What happen when change 2.2 to 1.81 ratio?

[erick11s]

Hi, what happen if i change my bravo 3 ratio 2.2 to 1.81 ratio ?

With 2:20 now my Top speed is 47 mph with propeler ss 22 pich - 400hp- wot 4800-5200 - 4500 lbs cobalt 272. Deadrise 20

With 1.81 what will happen.?

Top speed ? What pich i will go?

That I want is good aceleration better top speed, no problem if i need to change my pich propellers.

Thanks in advance for your help

Erick

 

[dubs283]

Re: What happen when change 2.2 to 1.81 ratio?

you'll lug the engine and cause damage, you may be able to make it up with a prop change to lower pitch but you'll sacrifice top end

more top speed would require a higher pitch prop which = more lugging, more damage

if you want a better hole shot and more top end you have one choice, more horsepower and torque

 

[erick11s]

Re: What happen when change 2.2 to 1.81 ratio?

Thanks fo your comment but I forget to mention something important, my engine is 4800-5200 rpm, but at 3/4 throttle I get to 5000 rpm with a torque that is heard strongly and if I try to go to top end suspect that I it would dangerously 5200 rpm, I check the original catalogues of Cobalt and this boat. as offered with different engines 454 mpi 375 hp or 502 415 hp (my current engine has 400 hp) in both cases with bravo 3 ratio is 1.81 and with bravo 1= 1.50, I think that in the past of this boat they changed the transmission to 2.20 and I feel it would be very tied up.

 

[Maclin]

Re: What happen when change 2.2 to 1.81 ratio?

If I understand the numbers in the post, you have a 27 foot Cobalt boat? With single engine? If so then I think that is why they have a 2.2 in there maybe.

Bravo3's usually have more like a 24-26 pitch. If you think you are hitting the rev limiter or could over-rev significantly now I would just try a 24 pitch set of props. If the engine can still hit close to the top recommended RPM with the 24 pitch then your speed will be closer to 50mph, just estimating.

If it is still up against the rev limiter with 24p then a 26p set may do the trick without having to change the drive ratio.

What may have happened is there was a 1.8 ratio drive installed initially, and then the 22 pitch propset makes sense, to help the engine rev and not lug with that 1.8 ratio.

 

[Maclin]

Re: What happen when change 2.2 to 1.81 ratio?

Mods, this may fit better in the Props section...?

 

[HT32BSX115]

Re: What happen when change 2.2 to 1.81 ratio?

Hi, what happen if i change my bravo 3 ratio 2.2 to 1.81 ratio ?

With 2:20 now my Top speed is 47 mph with propeler ss 22 pich - 400hp- wot 4800-5200 - 4500 lbs cobalt 272. Deadrise 20

With 1.81 what will happen.?

Top speed ? What pich i will go?

That I want is good aceleration better top speed, no problem if i need to change my pich propellers.

Thanks in advance for your help

Erick

Erick,

You have either a TACH or Speedometer (or both) that is not accurate.


Using a Prop-Slip calculator, using your above pitch, ratio and RPM, the above speeds are not possible.

Prop Slip Calculator

4800 RPM would give you a theoretical (NO-slip) speed of 45 MPH and with 10% slip, about 41 MPH.

5200 RPM would give you a theoretical speed of 49 MPH and with 10% slip 44 MPH

Even 10% slip might be a little optimistic with a heavy boat. you might be closer to 12%. (single prop slip is usually around 15% with a light planing type recreational hull big heavy boats (or boats with a "dirty" bottom) can have slip as high as 20% with a single prop)

I suggest using GPS (If you're not) to determine your speed. Get another tachometer and/or verify your current one is accurate.

It's extremely important that you use an accurate tach and speedometer to make measurements.

forget to mention something important, my engine is 4800-5200 rpm, but at 3/4 throttle I get to 5000 rpm with a torque that is heard strongly and if I try to go to top end suspect that I it would dangerously 5200 rpm,

If I'm not mistaken, you have a rev limiter that will prevent the engine from exceeding 5200. You will not damage the engine if you briefly operate it at 5200.

You need to determine that your TACH is correct before you do this.

Now, having said all that, I had a look at the Cobalt 272 owners manual and it appears that yours came with a 7.4L Mercruiser with a 2.00:1 drive and 26p props.

If your engine is really rated at 400hp, it's likely a 502 or a 454 MPI etc and should run ok with the 2.20:1 drive if you use 26 or 28p props.

If you REALLY do have 22p props on it, there is something seriously wrong with the engine or the drive ratio is way wrong OR the boat is seriously waterlogged and weighs a LOT more than it should.......

If you have the drive removed, I would verify the ratio by turning the input shaft and counting prop-shaft turns.

Then I would verify the tach or get a KNOWN tach for testing. Use GPS for speed measurements.

If you need a new drive and you have access to a 1.81:1, it would probably work ok with the 22p props, but running at high speed may cause the props to be turning at an RPM above the most efficient for those props.




Cheers,


Rick

 

[Fun Times]

Re: What happen when change 2.2 to 1.81 ratio?

Mods, this may fit better in the Props section...?

It's a toss-up as Mercruier gear questions seem to do well in both Non-Repair I/O and Inboards and Prop Questions and Topics.

Just hoping for the best outcome for erick11s as he started the thread in the Merc repair forum now moved three times.

Thanks guys for all that you do here to help out.