Gear ratio debate

[1970 Chieftain V]

What are the consequences (if any) of changing gear ratios? Mercruiser says that my inline 6 (160) should have a 1.68 gear ratio. What would happen if I changed that to 1.98? Is it purely a performance issue? Is it a reliability issue? I think I understand that prop shaft RPM will drop with the larger gear ratio. I can control engine RPM by changing prop pitch. I am not trying to second guess mercruiser I am just trying to understand their choice of gear ratios.

Jason

 

[Don S]

Re: Gear ratio debate

If you get the prop pitch out of a certain range, like 15 to 21. Then the performance suffers big time.
Sure you can prop it to get WOT rpm, but the proper ratio will also get you on plane easily, and the best speed.
Too much pitch, and you will be very slow getting on plane, and eventually you will get top end speed. But it will be a dog.

 

[1970 Chieftain V]

Re: Gear ratio debate

Does Merc make gear ratio recommendations based on the engines torque curve?

 

[Don S]

Re: Gear ratio debate

THat's part of it.

 

[1970 Chieftain V]

Re: Gear ratio debate

So in my example if I kept the prop pitch the same and WOT the same I would see a drop in the top speed of approximately 4 mph theortical speed. Would it also be slower out of the hole?

 

[JustJason]

Re: Gear ratio debate

How about accepting the fact that mercury figured it all out, and moving on.
Seriously.....
Merc coulda made the gear ratio 1.xx. As in whatever they wanted.
It's 1.68... and not 1.58 or 1.78 for a reason. And that reason is when you take EVERYTHING into the equation... power, gearing, propping.... 1.68 is what they felt was the optimal ratio for off the line, holeshot, and top end.

The shoulda/coulda/woulda theory game is the high performance world... And in that world you pay no matter what, and there are no promises... because it is all theory, and not laws.... remember, theorys can be wrong.

 

[Don S]

Re: Gear ratio debate

So in my example if I kept the prop pitch the same and WOT the same I would see a drop in the top speed of approximately 4 mph theortical speed. Would it also be slower out of the hole?

That's not how it works. If you change gear ratio's, you WILL not keep the same WOT (Wide Open Throttle) rpm, it's not where you want to move the lever too. It's at WOT.
In theory, if you find a prop that will keep your WOT rpm in the proper range, then you might not be able to get on step without a mile long run to do it.
As said above, This was all done for a reason. If one gear ratio would have worked for every engine, then why would Merc go to all the expense of makeing different ones. Read my first reply again, you seem to have not understood it. The pitch has to be within a certain range to work for general boating, starting, mid range, cruiseing, high speed on occasion.

 

[starsnstripers]

Re: Gear ratio debate

It seems as the ratio changes with horsepower increase? is there a table? like 120-170=1.68 170-210= ?ratio

 

[1970 Chieftain V]

Re: Gear ratio debate

How about accepting the fact that mercury figured it all out, and moving on.
Seriously.....
Merc coulda made the gear ratio 1.xx. As in whatever they wanted.
It's 1.68... and not 1.58 or 1.78 for a reason. And that reason is when you take EVERYTHING into the equation... power, gearing, propping.... 1.68 is what they felt was the optimal ratio for off the line, holeshot, and top end.

The shoulda/coulda/woulda theory game is the high performance world... And in that world you pay no matter what, and there are no promises... because it is all theory, and not laws.... remember, theorys can be wrong.

I don't want to move on! Like I wrote in my original post I am not trying to second guess what merc said is optimal. I am trying to understand what a change in gear ratio does performance wise.

 

[1970 Chieftain V]

Re: Gear ratio debate

It seems as the ratio changes with horsepower increase? is there a table? like 120-170=1.68 170-210= ?ratio

Well I am assuming that merc bases their gear ratio, in part, on the torque curve of a particular engine since horse power is a number calculated from torque. All the books I have read show a gear ratio for a particular engine and altitude.

 

[achris]

Re: Gear ratio debate

It's not just engine performance. It includes the performance of the props too... It's not just a matter of x hp range gets y gear ratio. There are many many things considered. Without a degree in several engineering disciplines there is no way of knowing how they work it out, but they do a good job....

The other thing you could do... ignore all of us, change the ratio and run the boat... then come back here bleating about poor performance... don't tell us you changed the ratio... sit back and laugh as we all run around like headless chooks trying to figure out why your engine don't work right!!!

You have 1.68:1. Live with it, move on and thank Merc for doing the calculations for you.....

Chris!!!!!!!!!

 

[achris]

Re: Gear ratio debate

... I am trying to understand what a change in gear ratio does performance wise.

Makes it bad.....

 

[achris]

Re: Gear ratio debate

It seems as the ratio changes with horsepower increase? is there a table? like 120-170=1.68 170-210= ?ratio

NO!!! I'll post the ratio chart in a few seconds....

Here is is now....

 

[wca_tim]

Re: Gear ratio debate

I beat this one to death some time ago and here's where i wound up...

1. A lot of it (majority) has to do with being in a propshaft rpm range that puts the average application at wot with a 17-23 pitch prop - both for availability and because of the loss of holeshot for a given amount of torque with higher pitched props.

2. if you go to a lower gear (as in from 1.5:1 to 2:1 increasing mechanical advantage) you'll get more torque at the prop, BUT, if you have to use a higher pitch prop to get your wot rpm in the right place, you will only lose overall because the prop is less efficient at using that torque to your advantage - especially in terms of getting out of the hole. The caveat here is that if you can over come the holeshot with a vented prop, you can win here if it's not too extreme because the venting gets your rpm's up on the holeshot and you get some more top end from the higher pitch prop. Wouldn't it be easier to just use the right gear ratio?

Note that this is for the "average" appication for a given engine. a boat that is grossly underpowered would benefit from a higher than standard gear ratio, just as a small light boat with a relatively large amount of power would benefot from a higher gear ratio.

3. I suspect on the higher torque / horsepower set-ups, such as going to 1.32:1 on the short lived big block apha set-ups may also have had something to do with the distribution of strain between the upper and lower gear housing, as well as an effort to gain gear tooth contact area in the upper gear box.

I'm no expert, but I bugged the crap out of a bunch of people asking some of these kinds of questions and followed up with some library work.

 

[1970 Chieftain V]

Re: Gear ratio debate

It's not just engine performance. It includes the performance of the props too... It's not just a matter of x hp range gets y gear ratio. There are many many things considered. Without a degree in several engineering disciplines there is no way of knowing how they work it out, but they do a good job....

Peace, I don't think I ever indicated Merc didn't do a good job. I think they do a marvelous job of selecting gear ratios to meet all around needs. I was hoping that with all the marine technicians on this board there might be some emperical data/observations of "if you increase gear ratio your hole shot gets better but top speed falls off" without having to go through all the calculations. I guess I need to work on a few more degrees to fully understand!

The other thing you could do... ignore all of us, change the ratio and run the boat... then come back here bleating about poor performance...

I don't think you would ever hear me complain about performance. I rarely run WOT. But it might be fun to watch some people.... well nevermind.

Thanks everyone for the discussion

 

[achris]

Re: Gear ratio debate

... But it might be fun to watch some people.... well nevermind.

I don't know about anybody else, but I don't find that sort of thing funny!!! I have given serious answers to some of your threads in the past. When people do things like this, they risk not being taken seriously in the future...

The techs on this forum spend a lot of time give good advise to people. That shouldn't be abused...

Chris.

 

[1970 Chieftain V]

Re: Gear ratio debate

I don't know about anybody else, but I don't find that sort of thing funny!!! I have given serious answers to some of your threads in the past. When people do things like this, they risk not being taken seriously in the future...

The techs on this forum spend a lot of time give good advise to people. That shouldn't be abused...

Chris.

That last part was a joke!! I would never not give the full picture when asking a question because I do appreciate the knowledge and time taken by the techs on this board.

Jason

 

[starsnstripers]

Re: Gear ratio debate

Chris, thanks for posting that chart. Good info there.

 

[achris]

Re: Gear ratio debate

Chris, thanks for posting that chart. Good info there.

Was rather hoping you didn't see that!! :redface::redface::redface:

 

[JustJason]

Re: Gear ratio debate

if you increase gear ratio your hole shot gets better but top speed falls off

if you keep the same diameter prop... and go from a 3 blade to a 4 or 5 blade prop.... and start with the pitch merc recomends in their prop calculator. then your statement would be true.

Seriously, and i'm not being rude here.... But if you spent 12 hours a day for a month reading everything you could about props..... you still wouldn't get it. I wouldn't either. 99% of the people here wouldn't get it. And that is just props.... let alone gear ratios.
But what I do get, and quite a few people get... is that mercury gets it. If i put my trust in their charts and formulas... and if i'm not dealing with high performance... 99% of the time using what merc says will get you there every time.
To be honest, your post is labeled "debate", and there really is no debate about it. It's nothing more than cold hard math. And there is ALOT of math involved in the overall package. I can't explain to you every gear ratio and proper prop and power package. Don, and achris can't either. But we can point you to the charts that have it all spelled out.