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Upgrade bigger motor

Hey, welcome to the forum.
Gearing is sort of a black art and not a science. That is, you can make generalities but not specifics about what is correct or not.
It may be that you will be happy with the larger engine with the setup that you have now. However, pushing the same boat with the same propeller and gearing won't make you go faster simply by adding more horsepower but it will get you on plane sooner and allow you a little more at the high end only because the engine isn't working as hard as the 4 cylinder.
So, you can change the drive to a lower ratio which makes the prop spins faster. That will make you go faster. You can also install a prop with more pitch or even larger diameter. That will also make you go faster. BUT, you'll find that somewhere is a point where the gearing and propeller will overwhelm the new engine's ability to spin which will put you right back in the same place you were with the current setup.
This is where the services of a professional propeller shop will pay off. Just like the tires on a car, the propeller is what transforms the horsepower into motion. Having the right propeller can make a big difference in how well the new package performs.
So, yes, you will benefit from new drive gearing but don't forget about finding the right prop too.
 
Smick said:
I want to upgrade my 3 litre mercruiser to a 4.3 litre do I have to change the leg as well which is an alfa 1
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I suspect you will need to go up 2” in prop pitch, assuming your current prop gets the 3.0 to 4600-4800 rpm.

I swapped a 4.3 for a 5.0 on my current boat, kept the 4.3 drive and it’s 1.81 ratio instead of swapping a1.62 drive. I needed to go from a 19” to 21” prop, rpms are perfect.
 
The engine (with some influence by the hull) will determine the final or over-all gear reduction.

Look up the Mercruiser's drive ratio chart.
You will see that the reduction for the 3L engine is different from that of the 4.3L engine.


In your case, going from a smaller engine to a larger engine, there is little risk in damaging your engine.
However, the reduction will not be correct.

Keep in mind that we cannot prop ourselves out of an incorrect final reduction.... it just simply does not work like that.



There is a science, and the Mercruiser engineers have not only designed it, they use it.
 
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Most typical gear ratio for a inline 4 cylinder is 1.94:1
Most typical gear ratio for a V6 15'-21' ft boat is 1.84:1

Based on that a prop pitch change may be all that is needed to achieve proper max RPM of 4800 for a V6 engine as he would be using a 1.94:1 with a V6.

Inline engines use a front motor mount and the boat has a stringer in place for this.
The V6 uses two motor mounts, on each side of the block. More than likely your boat does not have the stringers installed in the hull for this configuration so if this is the way you want to go you will need to remedy this.
Also many smaller boats with an inline 4 cylinder may not have the needed room at the front of the engine compartment for a V6 so that will need to be looked at.
 
There is a science, and the Mercruiser engineers have not only designed it, they use it.
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For someone who thinks that the engineers at Mercruiser are all idiots, this is a surprising statement.
But to tell truth to bull****, there is definitely an art to prop selection. it may be that you will have to try a few to hit on the correct one for your particular boat.
 
.........

o2batsea said:
For someone who thinks that the engineers at Mercruiser are all idiots, this is a surprising statement.
Please tell us who it was that thinks or said that!

But to tell truth to bull****, there is definitely an art to prop selection. it may be that you will have to try a few to hit on the correct one for your particular boat.
You can try props until the cows come home or until you are blue in the face.
Unless the final reduction is correct for the engine (with hull influence), you will never achieve optimal performance by trying prop after prop after prop after prop.
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RicardoMarine said:


Keep in mind that we cannot prop ourselves out of an incorrect final reduction.... it just simply does not work like that.
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this is just plain wrong , the experience I lay out in post #3 is evidence of that. I’ve used this boat for 5 seasons and works perfectly in all phases of operation - holeshot, cruise, WOT.

im sure there are more extreme examples where the engine swap say from a 3.0 to a 5.7. ( that would require multiple drive ratio ‘step changes’ 1.98 ratio to a 1.47 ratio) where this would nit work but there are plenty of examples like the OP asked where a one step change in prop pitch works.
 
Dieter, I am glad that you are able to make that work.
Perhaps you should contact Mercury Marine and suggest that they no longer need to produce quite as many different gear reduction drives.

Let me ask you this hypothetical question:

If you were offered a completely free brand new Mercruiser engine and stern drive package, and were given the option of choosing any final reduction stern drive that you preferred, what would you do?

Would you choose the one that Merc recommended for that particular engine, or would you choose one of your own choice?



.
 
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RicardoMarine said:
Dieter, I am glad that you are able to make that work.
Perhaps you should contact Mercury Marine and suggest that they no longer need to produce quite as many different gear reduction drives.

Let me ask you this hypothetical question:

If you were offered a completely free brand new Mercruiser engine and stern drive package, and were given the option of choosing any final reduction stern drive that you preferred, what would you do?

Would you choose the one that Merc recommended for that particular engine, or would you choose one of your own choice?



.
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rick why don’t you contact mercruiser since you seem to have sooooooo much time on your hands

stop obfuscating the OPs original question with long winded off topic posts. We are not talking about what drive ratios mercruiser offers or if I am ordering a new engine and drive combo.

like every boat ever made with a 3.0 it’s underpowered the op wants to upgrade his to a 4.3. Can I make this work with my existing drive? Yes likely have to change pitch.

no need to call mercruiser, unless you have too much time on your hands.
 
As a practical matter it seems that the 4.3 will accelerate in on plane much faster than with the 3.0 due to the much better torque characteristics of the 4.3, but will be limited in top speed vs what it would be with the factory gear ratio that would have come with the 4.3. Now if then we ncreasing prop pitch to allow proper max rpm will then compromise acceleration on plane due to running a much larger prop, that is a valid question....
 
louc said:
As a practical matter it seems that the 4.3 will accelerate in on plane much faster than with the 3.0 due to the much better torque characteristics of the 4.3, but will be limited in top speed vs what it would be with the factory gear ratio that would have come with the 4.3. Now if then we ncreasing prop pitch to allow proper max rpm will then compromise acceleration on plane due to running a much larger prop, that is a valid question....
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Can't know until the OP's boat is in the water and he can try a couple different props. Sometimes you can change diameter and keep the same pitch, or increase pitch one or two steps with same diameter or go up in diameter and down in pitch or, well, you see it can be quite confusing. Then there's a question of 3,4, or 5 blades. Cupped or no cup. Stainless or aluminum etc etc.
The proper way is to time it over a measured mile and find the one that delivers the best compromise of all the variables.
 
Here is the OP's question.
Smick said:
I want to upgrade my 3 litre mercruiser to a 4.3 litre do I have to change the leg as well which is an alfa 1?
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o2batsea said:
Hey fool, we are talking about propeller selection and not final drive ratio.
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See the original question in post #1.

Dieter said:
rick why don’t you contact mercruiser since you seem to have sooooooo much time on your hands
Judging from your rebuttals, you appear to have an equal amount of time.
I'm sure that the OP appreciates the time that any of us are able to spend here.... as long as it is productive.

stop obfuscating the OPs original question with long winded off topic posts. We are not talking about what drive ratios mercruiser offers or if I am ordering a new engine and drive combo.
see the OP's original question in post #1.

like every boat ever made with a 3.0 it’s underpowered the op wants to upgrade his to a 4.3. Can I make this work with my existing drive?
Thank you..... that is basically the OP's original question.
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louc said:
As a practical matter it seems that the 4.3 will accelerate in on plane much faster than with the 3.0 due to the much better torque characteristics of the 4.3, but will be limited in top speed vs what it would be with the factory gear ratio that would have come with the 4.3.
Louc nails it!

In post #7, Jack shows us the ratios for the two engines.
Most typical gear ratio for a inline 4 cylinder is 1.94:1
Most typical gear ratio for a V6 15'-21' ft boat is 1.84:1

Now if then we ncreasing prop pitch to allow proper max rpm will then compromise acceleration on plane due to running a much larger prop, that is a valid question....
This is why I suggested that we can't prop ourselves out of an incorrect final drive reduction.
We may be able to achieve some compensation... but generally we use the ratio that Merc recommends for a particular engine.
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Rick, just chill out. It's not about you and you being right about everything all the time. I hope with therapy that you can resolve your deeply concerning self esteem problems. I doubt that you ever will tho. You seem like the kind of person who has to overwhelm everybody with how much you know, and I doubt that you'll ever see how big of a problem that is for you. You have the dubious distinction of being the number one person complained about on this website. I guess that doesn't bother you at all.
You have this insatiable need to prove everyone else wrong all the time. Doesn't matter how big or small the issue, as long as me or Chris or Ghost post up on an issue you have to pounce on it and add your own topspin complete with graphics and quotes and links and all kind of stuff. You think it's being helpful but it's really just kinda sad.
Get some help. I mean it.

I'm sure the OP has long since skedaddled in fear. If I was posting for the first time and saw this ensuing sh!t show, I would too. This entire website has become the Rick-hates-everybody-who-doesn't-agree-and-has-to-prove-it center of the universe.
 
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You could not be more mistaken.
This is not about showing who is right or wrong. This is about getting the correct information to the person who started the thread.


What you are not seeing, is that these are our opinions ONLY. Yours, Jack's, Billy Bob's, or whoever.
You do not need to agree. You do not even need to read them. You can simply pass over them and move on.
But...... you chose to create an argument each and every time.
I'm going to call you out, and say that you are doing nothing more than "projecting".


Let's carry on like adults.
If you disagree, say so, but do it in an adult like manner, and explain your take in clear, understandable and complete terms.
Fair enough?



.
 
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Rick doesn't come across that way to me. All you guys are dynamite. Please get along. The information presented here is invaluable.
 
RicardoMarine said:
This is not about showing who is right or wrong. This is about getting the correct information to the person who started the thread.

.
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and the correct information is the OP can swap his engine to a 4.3 and go up in pitch on his existing drive and this will work very well for him. Any of the other horse $hit you have written like the snarky comment about me calling mercruiser and suggesting they reduce the number of drive ratios they offer, or stupidity like if I could have a engine / drive for free wouldn’t I want the correct ratio...is useless.
 
So one day....like well over 40 years ago, I bought a 69 Chev truck. Didn't like the Dana 60 open differential @ 4.11 ratio, so I found an Eaton 14 bolt posi rear which GM began using in 1973 models. It happened that it was a 3.73, which took some power away from the 350 V8 when loading the truck with heavy cargo and/or a trailer. So I built up a 283 and added some go fast goodies, like headers, cam, powerpack heads....etc. WRONG. So I found a worn out 400 small block and rebuilt it with a high torque cam....and using an Edelbrock torquer intake manifold, head upgrades, and other modest options. That motor developed too much power, so to "dial it in", I bought some big rear tires...12-16.5's. That marriage was perfect. I still have it today.
So, to "cut to the chase", the little 3.0 is fine with the "deeper" gears. Add more power.....if it's not too much, might just be able to dial it in with a prop. Really depends on what you want to do, how you load and operate your craft. The ratio difference is 1.94 to 1.81.....(see the clickable link in my first post).....should be able to make it up with your prop. Many more selections than in the case of tires on a truck.
 
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Here is my experience with gear ratios. It really depends on your boat. If you have a heavy boat, then keeping the higher reduction and installing a larger DIAMETER propeller will lower the prop slip and increase efficiency. On the other hand, if you want to go fast, then you need a smaller reduction, and less diameter. Putting a large pitch to compensate for a high reduction WILL increase the prop slip. Higher pitch props don't like to run slow and with a lot of torque.

A big example of this is suzuki outboards (yeah, I know outboards are a little different that sterndrives, but the gear ratio/pitch stuff stays the same, its an engine turning a prop). Their gear ratios are SO LOW (a 200hp turning a whopping 2.3:1!) that people don't buy them for small boats. They are very difficult to get right above 40 mph. I have one on my boat, and turning a 27p I get only 50 mph, a bizzare amount of prop slip. Other 200's run 60mph with a 24p on the same boat because they have gear ratios around 1.87. For higher speed low gear ratio with a big pitch prop is a $h*tty combo. However for heavy boats, people love the low gear ratio and have seen much better efficiency.

My recommendation to you: If you have a heavy boat, keep the drive and go up in diameter, this will reduce the prop slip and save fuel. If you have an average boat, then sell your drive on craigslist and get another one on craigslist. Maybe you could even find someone who wants to swap drives.
 
Keep in mind that Mercruiser was able to utilize the 1.5 to 1 ratio with both the 302 and the 350 motors. So prop selection must be a key
 
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