Alpha gear ratios

[SeattleMatt]

So I have a mercruiser 260 with alpha I on a bayliner 2655. I am looking at getting a used outdrive but it is off a 4.3. I am assuming it has 1.67 gear ratio? But I'm not sure.

What will happen if i put it on my boat? Can I make up for it with a bigger prop?
Or will it blow the gears?

 

[stonyloam]

Re: Alpha gear ratios

This should help http://www.seilermarine.com/info/mercruiser-drive-ratio-chart.htm I believe it is best to have the correct ratio.

 

[tfret]

Re: Alpha gear ratios

How can I confirm the ratio of the drive I have?

 

[stonyloam]

Re: Alpha gear ratios

How can I confirm the ratio of the drive I have?

Read this: http://www.mercstuff.com/drivegearratio.htm

 

[xltier]

Re: Alpha gear ratios

So I have a mercruiser 260 with alpha I on a bayliner 2655. I am looking at getting a used outdrive but it is off a 4.3. I am assuming it has 1.67 gear ratio? But I'm not sure.

What will happen if i put it on my boat? Can I make up for it with a bigger prop?
Or will it blow the gears?

i have known people who use a lower ratio for better hole shot for multiple skiers.prop it as big as u can and wastch the rpm`s very closely.u have a good size boat,u may be ok.

 

[SeattleMatt]

Re: Alpha gear ratios

Any idea what the stock gear ratio would be shipped on a 2655 like this? It is 27ft long and weighs 6800lbs loaded.

Would a 1.85 work?

 

[ziggy]

Re: Alpha gear ratios

How can I confirm the ratio of the drive I have?

from a thread a long time ago. can't remember who wrote it. but not me.

How do I check gear drive ratio?

Do all these steps in order.
1. Shut off the battery switch
2. Pull the spark plugs out of the engine.
3. Put the shifter in forward.
4. Turn the engine till the prop starts turning, continue turning till the timing mark is at TDC.
5. Put a mark on the prop hub and the drive housing that align with each other.
6. Turn the engine by hand while someone watches the marks on the prop, and count the revolutions the crankshaft makes to turn the prop 1 complete revolution.
7. If it took 1.5 revolutions of the crankshaft, then you have a drive ratio of 1.5:1 which is what is needed for a small block V8. (a 260).

Any idea what the stock gear ratio would be shipped on a 2655 like this? It is 27ft long and weighs 6800lbs loaded.

from what i understand, it's not about the boat, it's about the engine.

look at the links others posted for what your likely to have gear ratio wise.

personally, i wouldn't be changing gear ratio to anything other than what is called for by your engine/drive mfg. i've read some/many a thread where the wrong gear ratio is a cause for not reaching wot.

 

[SeattleMatt]

Re: Alpha gear ratios

Ok, thanks.

Interesting, I keep reading that a higher pitch prop is more efficient than lower. It seems like with a boat my size, top speed at 40mph, the prop or the drive have to be much lower than a boat that's 20ft.

Seems like the boat size would have something to do with it.

Not sure about "slippage" though.

 

[ziggy]

Re: Alpha gear ratios

Seems like the boat size would have something to do with it.

i would think so too. imho, you'd need to run a lower pitch prop to achieve wot specs compared to the fella with a 20' boat. the outcome is the same though. you need to reach wot spec. of your engine. hopefully on the high side of spec. with your normal load. you reach this spec. by adj. prop pitch, not gear ratio.

 

[SeattleMatt]

Re: Alpha gear ratios

So what would I get with a 1.84 and a 23 pitch prop on the 2655? would it blow the drive up?

 

[SeattleMatt]

Re: Alpha gear ratios

Well,

I'm going to buy it anyway because the price is right. When I get them both off, i will spin them to see the difference in ratio.

If my original is not 1.84, then I will just swap the lower units because they should be the same?

My drive just has a lot of hours on a big boat and saltwater. The new drive is going for $300 on CL and it's from a small FW boat.

 

[cannonford57]

Re: Alpha gear ratios

for the v8s its usually a 1.50 to 1 ratio...

 

[achris]

Re: Alpha gear ratios

Usual ratio shipped with a SBC is 1.5:1. You have a heavy boat, so going to a higher reduction and a slightly larger (more pitch) prop will be of benefit.

A bit of history and an explanation of ratios.
The overall ratio is determined by the combination of the upper and lower gears. Generally the lower stays standard (there are a couple of different gear sets for the lower but for the most part they are all the same). In the mid 90's Merc changed the tooth count in the lower housing to reduce gear noise, and it changed from a 1.65 lower to a 1.62 lower. Now, with that in mind you will see that the overall ratio changes because of it.
The 1.50 became 1.47, the 1.65 became 1.62, the 1.84 became 1.81, and the 1.98 became 1.94.

The upper box uses quite a few different sets, giving the overall ratio.
Generally the 4 cyl engines run 1.98/1.94, V6s run either 1.65/1.62 or 1.84/1.81, and V8s run 1.50/1.47.

Merc never made a 1.67...

If you put a 1.65/1.62 leg on you will need to increase your current prop by 10% (so if you currently have a 19" prop, you will need to go to a 21"). If you put a 1.84/1,81 leg on then you'll need to increase by 22% (if you currently have a 19", change to a 23").

All Alpha legs are good to about 300hp, so you shouldn't have any problem with the engine 'blowing up' the leg. The main thing it to make sure you have a prop on that allows the engine to reach the maximum recommended RPM at wide open throttle, which for that engine should be around 4800rpm.

Chris.........

 

[SeattleMatt]

Re: Alpha gear ratios

Thanks for all the information. I think the current prop is 17 or 15. I actually have a 21 and a 23 so i could try them out.

 

[180shabah]

Re: Alpha gear ratios

But watch your prop selection, as higher pitch units tend to be smaller diameter and designed with smaller faster hulls in mind. I think you will have major issues getting the thing to plane out with the higher pitch prop.