Drive Upgrade Questions

[Ramman]

Hey guys, I've been seriously thinking about upgrading the drive on my 1987 z-21, right now it has the alpha drive that im not happy with at all. I want to upgrade to a bravo, but Im not sure what is involved in doing this? or how big of a project it's going to be....Has anyone done this to their boat, or have any input for me?? THANKS for any help!

[gcarter]

I've not done this myself, but I believe two additional holes have to be drilled in the transom, at the top.
The rest should be straight forward. Exchange gimbal housings, mount the drive. The steering cylinder is the same I think. There's others here who have actually done it.

Oh yeah, you have to pull the engine.

[Pismo]

True, same transom hole, just need to drill two more holes. Need to remove everything, drive then engine then transom assembly, then install the bravo transom assembly, then put engine back with new coupler and then put on the drive. Not sure about the x dimension changes, I think it's the same x but not sure. Happy Bravo. What is wrong with the Alpha, other than the dog clutch tranny in the bullet and the shift interupt, it is a great piece. Keep it if you can. The Bravo is a bit slower heavier and shift is rather clunky but it is stronger. You must have the big block-Alpha combo which is prone to problems.

Cheers
SL

[gcarter]

I'm pretty sure the X is the same as I'm running a Bravo inner transom plate on my Alpha.

[Ramman]

Yea I do have the 454, I just bought the boat and found afterwards that drive has already been changed, Im not to happy with the performance, and because the boat won't push the 21 pitch prop it came with, I had to go buy a 19 pitch to get it to go anywhere. I figured a 21 foot boat with a 454 would go faster than 55 mph, Im just kind of dissapointed and looking at my options. Thanks for all your help:ribbon: :ribbon:

[Pismo]

A 22 with a 7.4L/310hp and a Bravo drive will do almost 65 so something is not right. To do only 55, the prop is only seeing about 220hp. that is very low for a 454. Some problem has yet to be found. Whatever problem that is will give you close to 10mph.

[Rootsy]

so how many rpm is the motor developing at WOT?

swapping from an alpha to a bravo won't magically hand you any speed... it WILL hand you longevity and piece of mind though.

in order to spin more pitch the same rpm you need to reduce the drag on the hull from teh water and the load you are trying to move forward.

imagine yourself standing against a big block of wood on a smooth floor.. you push and can move it a certain speed for teh strength your body can develop... then you spray some silicone (or other really slippery crap) on the bottom... wow you can move it faster... then you make it styrofoam and silicone the bottom and whew you are flying... SAME EXACT PRINCIPLE for a given hp trying to move a given hull through the water...

so fire us some particulars...

what prop (make, manuf, etc) and pitch (well we know 19)
what gear ratio in the sterndrive?
rpm the engine develops at WOT
are you using the power trim on the drive?
do you have trim tabs and are they being run in the down position?
how does the boat ride in general? bow high? bow low? squirrly??? does it porpoise?
where is the trim position at best WOT speed?
GPS speed?

we can help you track your issues down if there are in fact any :)

JR - i shoulda bought a bravo :yes:

[Ramman]

Alright here's the full story....I bought the boat about a month ago, it came with a 21 pitch mirage stainless on it. The first time I used the boat it would not move at all, no take off and no top end, at WOT I was under 4000 RPM's and the 4 barrel in the carb wasn't even opening because the motor would not rev up high enough to get them opened.. So I bought a 19 pitch mercury aluminum and put that on..The boat now gets up and goes like it should, at WOT im right at 4800 RPM's, I don't have a GPS, but the speedometer reads 53-55 mph at WOT, with 3 people in the boat. I know the speedometers are always off, but I had a realative with a boat who has GPS run next to me and I was right around 55 MPH. Im not sure what mt gear ratio is, after I bought the boat I found out that the previous owner had replaced the drive recently. I do know that with one revolution of the motor, the prop turns about 3/4 of a turn, whether that will help or not??...I do use the power trim, I usually run with it around half way up, after I get up on plane... The boat doesn't have trim tabs. All in all the boat ride's like it should, the bow is right where it should be, it dosen't ride high, with the trim half way, it shoots over the waves with no proplems... I still think the boat should push a 21 pitch with no proplem, I have realatives with 20 foot bowriders that have 305's in them and alpha drives and there pushing 21 pitch's with no proplem at all.. Which leads me to beleive that the previous owner put the wrong gear ratio on when he changed drives?? Which is also why I was thinking about upgrading to a bravo, to get a better gear ratio, so I can put a 21 pitch or higher prop on it, which I would think would get me more top end?? So thats my story, PLEASE HELP!!! I've got waverunners passing me! and thats not cool!! :splat: Thanks for all the help guys!!

[Formula Jr]

From my limited experience with the z21, I can say that this model boat is heavy for a 21. The Alpha/454 combo is not so good and is a drive killer. This boat needs a Bravo and a 498 to sing and dance. From your numbers you are running a 1.32 ratio drive which is correct for the 454. You should be running 4200-4600 at WOT.
So the 19 is right for your boat. You do not want a higher pitch prop with the set up you have unless you really do want to break the Alpha. Maybe try a stainless 19 or a stainless with a cup. You are doing good at 55 with the 19. Probably 2 more MPH with some prop tweaking.

[GKricheldorf]

Before i tore up my drive i was running a 4 blade 21 with the 1.32:1 ratio on a 454. I was not running WOT because i was breaking in the new motor. That 4 blade 21 was performing really well. We were planing faster then the 3 blade 19. We were seeing 44 mph gps @3500 rpm

[Ramman]

GKricheldorf......What drive was on your boat? I just don't understand how boats with 305's in them are able to push 21 pitch props and a 454 can't????

[GKricheldorf]

Ramman,

I have an Alpha One on it. Not a very good combo with the 454, but that is what Merc did on a few boats in 87.

Maybe some of the difference is because I recently repowered. I picked up a 97 Gen VI 454 with the roller lifters and Vortex heads. There is defiantly more power. I also ditched the cast iron exhaust manifolds/risers and put in GLM aluminum ones. Sounds much better and lost 100 lbs. It is still carbureted with the Quadra Jet 4bbl.

Have you done a compression check on your motor? Maybe it’s getting tired??? My old motor was a Mark IV and original with 318 hrs on it before hydro locked. It was rated at 330 hp.

[Ramman]

Not sure exactly what motor is in it, I know its the 454 mag 330 hp... Where are the hour meters on these boats?? I haven't done a compression check (although I should) the boat runs perfect, and sounds strong, With the 19 pitch on it, it gets up and goes just fine.. I still think the previous put the wrong gear ratio on when he changed the drive, I've been looking on the drive for the ratio but I can't find it...

[Pismo]

There is still the simple issue of horsepower, regardless of drive ratios or prop pitch, the prop is only seeing about 220ish hp. That is a problem. Should be at least 300 yielding about 60-65mph. Other problems could be the hull is soaked with water so carrying hundreds of extra pounds of weight, but this does not seems to be common is Donzis, or the bottom is so f**ked up it has increased the load greatly. Still unlikely. Puzzling..........

SL

[Ramman]

Pismo... I know for a fact that the hull is not soaked, the boat is in excellent shape for being 17 years old, The owner I bought it from had since it was new, and it was used on a smaller inland lake it's whole life. So I dunno I think im gonna take it to the donzi dealer and let them figure it out.

[Ramman]

Well I finally got the z-21 fixed! The timing was off by 7, it was supposed to be at 8, and it was at 2.. Now it's pushing the 21 pitch with no problems at all! And holy S**t I see why donzi is #1!! 62 mph on the speedo...My RPM's are a little low though, with the drive trimmed, Im barely pushing 4200, so I think Im gonna try to find a smaller diameter 21....THANKS for all the help guys!!