Best ratio for 4.3MPI swap into 20ft Caribbean

[racingline]

Hi Everyone,

I'm wondering what is the best outdrive ratio for a 4.3MPI 220hp engine going into a 20ft Caribbean Adventurer runabout. I've attached a pic for reference.
It weighs 2740lbs with the current 470 setup, so with the 4.3 should be around 2850lbs at the max.

I have a 1.84 and a 1.5 in my garage to choose from now, but if everyone thinks the 1.62 would be best, then I can try and find that.

The boat will be used for towing a fair bit, so a good hole shot would be the preference over top speed if I had to choose.
I currently have a Solas 14 1/8 x 19 4 blade SS prop, so while not a huge factor, if that did suit either of the current legs I have, then that would be easy.

What do you guys think?

Thanks!!

 

[Scott Danforth]

Use the 1.84 and change the u-joint shaft, then adjust the prop pitch

 

[nola mike]

Use the 1.84 and change the u-joint shaft, then adjust the prop pitch

You shouldn't need to change the u-joint shaft. For low-end pop, might even keep your current prop, but you'll have to water test it to see. Assuming you have a 1.84 on there already?

 

[racingline]

Hi Scott,

Thanks for the reply.

When you say change the u-joint shaft, is that just for maintenance reasons? Or is there a stronger or different version available?

Cheers.

 

[racingline]

You shouldn't need to change the u-joint shaft. For low-end pop, might even keep your current prop, but you'll have to water test it to see. Assuming you have a 1.84 on there already?

Hi, Yes the boat came with the 1.84, but have removed currently for the swap, so it was just what was the best ratio choice for reassembly

 

[nola mike]

I actually went down from a 21 to a 19 when I did my swap, gained about 500 RPM. I should have more, but still don't think my carb is dialed in correctly. The 4.3 runs smooth and effortlessly with a ton of torque at 3500 RPM cruising, Doesn't matter whether I have one or eight aboard.

 

[achris]

Use the 1.84, but 19" might be a bit short. When I had my 4.3LX (180hp)I had a 1.84 drive and was swinging a 21" to about 300rpm over the top. Boat drove great, but it would have handled a 23" quite easily... Also Solas props are badly inefficient, you'd probably need a 25" in that brand.

I think the 19" would be too much for the 1.5 drive.. I have currently a 1.62 behind the same engine you're dropping in, and that can only turn a 17"...

And, those boats are quite heavy (solid), so a 4 blade may perform better.

Chris...

 

[Scott Danforth]

You shouldn't need to change the u-joint shaft. For low-end pop, might even keep your current prop, but you'll have to water test it to see. Assuming you have a 1.84 on there already?

I thought the input yoke is different length between inline motors and the V motors

 

[achris]

I thought the input yoke is different length between inline motors and the V motors

If it's pre-83, then yes the yoke will need to be changed. From 83-on, all yokes the same length.

 

[Scott06]

Use the 1.84, but 19" might be a bit short. When I had my 4.3LX (180hp)I had a 1.84 drive and was swinging a 21" to about 300rpm over the top. Boat drove great, but it would have handled a 23" quite easily... Also Solas props are badly inefficient, you'd probably need a 25" in that brand.

I think the 19" would be too much for the 1.5 drive.. I have currently a 1.62 behind the same engine you're dropping in, and that can only turn a 17"...

And, those boats are quite heavy (solid), so a 4 blade may perform better.

Chris...

Agree with this. When i swapped engines 4.3=>5.0, i kept the 1.81 drive went from a 19” to 21” pitch works perfectly

 

[Bondo]

I thought the input yoke is different length between inline motors and the V motors

Ayuh,.... To add to what Chris said, it's Only the GM in-lines with the MC-1 drives, that had the shorter input shafts,.....

 

[Scott Danforth]

Ayuh,.... To add to what Chris said, it's Only the GM in-lines with the MC-1 drives, that had the shorter input shafts,.....

learned a new thing.....