5.0 merCruiser swap

[Wagner jardd]

I have a 17ft 1994 Larson that had a 4.3 merCruiser in it. I had to swap it out with a 5.0 merCruiser but kept the same 4.3 alpha one out drive. The boat runs good but won’t go over 30mph and the rpm gage is reading 5500 rpm at full throttle. Any idea what’s going on?

 

[Dieter]

I have a 17ft 1994 Larson that had a 4.3 merCruiser in it. I had to swap it out with a 5.0 merCruiser but kept the same 4.3 alpha one out drive. The boat runs good but won’t go over 30mph and the rpm gage is reading 5500 rpm at full throttle. Any idea what’s going on?

the 5.0 and 4.3 will use different gear ratios in the drive. Typically the 4.3 used a 1.81 or 1.84 drive , 5.0 a 1.62/1.65 drive. So you have too much gear reduction, hence revs are way up.

You may be able to go up in prop pitch to accommodate this. I did a 4.3 to 5.0 swap in a Sea Ray 200 kept the 1.81 drive ratio and went from a 19" pitch3 blade to 21" pitch 4 blade, and have great hole shot for pulling skiiers. A decent rule of thumb is for every 2" pitch up or down the wot rpm will go up or down 400 rpm. Of course with all the different design props- stainless, Al, 4 blade vs 3 this isnt always 1:1.

Given you have such a light boat - you may need to go up 4" in pitch from what you are running now as a boat with that hull and engine may have wanted 1.47 drive ratio from the factory. I would start with the prop pitch changes and worst case you may have to get a new upper drive which is what has the gear ratio.

What prop are you running now

 

[kghost]

Normally 5.0 and 5.7 alpha use a 1.5:1 ratio. The newer gen Ii's its a 1.47:1.

What ever the pitch of the prop this on it, go up 2 pitch numbers.

Example (aluminum 3 blade)- old prop, 19 pitch, New prop 23 pitch.

One pitch typically will alter RPM by ~ 200-300 rpm.

You are currently 500+ rpm over specification.

Top speed cannot be determined that easy.
I would say with all things being correct you should easily hit 50+ mph in a 17 ft boat.
18 ft Checkmates with a 5.7 do 60+ all day.

 

[kghost]

After reviewing the manual,

5.0 L 200 HP (2 bbl Carb) can have either 1.50:1 or 1.65:1. 1.65:1 is listed as standard.
5.0 LX 230 HP (4 BBl Carb) is 1.50:1 Standard

The chart is from Gen I manual as the gen II manual does not show engine listings with gear ratios

 

[Wagner jardd]

Normally 5.0 and 5.7 alpha use a 1.5:1 ratio. The newer gen Ii's its a 1.47:1.

What ever the pitch of the prop this on it, go up 2 pitch numbers.

Example (aluminum 3 blade)- old prop, 19 pitch, New prop 23 pitch.

One pitch typically will alter RPM by ~ 200-300 rpm.

You are currently 500+ rpm over specification.

Top speed cannot be determined that easy.
I would say with all things being correct you should easily hit 50+ mph in a 17 ft boat.
18 ft Checkmates with a 5.7 do 60+ all day.

can you just change gear ratio in lower unit? Or do you have to change prop?

 

[Wagner jardd]

the 5.0 and 4.3 will use different gear ratios in the drive. Typically the 4.3 used a 1.81 or 1.84 drive , 5.0 a 1.62/1.65 drive. So you have too much gear reduction, hence revs are way up.

You may be able to go up in prop pitch to accommodate this. I did a 4.3 to 5.0 swap in a Sea Ray 200 kept the 1.81 drive ratio and went from a 19" pitch3 blade to 21" pitch 4 blade, and have great hole shot for pulling skiiers. A decent rule of thumb is for every 2" pitch up or down the wot rpm will go up or down 400 rpm. Of course with all the different design props- stainless, Al, 4 blade vs 3 this isnt always 1:1.

Given you have such a light boat - you may need to go up 4" in pitch from what you are running now as a boat with that hull and engine may have wanted 1.47 drive ratio from the factory. I would start with the prop pitch changes and worst case you may have to get a new upper drive which is what has the gear ratio.

What prop are you running now

The prop says 14 1/4 x 21

 

[Dieter]

can you just change gear ratio in lower unit? Or do you have to change prop?

You can swap gears but it’s pretty involved with merc specific tools.ratio is in the upper half of the drive not lower half. Generally folks will buy a new SEI for less than you can have gears swapped or buy a used upper.

I would verify the tach is accurate. If it is you could try a 23 but that may not be enough pitch to get rpms down. You can probably find some props in a 25” and try as well.

Is current 21” a basic aluminum like a black max or is it stainless?

 

[Wagner jardd]

You can swap gears but it’s pretty involved with merc specific tools.ratio is in the upper half of the drive not lower half. Generally folks will buy a new SEI for less than you can have gears swapped or buy a used upper.

I would verify the tach is accurate. If it is you could try a 23 but that may not be enough pitch to get rpms down. You can probably find some props in a 25” and try as well.

Is current 21” a basic aluminum like a black max or is it stainless?

Stainless

 

[Wagner jardd]

Stainless

If I do want to buy a different lower unit, what do you recommend?

 

[Dieter]

Stainless

Reason I asked is some of the larger stainless props like Revolution 4 or enertia with a lot of blade area and cup bite more and will bring rpms down as they have less slip than a vengeance prop.

I think probably in a hull that light probably be a happiest with a 1.47 drive ratio. Do you know if the drive is correct for a 4.3 with a 1.81 or 1.84? No way some one slipped a 1.98 drive off a 3.0?

Is your 5.0 a newer vortec head engine ? 2 bbl or 4 bbl carb?

 

[kghost]

Just to clarify,.

You would only need the upper gear case to get your ratio for a V8.

Most, Not all, lower units come standard gear ratios.

The Upper gear case has only 2 gears.

If you have the $$$ or know someone who has the proper tools you could purchase new upper gears for the proper ratio.

I agree, Starting with a 21 pitch stainless steel is not as easy to go up much past 25 pitch...but you would not be the first.

Maybe if you know someone with a much bigger boat with a bravo 1 type outdrive on a 27+ ft boat, they may have a 25 pitch stainless to try and see if it gets you where you need/want to be.
Otherwise do some research for 25 pitch props.

 

[kghost]

Did a quick search and this showed up,
Used 25 pitch stainless steel 3 blade.

You would need the correct thrust washer and nut end bronze retainer.

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