Gear Ratio Best option

[coopin87]

Ok guys im trying to decide if I should change my gear ratio and if so what to. It is currently 1.62:1 that has a 5.7 with roughly 300 hp bolted to it. This weekend I ran it with a 21 pitch aluminum prop and rpms were at 5500 (acurrate confirmed rpms at 4k with two other boats). Not sure on my exact speed roughly 45 mph but that is just guestimating since I didnt have the gps with me.

Bottom line here is im trying to get the most out of my boat for the best cost. I do have options but not sure the best most economical route. Different upper; bigger prop, maybe a venting prop (heard about people using these on bass boats not sure if it would work for me). Im open to suggestions.

Sorry for the rant guys Im just trying to cover my basis before I dump any more money into her.

 

[Bt Doctur]

Re: Gear Ratio Best option

thats not that bad for an AIR BOAT ENGINE and reduction box.
what is this motor supposed to be in.

 

[Don S]

Re: Gear Ratio Best option

We just went through this in your other post about your rpm, gear ratio, and props, no since doing it again.
Here is your other thread. http://forums.iboats.com/showthread.php?t=555337
Which came after this thread http://forums.iboats.com/showthread.php?t=537710

Your V6 engine was replaced with a V8, you now have the V8 built up a little more than stock. You still have a V6 gear ratio, and no you will not fix it with a prop change as stated previously. You might get the rpm in the proper range (I doubt that too) with more pitch but you can't find one with enough pitch. Mercruiser used different gear ratios with different engines for a reason, that is to keep the props with-in a useable range for general use.

Change the gear ratio if you want your boat to ever act like it should...............

 

[achris]

Re: Gear Ratio Best option

Sounds like a non standard engine... What is the spec on maximum revs? (what engine speed does it make the most power?) Prop to that. I'd have a guess and say that 5500 might be a bit to high. It also depends on what you want from the boat, and what the torque curve looks like. If the torque curve is flat enough, then a change to a 1.5 ratio might get the revs down and make the boat a bit quicker.

Without a lot more information (about the engine, boat and what you want to achieve) its really hard to advise.

Chris......

 

[coopin87]

Re: Gear Ratio Best option

So what upper should I go with then Don? 1.50:1? I know there are a few others out there as well I just want to get it right for what I have now.

Trying to take your advice:

Why is there never enough time to do it right the first time,
But always enough time to do it again?

 

[coopin87]

Re: Gear Ratio Best option

Sounds like a non standard engine... What is the spec on maximum revs? (what engine speed does it make the most power?) Prop to that. I'd have a guess and say that 5500 might be a bit to high. It also depends on what you want from the boat, and what the torque curve looks like. If the torque curve is flat enough, then a change to a 1.5 ratio might get the revs down and make the boat a bit quicker.

Without a lot more information (about the engine, boat and what you want to achieve) its really hard to advise.

Chris......

max rpm's are supposed to be 4400-4800
What I want from the boat:
consistancy
decent hole shot
decent top speed
(I know between the two there is alot of give and take)
Boat is a 95 marada mx3
not sure on torque curve :/

 

[Don S]

Re: Gear Ratio Best option

I gave you a link to the proper upper (1.47 or 1.50 same thing) in your other thread. But you wanted to go looking for props and smart tabs.

Remember this reply from your other thread?

The best thing to do is get a new SEI upper unit for your drive, or get the proper gears in the upper. If your tach is accurate and you are actually getting 5500 rpm, then you are over the max rpm by 700 rpm. Since an inch of prop pitch changes about 100 to 150 rpm you will need a 25 to 28" pitch prop. Which is going to kill your hole shot and getting on plane.

http://www.sterndrive.cc/mm5/mercha...gory_Code=MercruiserAlphaGenIIUpperSterndrive

 

[coopin87]

Re: Gear Ratio Best option

My appologies Don. Still trying to learn all this stuff. I did not completely understand how props/gear ratio/ tabs ect worked and still dont completely. But if thats what your saying I need to do then I guess thats what needs to be done.

What should I expect after I replace it from 1.62 to 1.5 as far as speed hole shot ect?

So my current lower will work with the new upper? I take it they are all the same?

 

[Don S]

Re: Gear Ratio Best option

What should I expect after I replace it from 1.62 to 1.5 as far as speed hole shot ect?

So my current lower will work with the new upper? I take it they are all the same?

That is the kind of question you ask in the prop forum. The guys have prop calculators, slip calculators that work on speed, gear ratios, etc. that show estimated speed etc.

Basically props for general boating have a range of 17" to about 23" of pitch. Above or below those, you either loose hole shot or top end. The higher the pitch, the less hole shot.
To stay within those basic limits, they use gear ratios to deal with different engine torque and hp.

Yes, just changing the upper or the gear ratio in the upper is all you need. The lowers are the same.

 

[coopin87]

Re: Gear Ratio Best option

Thanks again don I will post in the prop section to find out what I should expect then. Going to go ahead and order the upper 1.50 that you posted the link to.

 

[coopin87]

Re: Gear Ratio Best option

I have found a used alpha one outdrive with 1.5:1 gear ratio going to try and go take a look at it sometime before the weekend is over. Anything I should look for to tell what kind of condition it is in?

 

[achris]

Re: Gear Ratio Best option

I have found a used alpha one outdrive with 1.5:1 gear ratio going to try and go take a look at it sometime before the weekend is over. Anything I should look for to tell what kind of condition it is in?

If it's not a Gen II (you haven't told us the year :facepalm, make absolutely sure that the water pocket cover doesn't have any signs of melting, and that the gasket isn't leaking.... If you have a pressure tester setup, then take that with you and pressure and vacuum test the drive... Check the condition of the oil. If it's BRAND NEW, be suspicious. If it's not brand new, make sure it's not water contaminated or burnt... And make sure the propshaft isn't bent...

Chris......

EDIT: get the serial number and we can check the year...

 

[nola mike]

Re: Gear Ratio Best option

Also, take off the top cover and make sure the upper gears look ok. And obviously put the drive in forward and reverse--make sure that the prop turns the shaft in both directions when you change gears.

 

[Don S]

Re: Gear Ratio Best option

You have an Alpha I Gen II. The older Alpha I will not work.

 

[achris]

Re: Gear Ratio Best option

You have an Alpha I Gen II. ....

And you know that how? He said the BOAT was '93... As we are apt to do here, not assume the engine and the boat are the same era...

So, OP.... what year is the engine and drive? (serial numbers are better if you have them)

Chris.....

 

[Don S]

Re: Gear Ratio Best option

And you know that how? He said the BOAT was '93... As we are apt to do here, not assume the engine and the boat are the same era...

Chris.....

It's also been a 96 and at the end of a 230+ thread turned into a 95 plus or minus. But there were pictures.

See reply #3 of this thread :facepalm:

 

[coopin87]

Re: Gear Ratio Best option

It's also been a 96 and at the end of a 230+ thread turned into a 95 plus or minus. But there were pictures.

See reply #3 of this thread :facepalm:

Will I be able to tell the difference right away from gen I to gen II? Sorry for any confusion. I know what mine looks like and can compare it lol thats about all I know about outdrives which is why im in this situation in the first place. If I would have known as much as I do now from you guys I would have probally waited and bought a different boat lol. I guess you live and learn.

Not sure where I stated it was a 93 but the boat is a 95 plus or minus. I think the title says 96.

 

[joewithaboat]

Re: Gear Ratio Best option

Will I be able to tell the difference right away from gen I to gen II? Sorry for any confusion. I know what mine looks like and can compare it lol thats about all I know about outdrives which is why im in this situation in the first place. If I would have known as much as I do now from you guys I would have probally waited and bought a different boat lol. I guess you live and learn.

Not sure where I stated it was a 93 but the boat is a 95 plus or minus. I think the title says 96.

The way i tell visually real quick is the trim cylinder location and mounting method...

Gen 2 anchor location is more toward the middle and held on with clips that have plastic covers on them.

Gen 1 anchor location is more toward the rear edge of the drive and held on with nuts.

Hope that helps, you could just give us a picture.

 

[Don S]

Re: Gear Ratio Best option

.

 

[joewithaboat]

Re: Gear Ratio Best option

That is a great set of side by sides !