BRAVO 3 RATIO

[Paul2024]

I need to fit a new, as in refurbed but new to me, Bravo 3 drive onto my 7.4L. The sterndrives come with various 'Ratios' (2.2:1, 1.5:1) which I understand are determined by a combination of boat LOA, weight, etc, but I'm unable to find anything definite.
Boat = Mariah Z280. LOA = 27'10" (8.48m). Weight = 7200lbs (3266kg)

Any advice/pointers would be really appreciated. Thanks

 

[Fun Times]

Do you happen to have the serial number of the original bravo 3 Stern drive at all? It sometimes may help determine what it came non altered with.

 

[Paul2024]

Thanks Fun Times. I probably missed the most important point from my original thread - the 7.4L that has been recently fitted replaced a 7.3 dtronic which was mated to a Bravo 3 diesel leg. The engineer that fitted the 7.4L told me it would work with the diesel B3, but it doesn't, and he has disappeared. So my way forward is to fit a new B3, but I need to ensure I get the right ratio

 

[Scott06]

Thanks Fun Times. I probably missed the most important point from my original thread - the 7.4L that has been recently fitted replaced a 7.3 dtronic which was mated to a Bravo 3 diesel leg. The engineer that fitted the 7.4L told me it would work with the diesel B3, but it doesn't, and he has disappeared. So my way forward is to fit a new B3, but I need to ensure I get the right ratio

You mean it would not bolt up or the rpms were wrong? If rpms at wot what did you get ? you maybe able change out props or re gear your existing drive

 

[Paul2024]

You mean it would not bolt up or the rpms were wrong? If rpms at wot what did you get ? you maybe able change out props or re gear your existing drive

Hi Scott. It fitted okay & initially went into forward & astern, albeit a bit clunky, possibly due to having to keep the idle fast to stop it cutting out with the gearing in the diesel drive? When we took it out for a test run it wouldn't go into astern. We returned to the berth and haven't run at speed yet. My new engineer looked at it & confirmed there's something not operating properly in the drive (I was hoping it was just cable adjustment). The boat's now got to be lifted, drive taken off and possibly stripped to find out what's wrong.
So, my thoughts are do I go through the cost & time of re-gearing the existing drive or just spend that money on a drive purpose built for the engine, and sell the diesel drive?
I'm looking at both options, but I first need to know what ratio sterndrive would be best for my boat/engine setup
Thanks for your advice

 

[Scott06]

Hi Scott. It fitted okay & initially went into forward & astern, albeit a bit clunky, possibly due to having to keep the idle fast to stop it cutting out with the gearing in the diesel drive? When we took it out for a test run it wouldn't go into astern. We returned to the berth and haven't run at speed yet. My new engineer looked at it & confirmed there's something not operating properly in the drive (I was hoping it was just cable adjustment). The boat's now got to be lifted, drive taken off and possibly stripped to find out what's wrong.
So, my thoughts are do I go through the cost & time of re-gearing the existing drive or just spend that money on a drive purpose built for the engine, and sell the diesel drive?
I'm looking at both options, but I first need to know what ratio sterndrive would be best for my boat/engine setup
Thanks for your advice

Ok the problem is a little more complex...

I am not the right person to SWAG a ratio for your application. Maybe @alldodge can help?

SEI just started making aftermarket B3's https://www.sterndrive.cc/Mercruiser-Bravo-Applications.html

It may be worth diagnosing your current drive if only to hone in on a drive ratio if it is something inexpensive or simple... could also be a waste of time... likely what you mention is the cone clutch being worn. Possible you could also invest in just a new upper if your lower is good and can work with what ever gear ratio you need.

My buddy with a 28 ft bayliner 7.4 mpi runs a 2.20 ratio with 24" props. might be close to what you have.

Do you have a prop shop you can exchange props with ?

 

[Bondo]

Ayuh,.... Welcome Aboard,..... The motor should idle down to 650/ 700 rpm, regardless of the drive's ratio,...

Other than the poor idle, it sounds like you've got some sorta drive issue, which could be in the upper, where the cone- clutch is, or it the lower unit,...
Either way, you might just need one or the other, rather than both,....

 

[alldodge]

Agree with others, need to take the drive apart to find out what failed.

Also need to find out why the motor would not idle. Engaging the drive at higher revs can break the gears.

Once drive is apart, count the teeth of each gear and then we can determine what ratio it is/was

 

[alldodge]

7.3 Dtronic Max WOT 3950
7.4 BBC Max WOT 4800 (but don't know which 7.4)

 

[Paul2024]

Thanks all for your advice. I'll discuss with my engineer to agree best way forward

 

[Paul2024]

Belated update (I've been away) on this one.

Took the drive off & on stripping the drive my engineer found the lower gear had broken up, see pics. Problem now is that debris could have dropped down and caused more damage throughout the drive. Question is strip and re-build or buy a new unit?

Very few B3's come up for sale in the UK so I'm thinking about the SE1 drive as Scott said above.

 

[alldodge]

I would go SEI

 

[Scott06]

I would go SEI

Agreed seem to fit the bill for reasonably cost effective approach plus good warranty.

 

[04fxdwgi25]

On my 22', running a 454 mag and B3 with 2.0 drive and 28" props.
On a 28 footer, I believe they call for the 2.2 drive ratio