Mercruiser gear ratios

[rolmops]

I have had trouble getting my engine -a 1984 140 mercruiser- to run at around 4500 rpm at wot while pushing a 19 foot islander with a 19 pitch prop.
After a complete rebuild I managed to get it up from around 3000 to 3400 rpm, nowhere near the 4500 point.
So I did some checking on lower unit gear ratios and found that for the 140 there is 1.94/20 24, for the 165 six cylinder there is 1.62/24-24. And for the 170 4 cylinder there is 1.81/17-19.
This led me to wonder if I have the wrong gear on my setup and maybe changing out the gear ratio (or the engine) might give the right rpm for my setup.
How can I find out what gear ratio my outer unit has? and does anybody know what rpm to expect when different gears are mixed up with different engines?
As always,thank you for your comments and opinions,Cornelis.

 

[alldodge]

I have had trouble getting my engine -a 1984 140 mercruiser- to run at around 4500 rpm at wot while pushing a 19 foot islander with a 19 pitch prop.
After a complete rebuild I managed to get it up from around 3000 to 3400 rpm, nowhere near the 4500 point.
So I did some checking on lower unit gear ratios and found that for the 140 there is 1.94/20 24, for the 165 six cylinder there is 1.62/24-24. And for the 170 4 cylinder there is 1.81/17-19.
This led me to wonder if I have the wrong gear on my setup and maybe changing out the gear ratio (or the engine) might give the right rpm for my setup.
How can I find out what gear ratio my outer unit has? and does anybody know what rpm to expect when different gears are mixed up with different engines?
As always,thank you for your comments and opinions,Cornelis.

I find the 1984 140 WOT rpm is between 4200-4600. With a 19 pitch prop turning 3400 rpm, and a 1.94 ratio drive, and a 10% slip puts you at 28mph. If you have the HP to turn it at 4500 rpm you would be running 37mph. To determine the drive ratio without taking it apart, remove the drive , hold it in gear with the shift foot and rotate the drive shaft and count the rotations.

What is the diameter of your prop. You could change to a 15 pitch which should bring you up 800 rpm or a bit more and could improve your top end around 29mph

 

[Bondo]

I have had trouble getting my engine -a 1984 140 mercruiser- to run at around 4500 rpm at wot while pushing a 19 foot islander with a 19 pitch prop.
After a complete rebuild I managed to get it up from around 3000 to 3400 rpm, nowhere near the 4500 point.
So I did some checking on lower unit gear ratios and found that for the 140 there is 1.94/20 24, for the 165 six cylinder there is 1.62/24-24. And for the 170 4 cylinder there is 1.81/17-19.
This led me to wonder if I have the wrong gear on my setup and maybe changing out the gear ratio (or the engine) might give the right rpm for my setup.
How can I find out what gear ratio my outer unit has? and does anybody know what rpm to expect when different gears are mixed up with different engines?
As always,thank you for your comments and opinions,Cornelis.

Ayuh,.... Have ya tried a 17" or 15" prop,..??

If I hada Guess, Ya might well have a 1.84:1 or a 1.65:1 on there,....

The ratio, may, or may not be stamped on the upper, in the upper port-side corner of the housin',....

You should have the 1.98:1 ratio,.... I once put a 1.50:1 on that motor, 'n could barely pull 2800/ 3000 rpms, with a 15" prop,...

 

[rolmops]

I will try a 15 pitch prop this spring after the ice is out of Lake Ontario and the stray trees and docks and telephone poles and whatever else the streams flush into the lake will have cleared up so I can open her up without the risk of ending up in a tree 5 miles off shore.

 

[airshot]

I would do a quick go thru your motor to be sure all is in proper adjustment such as timing, compression etc. After a rebuild something could have been missed when set back up. As mentioned a 1.98 ratio is the recommended ratio. Another thing to check is make sure your boat is not water logged with water soaked foam under the floors as this added weight will definitely keep the rpm's down. As a comparison I have a 1983 22' Islander with the 140 hp and a 1.98 ratio outdrive and it does turn 4500rpm with a 19" pitch at 34 mph (gps). You definitely have something going on and need to look at all options. My outdrive has a small code number stamped at the top of the decal on the side, that identifies the ratio. It was the good folks on this site that helped me get the proper ratio for my boat.

 

[rolmops]

I would do a quick go thru your motor to be sure all is in proper adjustment such as timing, compression etc. After a rebuild something could have been missed when set back up. As mentioned a 1.98 ratio is the recommended ratio. Another thing to check is make sure your boat is not water logged with water soaked foam under the floors as this added weight will definitely keep the rpm's down. As a comparison I have a 1983 22' Islander with the 140 hp and a 1.98 ratio outdrive and it does turn 4500rpm with a 19" pitch at 34 mph (gps). You definitely have something going on and need to look at all options. My outdrive has a small code number stamped at the top of the decal on the side, that identifies the ratio. It was the good folks on this site that helped me get the proper ratio for my boat.

On this rig,I have replaced the transom,the flotation foam (with poured in extension closed cell), the floor, the seats and the doghouse. The engine is a complete rebuild with new electronic ignition and rebuild carburetor.Everything works as it should. By now the time has come to look at other factors such as a bottom repaint and whether or not I have the right gear in the outer housing.
Considering all the money I spent on it,I should have bought a 15 year newer Islander and I still would have money to spare..
I will check the outdrive for code numbers. Maybe that will solve the problem.