near 600 hp,thats a race engine and would never last in a boat.The engines displacement is only 3.0l and it revs past 6000rpm so it wont generate that much torque at lower revs.Im thinking about going a bit large on the turbo to keep the exhaust pressure low and not generating to much torque on lower revs,lag is really not an issue when you only have one gear.So yeah,its definitely in the grey area.Peak hp is at 4600-5000 range and thats where I want it to rev at full speed.Pretty same as the boat gas engines.

HT32BSX115 · Mar 23, 2011

Re: Alpha one to bravo 1

Yeah......with a 3L engine, and a LOT of boost, you might break the engine before you break the drive.....AND if you don't do the exhaust right, you might just start a fire in the engine room if it's not insulated correctly! You'll have EGT's in the 1000+ (F) range at full boost !

The exhaust side of the turbo will be pretty (RED) hot unless it too is jacketed....

MCL · Mar 23, 2011

Re: Alpha one to bravo 1

300+ hp will require about 1.5 bar boost.The engine can endure much more hp&boost,so I dont think I will have a reliability issue.The manifold will be FW cooled but I plan on using a non jacketed turbo.Sure there is a fire risk,but its running on diesel so no risk with fumes,and all the fueling is on the other side of the engine.Only way to get a fire is getting oil/fuel directly on the turbo.

Dont worry.I will report back in a year or two to tell if its all well,if I blew the drive or burned the boat down

.

TilliamWe · Mar 23, 2011

Re: Alpha one to bravo 1

HT32BSX115 said:
...If I'm not mistaken, the "X" drives ARE the diesel drives. ......possibly required because of the "harder" torque pulses of a diesel (at much lower RPM presumably)...

I am nitpicking here, but your statement isn't worded 100% correct. Not ALL "X" drives are installed behind diesels. The 496MAG HO and now the 8.2MAG HO get "X" series drives standard. And some of the lower HP Cummins/Mercruiser diesels get regular Bravo drive (I think there is even an engine or two that can get an Alpha!).
But you are correct, a diesel in the power range that he is talking would definately get an "X" from the factory.

HT32BSX115 · Mar 23, 2011

Re: Alpha one to bravo 1

But you are correct, a diesel in the power range that he is talking would definately get an "X" from the factory.

Yes....and actually, I think I remember seeing the "little" diesels with Alphas behind them......... (I think Don set me straight on that last year sometime!!)

Alpha one to bravo 1

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