Need help on lower unit interchange on 1985 Johnson 90hp

[FBPirate95]

My outboard has the following lower unit:

http://www.sterndrive.info/outboardmotor/v4_90_small_gearcase.html

I have obtained this lower unit:

http://www.sterndrive.info/outboardmotor/type_o_v4_v6_large_gearcase_id.html

I have swapped the lower units and everything appears to be fine except the shift rod length. I'm not able to achieve the 21 27/32 inch shift rod length on the newer lower unit. I was thinking I could extend the shift rod so I could get the right length. My question are: Is there any problems with running the larger lower unit on my 90hp? Are the gear ratios the same? Will it negatively effect the performance of my outboard?

 

[Daviet]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

Check the Top Secret File sticky, they show an interchange guide.

 

[FBPirate95]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

Check the Top Secret File sticky, they show an interchange guide.

I checked, but can't tell from the info.

 

[FBPirate95]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

More info on the unit. I believe its from around 1988-1990 and possibly a 120 - 140hp unit. But I'm not certain. The original unit I have is a 13 spline prop shaft and this one is a 15 spline shaft.

Here's a picture. If anyone can help ID or help me determine if this will run ok on my 90 hp it would be much appreciated.

 

[FBPirate95]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

I've done some more research and found that my original lower unit appears to have a 2.0:1 gear ratio (0.50) The replacement unit has a 2.25:1 gear ratio (0.44).

Based on this I know my outboard will need to turn a higher RPM to get the same RPM at the prop compared to the original unit. The way I see it, the math works out to be:

Original unit:
5000 rpm on the flywheel = 2500 rpm at the prop (5000 x 0.50)

Replacement unit:
To get 2500 rpm at the prop the flywheel will need to turn at 5680 rpm (2500 / 0.44)

Thats an increase of 680 rpm.

In order to lower the rpms back to the original 5000 rpms, and to keep the distance traveled by the boat per prop revolution equal to the original lower unit, I would need to increase the pitch of the prop.

By my understanding there is roughly a 300 - 400 rpm drop per 2 inch increase in pich.

Therefore to compensate for the 680rpm increase due to changing gear ratios I would need to increase the prop pitch by 4 inches.

Assuming I am runing a 15in pitch, I would need to get a 19in pitch.

Is this a correct method of thinking for compensating for the gear ratio change?

 

[ezeke]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

To get the same theoretical speed at the same RPM as your 15 pitch prop you would need to go to 17 pitch.

However, your engine should be propped to reach 5800 RPM at WOT with your average load so that you don't lug the engine. You can do that with a 15 pitch prop and achieve the same MPH as you are now reaching at 5,000 RPM by going to 5,800 with the new drive.

Your old shift rod does not work in the new drive?

 

[FBPirate95]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

To get the same theoretical speed at the same RPM as your 15 pitch prop you would need to go to 17 pitch.

However, your engine should be propped to reach 5800 RPM at WOT with your average load so that you don't lug the engine. You can do that with a 15 pitch prop and achieve the same MPH as you are now reaching at 5,000 RPM by going to 5,800 with the new drive.

Your old shift rod does not work in the new drive?

No the shift rod doesn't work. The replacement drive is what they call a "larger gearcase". Meaning the distance from cavitation plate to prop shaft center is 1 inch longer than the regular gearcase. Also, the diameter of the prop shaft housing is larger to contain the larger gears.

I'm having a machinist at work make an adapter that will lengthen my shift rod by 1 inch to compensate for the larger gearcase. So that should solve that problem.

I just want to make sure that running the larger gear ratio won't over burden the 90hp, or negatively impact the performance of the boat.

Thanks for the feedback above on the props.

 

[metriccrescentwrench]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

That lower unit bears a striking resemblance to the OMC V6, though I'm pretty sure they had a 1.86 gear ratio. The V6 lowers have a 15 spline prop shaft as well.

 

[ezeke]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

The measurement that is critical is not from the A/V plate but rather the outer mating surface of the gearcase to the center of the hole at the top of the shiftrod, with the gearcase in neutral.

The 20" large gearcase on the looper motors had this measurement at 21-15/16, compared to 21-27/32 on the 20" crossflow motors with the small gearcase.

That's only a 3/32 difference, which should be adjustable by turning the shiftrod.

 

[ezeke]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

That lower unit bears a striking resemblance to the OMC V6, though I'm pretty sure they had a 1.86 gear ratio. The V6 lowers have a 15 spline prop shaft as well.

The unit is sold as Type "O" and can be fitted either way; 1.85 or 2.25. http://www.sterndrive.info/outboardmotor/1979_up_v4_v6_drawing.html

 

[FBPirate95]

Re: Need help on lower unit interchange on 1985 Johnson 90hp

The measurement that is critical is not from the A/V plate but rather the outer mating surface of the gearcase to the center of the hole at the top of the shiftrod, with the gearcase in neutral.

The 20" large gearcase on the looper motors had this measurement at 21-15/16, compared to 21-27/32 on the 20" crossflow motors with the small gearcase.

That's only a 3/32 difference, which should be adjustable by turning the shiftrod.

The most I could get was about 21 1/2 inches before it would fully unscrew from the unit. I even tried swapping rods, and it still didn't work. I went ahead and hooked everything up and tried shifting it by hand at the motor and I can only get forward & neutral. Wouldn't go down far enough to pick up reverse.