Mercruiser out drive compatibility

[Ghg]

Is there any difference between the outdrives of 1967 Mercruiser 4 cylinder and same year Mercruiser 6 cylinder? Any gear ratio difference?

 

[alldodge]

There is no difference other then the possibility of a different ratio
Most will be different ratios if installed on the same model boat, but if different boats can have different ratios

 

[Bondo]

Is there any difference between the outdrives of 1967 Mercruiser 4 cylinder and same year Mercruiser 6 cylinder? Any gear ratio difference?

Ayuh,...... Welcome Aboard,.... The housings should be the same,....
The gear ratios will be different though,.....
The I-4 should be 1,98:1,....
The I-6 could be 1.84:1, or 1.65:1,......

 

[Ghg]

How will it effect the performance?

 

[Bondo]

How will it effect the performance?

Ayuh,..... Which way are you going,..??
I-4 to I-6, or I-6 to I-4,..??..??

 

[Ghg]

Going from I 6 to I 4

 

[Bondo]

Going from I 6 to I 4

Ayuh,.... Then you'll need to drop some prop pitch,.....
Donno how much though, depends on which ratio the drive is,.....
What prop pitch were you running on the I-4, when it had the proper ratio,..??
Hopefully, you'll be able to find a prop small enough,.....

 

[Ghg]

It had 23 stamped on the prop, is that the pitch?

 

[Scott06]

Ayuh,.... Then you'll need to drop some prop pitch,.....
Donno how much though, depends on which ratio the drive is,.....
What prop pitch were you running on the I-4, when it had the proper ratio,..??
Hopefully, you'll be able to find a prop small enough,.....

If he is going from a 6 cylinder to a 4 cylinder drive doesn't he need to increase pitch?

 

[alldodge]

It had 23 stamped on the prop, is that the pitch?

Might be, need to look further

If he is going from a 6 cylinder to a 4 cylinder drive doesn't he need to increase pitch?

No, would need to decrease pitch

Might even think about not doing it depending on what boat it is. If the boat is heavy for a 6cyl then it will be a dog with a 4cyl, even with less pitch

 

[Ghg]

Is there a way to determine the ratio?

 

[alldodge]

Is there a way to determine the ratio?

Remove drive and prop.
Mark prop shaft and drive shaft (grease pencil or marker)

With drive in forward or reverse rotate drive shaft 10 revolutions while also counting prop shaft rotations.

Divide prop into drive

Exp 10/5= 2.0
10/5.5= 1.81

 

[Scott06]

No, would need to decrease pitch

Might even think about not doing it depending on what boat it is. If the boat is heavy for a 6cyl then it will be a dog with a 4cyl, even with less pitch

I might be getting stupid but I thought he was not switching engines but rather drives - going from a 6 cylinder drive (1.65 or 1.81) to a 4 cyl drive (1.98) so increasing the gear reduction

 

[alldodge]

Going from I 6 to I 4

I might be getting stupid but I thought he was not switching engines but rather drives - going from a 6 cylinder drive (1.65 or 1.81) to a 4 cyl drive (1.98) so increasing the gear reduction

He is going from a 6 to 4 so the drive will be higher ratio

 

[Scott06]

He is going from a 6 to 4 so the drive will be higher ratio

snip

 

[alldodge]

Ayuh,...... Welcome Aboard,.... The housings should be the same,....
The gear ratios will be different though,.....
The I-4 should be 1,98:1,....
The I-6 could be 1.84:1, or 1.65:1,......

Using Bondos post
He will have a 1.84 or 1.65 drive

Installing a 4cyl means less power to spin prop that fast. The 4cyl would have a 1.98

The 1.98 woukd spin prop slower. So needs to reduce pitch to compensate

 

[Bondo]

Using Bondos post
He will have a 1.84 or 1.65 drive

Installing a 4cyl means less power to spin prop that fast. The 4cyl would have a 1.98

The 1.98 woukd spin prop slower. So needs to reduce pitch to compensate

Oh boy,..... Now I'm confused,.....
I read Ghg's post to say, he's going to a taller geared drive, with the smaller motor,....
Maybe I missed the boat here,.....

Ghg, What motor do you have,..??..??
Ghg, Which drive do you intend to run,..??..??

 

[Ghg]

I’m not switching from a 4 cylinder to 6 cylinder engine, I’m considering buying a outdrive that is on a 6 cylinder for my 4 cylinder motor, same year engines

 

[alldodge]

Ok my bad
But ratio is still going higher

 

[Scott06]

I’m not switching from a 4 cylinder to 6 cylinder engine, I’m considering buying a outdrive that is on a 6 cylinder for my 4 cylinder motor, same year engines

Ok that’s what I was getting at , assuming the original prop hit the correct wot rpm you will need a higher pitch prop to account for the higher gear reduction in the 4 cyl drive.

as an example I repowered my current boat.it originally had a 4.3 V6 with a 1.81 ratio drive. I upgraded to a 5.0 V8 that from the factory would have had a 1.62 ratio drive. I kept the 1.81 with the bigger engine went from a 19” to 21” pitch prop and it worked just fine. You will need to do the same and step up in pitch to make it work