4.3 L Mercruiser engine HP ID and drive unit questions
 

I have a 1990 1850XL with a 4.3 L Mercruiser engine. I need to figure out the HP rating.
I also have a drive unit dilemma. I have discovered that the original alpha one drive with a 1.84 drive ratio was replaced with an older model 165 unit having a 1.65 drive ratio. The installed unit requires repair. I am looking for info/ pros and cons for repairing the 165 model vs going back to a rebuilt original alpha one with a 1.84 drive ratio. I am not sure how much effect on performance this will have.
This also brings prop selection into play.

Lots of questions.
Tks.

Re: 4.3 L Mercruiser engine HP ID and drive unit questions
 

1.84/1.65 = 1.11 and about the same as comparing a 21 to 19 pitch props since 21/19= 1.105. My point is if you go back up to 1.84 ratio you could probably also go up from a 19 to 21 (or 21 to 23) for example.

What you really should know is your current WOT RPM with what you have. If the 1.65 drive resulted in a little low RPM you had the wrong prop and the 1.84 change would bump that back up and let you go back up to proper WOT speed with current prop and better performance. The WOT recommended range is usually 4400 to 4800 and that's also 1.09 ratio for the range you can live within.

If you have a nice SS prop in good shape but your WOT has been low the 1.84 is the better choice.

If you have a beat up aluminum prop then make your outdrive choice, run with current prop and see what you get, then change the prop pitch to get a better match to WOT speed.

Props are super easy to change. I run a 21 if it's just my wife and I going for a ride (mostly) but put on a 19 if we have a full load and pull tubes. My Alpha is a 2:1 and we have the 135 hP 3.0 liter. Those two scenarios are actually enough different to warrant running a different prop but the 19 was original.

Re: 4.3 L Mercruiser engine HP ID and drive unit questions
 

Thanks. I made the decision to go back to the 1.84 yesterday. It was the original configuration from the factory. I do have a basically brand new aluminum 3 bladed 19 prop from an old boat of mine and a beat to hell 4 blade from the previous owner ( not sure but I think it is a 19 as well). I plan to start with the 3 blade and see what it gets me. Thanks again for the input.