wrong ratio drive?

[nola mike]

Finally got my "new" outdrive on, and confident enough in the boat for a few WOT runs. Noticed that I'm turning ~4750 rpm/44 mph using my same 23p prop. Old drive would do 4600/49 mph. Guy said the drive was from a 4.3l. S/n isn't readable, and no other markings. Did I get a 2:1 ratio drive instead of a 1.84? I would think there'd be more than a 150 rpm increase, but I don't know. Either way, any reason not to put a 25p on it? Alpha 1 gen 1 btw.

 

[HT32BSX115]

Re: wrong ratio drive?

Well,


You're gonna have to indicate what engine you're running, what you're using for speed measurement (GPS?), and determine if your tach is accurate. (what's your drive ratio?)

Then you can use a prop-slip calculator to figure out where you are.....

12-15% slip is normal for most single prop drives and run-a-bout boats....

A higher ratio drive doesn't automatically mean you're going to go faster.....


Regards,


Rick

 

[Bondo]

Re: wrong ratio drive?

Did I get a 2:1 ratio drive instead of a 1.84?

Nope,.. It would be Well Over 5500rpms...

 

[achris]

Re: wrong ratio drive?

You may have a 1.94:1 drive.... Some 4.3s were shipped with them.

 

[45Auto]

Re: wrong ratio drive?

I doubt if you have a 2.0 drive. Speed and RPM difference is probably due to temperature, location, GPS vs speedo, current, wind, boat loading, etc.

Going from a 1.84 to a 2.0 should put you right at 5000 RPM with the same prop:

Old RPM/Old Ratio = New RPM/New ratio

4600 RPM/1.84 ratio = X RPM/2.0 Ratio

Solve for X: X = (4600/1.84)*2.0 = 5000 RPM

No reason not to put a 25" pitch prop on it if that gives you better performance. Don't be surprised though, when it doesn't .....

 

[nola mike]

Re: wrong ratio drive?

Hmmm. Maybe a 1.94 then. All other variables except drive were the same. Tach is boat tach, but again, same as last year. Speed=speedo verified by GPS.
Now I'm curious...I didn't know about the 1.94. Can I just pop the top and count gears to find out? Wil this be a problem or can I up prop?

 

[45Auto]

Re: wrong ratio drive?

I would guess that something has changed besides your drive ratio (maybe fuel load, trim, etc). Going from a 1.84 at 4600 RPM to a 1.94 or 2.0 at 4750 RPM shouldn't cause a 5 MPH difference in speed.

Your original set-up at 4600 RPM and 49 MPH showed just over 10% slip. Using the same slip at 4750 RPM with 1.94 and 2.0 ratios shows about a 2 MPH difference, not 5 MPH

 

[achris]

Re: wrong ratio drive?

Hmmm. Maybe a 1.94 then. All other variables except drive were the same. Tach is boat tach, but again, same as last year. Speed=speedo verified by GPS.
Now I'm curious...I didn't know about the 1.94. Can I just pop the top and count gears to find out? Wil this be a problem or can I up prop?

yeap, just undo the 4 screws and count teeth. Screw torque is 20 lb ft. Be careful of the thin 'o' ring under the lid. I posted a teeth/ratio chart a week or so back.... Do a search on mystery ratio, I think.

Chris..........

Edit: Found it.... http://forums.iboats.com/showthread.php?t=385540&highlight=mystery+ratio

The chart is for the ratio with the older 17/28 lower. If you have a later model lower, divide the chart ratio by 1.0185. That will give the new ratio.... So if the top box tooth count is 20/24, then the ratio with the older lower would be 1.98. if you divide that by 1.0185, then you have 1.94. Merc changed the tooth count on the lower to a 14/23 in the late 90's, hence the slightly different overall ratio.

 

[nola mike]

Re: wrong ratio drive?

thanks chris, i'll check that out later in the week.
@45: that 44 mph may have been at 4600 rpm. i didn't keep it over-revved for long.

once again though, the fact is that it is over-revving--any problem with going with a 25p prop?

 

[nola mike]

Re: wrong ratio drive?

just as an update, i got a 25p prop for the drive. last time out with a full tank of gas i'm at 47 mph @ 4500 rpm. tried counting the teeth, but it was tougher than it looked and i got impatient. definitely not a 1.84 though.

 

[fishinpa]

Re: wrong ratio drive?

yeap, just undo the 4 screws and count teeth. Screw torque is 20 lb ft. Be careful of the thin 'o' ring under the lid. I posted a teeth/ratio chart a week or so back.... Do a search on mystery ratio, I think.

Chris..........

Edit: Found it.... http://forums.iboats.com/showthread.php?t=385540&highlight=mystery+ratio

The chart is for the ratio with the older 17/28 lower. If you have a later model lower, divide the chart ratio by 1.0185. That will give the new ratio.... So if the top box tooth count is 20/24, then the ratio with the older lower would be 1.98. if you divide that by 1.0185, then you have 1.94. Merc changed the tooth count on the lower to a 14/23 in the late 90's, hence the slightly different overall ratio.

???????????

 

[achris]

Re: wrong ratio drive?

???????????

If you're querying the later part of the post you'll need to go back and find another one of my posts a few days later then that one. I explain, in detail, what happened with the lower ratio change.

Chris......

Cant imagine why so many wont or dont add their geographic location to their profiles !

Yep, me too....

 

[achris]

Re: wrong ratio drive?

???????????

This is it....

http://forums.iboats.com/showpost.php?p=2667638&postcount=13