outdrive ratio question

[Quadruplecccc]

i have a 1976 mercruiser 233hp with an alpha drive pushing a 24' crestliner. drive option calls for a 1.47:1 gear ratio. I have acquired a spare outdrive that may be a 1.50:1 gear ratio.

that being said: will I run into any noticeable differences using a slightly higher gear ratio?

 

[Eric 3.7LX Alpha 1]

Re: outdrive ratio question

Not that close in ratio:

 

[Quadruplecccc]

Re: outdrive ratio question

according to that chart I should be using a 1.32:1 ratio? please explain what happens to engine rpms if either one would be used on same boat
Lower gear ratio = higher rpm???

 

[Bondo]

Re: outdrive ratio question

according to that chart I should be using a 1.32:1 ratio? please explain what happens to engine rpms if either one would be used on same boat
Lower gear ratio = higher rpm???

Ayuh,.... I believe the 1.32:1s are out of production,...

Ya gotta run the 1.47:1 / 1.50:1, 'n a higher pitched prop,...

 

[Eric 3.7LX Alpha 1]

Re: outdrive ratio question

The Model 233 is a Ford 351 engine. Is that the engine you have? or your HP?

Either ratio would cause the engine have Higher RPM's at a given speed that would have with the 1.32.

They would both give you a lil' higher WOT RPM.

 

[Quadruplecccc]

Re: outdrive ratio question

yes it is a 351. the drive that is on is the 1.47 with a 17p prop. last yr I was achieving 4k rpms at wot. now only getting 3600 rpms at wot.
was going to go to a lower pitch when I discovered the prop is seized to shaft. I chose to swap outdrives so wouldn't be disabled for long and can work on gettin prop off in garage.
the prop is old so I was blaming that for the lugging since I cant find any other problems. I have a 15p that looks to be in better shape to try.
I'm getting the impression that I should have a higher pitch on there though?
could it be that I actually have the 1.32 on there and didn't know it?

 

[achris]

Re: outdrive ratio question

A battered prop will (in most causes) cause the revs to be higher with a lower boat speed.

 

[Quadruplecccc]

Re: outdrive ratio question

just got done swapping outdrives. ran outta light so 2morrow I will put on the 15p and hopefully can get to a lake to run it and compare performance. will take a 19p along just in case there's a drastic change

 

[achris]

Re: outdrive ratio question

With the drop in revs, I'd also be doing some engine diagnostics. Find out why the revs dropped. Changing to a smaller prop will bring the revs back up, but might just be covering up the real reason....

Standard engine checks..
1. Compression tests.
2. Check spark plugs, replace if needed.
3. If points, replace, along with condenser and rotor. Check and adjust, if needed, points cam dwell angle.
4. Inspect distributor cap and leads, replace if necessary.
5. Check spark quality (needs to be a nice BLUE spark drawn over 1/2").
6. Set timing. Check timing advance is working correctly.
7. Check fuel pump pressure.
8. Check for full opening of the carb. Also ensure choke is opening fully when the engine is warm.

Chris....

 

[Quadruplecccc]

Re: outdrive ratio question

swapped drives and found it is not pumping water. had to put my old one back on and put the 15p prop on. ran it on the lake with no trbl but as expected it achieved 4k rpms and topped out at about 27mph.
changed plugs before running it too. runs fine. just not where I think it should be. maybe im expecting too much?