Changing gear ratios...

[halltimothy]

I'm changing out lower units on my Johnson 140 Looper. My old LU had a 2.25:1 gear ratio and ran at 5800 RPM WOT with a 14x21 SS prop. New lower unit I'm putting on is a true V6 lower unit with a 1.86:1 ratio. According to my quick math the size I should put on would be roughly a 15x17 SS. Just wanted to get others' opinions/suggestions

Thanks

 

[Frank Acampora]

Re: Changing gear ratios...

Very likely, even with a prop change, that gear ratio will reduce performance. I can't speak directly about OMC, but on all my Force engines, changing lower unit gear ratios from 1.73 to 2.00 and going up 2 inches in pitch results in increased performance. They usually gain 3-8 MPH top speed. Thus, going the other way should result in decreased performance. In fact, when I lost the 2.00 gears on my Chrysler 140 I replaced the lower unit with a 1.7 ratio and it just strangles the engine.

 

[MikDee]

Re: Changing gear ratios...

IMO, I agree with Frank,,,

 

[Maclin]

Re: Changing gear ratios...

I also think that is too big a jump to cover with prop pitch unless you are putting it onto a different and lighter boat.

 

[Bondo]

Re: Changing gear ratios...

Ayuh,.... After doin' a few "Experiments" with Merc alpha drives, on my barge,...

I've found that the mathamagical equation answers don't always happen in real life, in the water,...

.39:1 Is a steep jump/ drop,...

 

[dazk14]

Re: Changing gear ratios...

Agree with above, however I realize the V6 lowers are much easier to find than the odd ball 140 looper large gearcase lower with it's 2.25ratio. The 120 looper - same displacement were normally fitted with the small gearcase.

That said, given that you're pulling a 21pitch, it must be a fairly light boat, so you would NOT want a large diameter prop ~15".
Stick with the ~14" versions of props. I'm guessing you run ~45mph.

A 14"x17" would be a good place to start.

 

[jestor68]

Re: Changing gear ratios...

You should test the current prop with the new ratio and adjust from there.

What kinda boat are we talking about?

 

[dazk14]

Re: Changing gear ratios...

You should test the current prop with the new ratio and adjust from there.

Since you own the prop, that's great.

Also note, I think you may fall so far out the peak power band that the rule of thumb with pitch won't be applicable... You will likely see more than 150/200 rpm increase per inch drop in pitch.

I won't be surprised(WAG) to see the 21" run 4600rpm and a 4" pitch drop will probably yield in the neighborhood of 1,000rpm increase.

I don't have the WOT rpm range for that motor in front of me.

 

[Bondo]

Re: Changing gear ratios...

Since you own the prop, that's great.

Also note, I think you may fall so far out the peak power band that the rule of thumb with pitch won't be applicable... You will likely see more than 150/200 rpm increase per inch drop in pitch.

I won't be surprised(WAG) to see the 21" run 4600rpm and a 4" pitch drop will probably yield in the neighborhood of 1,000rpm increase.

I don't have the WOT rpm range for that motor in front of me.

Ayuh,... With the smaller motor, goin' to taller gears,...

I'm thinkin' yer guess might be not low enough,...

Maybe a 14x15, or smaller yet,...

In comparison, I had a 3.0l Merc alpha, that with it's 1.98:1, 'n a 14odd"x 19" prop would pop outa the water, 'n run 'bout 30 mph at wot,...

That same motor, with a 1.50:1 outdrive, even a 15" pitch prop it couldn't get up on plane,....
Just Bogged out at 2500 rpms or so,...

Side story,...
That day with the little motor, 'n the tall drive, I was tryin' a 15x15 prop, 'cause it was the Only 15" pitch I could find locally(pre-internet days),....
As I said, it just Booooogged Badly,...

Pulled into a Bud's dock, pulled the brand new prop off, 'n headed to 'is workshop,...
I found his jigsaw, 'n commenced to carvin' a full inch off each blade,...
Figured if I couldn't find a shorter pitch, I'd try a smaller diameter,...

Anyways,... After cleanin' up my mess, I bolted it back on, 'n gave it a Test run,....
Same Booooog,.... mighta picked up a hundred or two revs, but virtually, No change,...

In the end, I went with Bigger Horsepower, insteada findin' the Right drive ratio,.... :laugh:

 

[dazk14]

Re: Changing gear ratios...

gotta brush up on my Bondo readin' skills.LOL All good. Punching in Bondo's original setup 1.98, 19"Pitch, 30mph, 15%slip, yields nets only 3884 WOT RPM max.
That rig is starting as a Bowser:whip:<-----Bondo. LOL

 

[Bondo]

Re: Changing gear ratios...

gotta brush up on my Bondo readin' skills.LOL All good. Punching in Bondo's original setup 1.98, 19"Pitch, 30mph, 15%slip, yields nets only 3884 WOT RPM max.
That rig is starting as a Bowser:whip:<-----Bondo. LOL

Ayuh,.... My bad, for tryin' to remember, off the top of my head, what I did, nearly 20 years ago,.....

 

[Bondo]

Re: Changing gear ratios...

Can I get an AMEN!!!

How 'bout an Ayuh,.... :lol:

 

[dazk14]

Re: Changing gear ratios...

And the congregation :hail: shouts back an "Ayuh!" to the Good Reveren' Bondo.

 

[phillnjack2]

Re: Changing gear ratios...

some of the folks above must of had some real crappy engines when they tried their props.

the 140 loop charged engine will have no problems at all dealing with a 1.86 gear ratio !!!!!!!!
people don't seem to know that some of these engines had ratios very similar when they first came out.
now even the much older 140hp crossflow had 2.1 reduction and was only 140 at the crank not the prop !!!
I have a 13/3/4 x 23 pitch stainless prop, these props were originaly made back in the early 1970's for the 1.6 litre 140hp engines
that ran 2.1 reduction gearboxes.
Now comparing an old 140 crossflow to a 140 looper is crazy, old was rated at crank and looper at the prop.
the looper is miles more powerfull and will have no trouble at all pushing a 14 x21.

 

[dazk14]

Re: Changing gear ratios...

some of the folks above must of had some real crappy engines when they tried their props.

the 140 loop charged engine will have no problems at all dealing with a 1.86 gear ratio !!!!!!!!
people don't seem to know that some of these engines had ratios very similar when they first came out.
now even the much older 140hp crossflow had 2.1 reduction and was only 140 at the crank not the prop !!!
I have a 13/3/4 x 23 pitch stainless prop, these props were originaly made back in the early 1970's for the 1.6 litre 140hp engines
that ran 2.1 reduction gearboxes.
Now comparing an old 140 crossflow to a 140 looper is crazy, old was rated at crank and looper at the prop.
the looper is miles more powerfull and will have no trouble at all pushing a 14 x21.

I agree with your 140hp comparisons, however, doing some gear ratio math, he'll lose 900rpm on paper, I'm not understanding your comment of'" no trouble at all pushing a 14x21" Prop.

 

[Faztbullet]

Re: Changing gear ratios...

Been there done that with a customer... Reducing prop pitch will allow it to reach recommended rpm at WOT but it will be a dog on hole shot and up to the 3000 rpm range.

 

[halltimothy]

Re: Changing gear ratios...

I think I'm just gonna stick with the proper gear ratio (2.25:1) on this one. Only problem, can't seem to find a V4 lower unit that has the large hub aka the Bigfoot. Has anyone had success putting a regular V4 Looper lower unit on the 140 Loopers? I've found one that came off a 90. Would this be too much power going through the gearcase? Boat weighs anywhere from 2500-3000 lbs depending on what I'm doing that day.

Thanks

 

[halltimothy]

Re: Changing gear ratios...

I think I'm just gonna stick with the proper gear ratio (2.25:1) on this one. Only problem, can't seem to find a V4 lower unit that has the large hub aka the Bigfoot. Has anyone had success putting a regular V4 Looper lower unit on the 140 Loopers? I've found one that came off a 90. Would this be too much power going through the gearcase? Boat weighs anywhere from 2500-3000 lbs depending on what I'm doing that day.

Thanks

 

[Faztbullet]

Re: Changing gear ratios...

The early loopers had the small gearcase and they will hold up as long as you dont have a hot foot....

 

[phillnjack2]

Re: Changing gear ratios...

Dazk14
I agree with your 140hp comparisons, however, doing some gear ratio math, he'll lose 900rpm on paper, I'm not understanding your comment of'" no trouble at all pushing a 14x21" Prop.

what I am saying is the old 140 crossflow of just 1.6 litre can push a 14x23 prop easy with 2-1 ratio.
so a 2 litre looper that is way more powerfull in all aspects will push a 14x21 with ease using a 1.86 ratio, unless its worn out.
when you look at people racing the v4 loopers they use much bigger props and get the ratio or around 1.8-1 on smaller lower units as they can.
smaller gearbox = less drag, and unless your making an extra 100hp from the powerhead you wont strip the box of any gears.