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#400966 - 04/28/08 10:52 PM Volvo-Penta SX Ratio Problem  
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Ignatz Offline
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I had my outdrive "fixed" at a repair shop last summer. It only needed the skeg repaired. Later I found out my lower unit was actually sent out to another place. No problem I thought.

When I ran the boat it seemed I couldn't get the same rpm and speed out of it as I used to, but I was having problems with my tach so I dismissed it at the time. I had all winter to think about it and I kept thinking maybe they swapped my outdrive with someone else's since the motor still sounded great.

This weekend I pulled it off and confirmed my thought as I determined the ratio was 1.43:1 to what I had as a 1.79:1. The 1.43 ratio seems more like something used on bigger motors than my 4.3GXi.

I don't want to deal with the repair shop and I was just thinking to re-prop with a new prop since I only have one prop anyway. My prop was a 21 pitch and doing some math I was thinking of using around a 17 (1.43/1.79*21).

Can you use a simple math conversion like this to come up with a new pitch?

Is there anything wrong with using obviously wrong drive ratios as long as you come up with a prop in the commonly available pitches?


2005 Chaparral 190 SSI - Volvo 4.3 GXi-SX
2005 Jeep Grand Cherokee - V6
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#401015 - 04/29/08 07:37 AM Re: Volvo-Penta SX Ratio Problem [Re: Ignatz]  
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Dave R Offline
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Raymond NH
While you can use a smaller pitched prop to get the engine closer to it's optimum WOT RPM range, the drive will probably be more efficient turning the original steeper pitched prop at a slower RPM. You may lose a little MPG and top end MPH.

Another fix would be to drop a 5.7 GXi in place of the 4.3...


"Mischief Managed"
2000 Regal 2550 LSC
7.4 MPI Bravo 3
#401023 - 04/29/08 08:40 AM Re: Volvo-Penta SX Ratio Problem [Re: Dave R]  
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seabuddy Offline
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Depending on the boat and the load you should lose hole shot strenght and gain alittle in top speed once you re-prop so the engine is within its rpm WOT range when the pedal is to the metal. The heavier the load and boat, the more you should feel the loss of "getting going". The top speed change should be minor.

Perhaps think of this oversimplied car setup with a manual transmission. Start in first gear and go. Start in second gear from the same dead stop in second gear. Now do both on a steep hill or ramp. These transmission gears represent the reduction gears in the boat both old and new. Now change the tire size and try those second gear starts again. You should be able to improve the starts with the tires (think "Prop") but most likely not so much that you are back to where you where with the first set-up unless you where mis-propped to begin with. This example way over-states the difference, but it may help in understanding what you might be stuck with.

Its been done, but usually not by/with folks that do water sports or have a bigger size, heavier boat for the engine. If its a light boat and you do not load up the boat and/or do water sports, it turns out OK.

#401030 - 04/29/08 09:06 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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seabuddy Offline
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When and if you re-prop and if your engine and drive are out-of-warranty think about a Volvo branded Four Blade and pick one that perhaps lets the WOT rpm be close to 5,200 - 5,500 rpm (over the factory range) for the most getting-going improvement. I do not mean to suggest that you run at these type of engine speeds (lots of wear) or expect anyone to do warrantee work on such a set-up.

Most V-6s that I have experience with have alot of torque for their displacement size, but it peaks way up high on the engine rpm scale (around 4,000 rpms) and you want peak torque to have the most getting-going for many of the times you take off. The Volvo 4 blade sized to this rpm spec should allow the engine to spin faster under getting-going situations but its got good bite, too. Remember, not all 4 blades are the same. Go to a Merc brand High Five for even more getting-going, but it may bog down the V-6 engine. Good top speed with the Volvo 4, not so much with the Merc High Five, in this situation, IMO.

Try not to really shorten your engine life by running your boat with the engine reving too high.

#401032 - 04/29/08 09:08 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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seabuddy Offline
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I agree with Dave R on the milage for what I am talking about.

#401042 - 04/29/08 09:42 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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seabuddy Offline
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But its a small difference.

#401062 - 04/29/08 10:41 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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seabuddy Offline
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Hard for me to say about the top speed.

#401065 - 04/29/08 10:46 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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Maclin Offline
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Argyle, TX
On a more philosophic level, I think I would get it back like it was stock. Think about all the questions that got raised by the change and then think how you would explain all that to the next buyer. You (and subsequent owners) now cannot follow any other similar boat's advice on props and related propulsion and RPM issues. Just wanted to get that out there....

#401067 - 04/29/08 10:51 AM Re: Volvo-Penta SX Ratio Problem [Re: Maclin]  
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seabuddy Offline
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part of the orginal posting...

Quote:
Is there anything wrong with using obviously wrong drive ratios as long as you come up with a prop in the commonly available pitches?


Yes, see all of the above following posts.

#401079 - 04/29/08 11:06 AM Re: Volvo-Penta SX Ratio Problem [Re: seabuddy]  
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lvikingm Offline
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Gallatin,Mo.
If I remember right the gear ratio is always in the upper unit !!! Sound as if the whole unit was changed not just the lower, !!! Check your warranty card as the serial number plate is always on the upper unit and that is where your gear ratio is as well. You really need to find the correct gear ratio as it will never run the same


Larry
#401266 - 04/29/08 07:41 PM Re: Volvo-Penta SX Ratio Problem [Re: lvikingm]  
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Ignatz Offline
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Ignatz  Offline
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You guys are the best. Thanks for all the responses.

Concerning the gears and serial number, Yes the number is on the upper half, I didn't see any sort of numbers on the lower half. I visited the shop during the repair and physically saw the lower half missing and that's when they told me they sent it out.

When I pulled it off I checked the ratio by spinning the shaft and counting the revs at the prop. It came up exactly to 1.43:1.

I love mp V-P, but I can't believe they don't put matching serial number plates on both sections.

I was actually amazed that with such a wrong combination the boat still managed 40mph @4,000rpm. It originally did 47mph @4,900rpm. I think I'll give it a try with the new prop and see what happens.


2005 Chaparral 190 SSI - Volvo 4.3 GXi-SX
2005 Jeep Grand Cherokee - V6
#401270 - 04/29/08 07:53 PM Re: Volvo-Penta SX Ratio Problem [Re: Ignatz]  
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Maclin Offline
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Argyle, TX
I thought it was a combination of the two sets of gears, have to calculate the net reduction. It is that way on some drives, not sure about yours.

#401292 - 04/29/08 09:40 PM Re: Volvo-Penta SX Ratio Problem [Re: Maclin]  
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athiker Offline
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Lake Norman, NC
I don't understand not wanting to "deal with" the repair shop. Buy a new prop?...that's kinda ridiculous! What's the story?

I can see letting some things slide, but this is no small thing if it affects your boat's performance so much. You have a pretty new boat there too...at least the parts you know are original!


2000 Cobalt 206 5.7L EFI Alpha 1
#401427 - 04/30/08 12:22 PM Re: Volvo-Penta SX Ratio Problem [Re: athiker]  
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Frantically Relaxing Offline
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Just using some simple math, a 1.79 ratio prop shaft spins at 79.9% the speed of a 1.43 ratio prop shaft at the same engine speed.

Ergo, take what WAS your prop pitch and and multiply by .8 and you should be in the ballpark:

Old / New
23" - 18"
21" - 17"
19" - 15"

You can also try different diameters. Personally, I think the main reason they make different ratio's is because a big engine needs a high-ratio drive to accomdate its capability without running into the need for ridiculously high prop pitches --And conversely, putting a 13" pitch prop on a 4-banger with a high ratio drive just doesn't sound right, even though it's my belief that it wouldn't make a bit of difference...Example--our Bayliner has a 14.5 x 19" prop behind a 2:1 ratio Alpha. Even at our altitude, I can over-rev the engine with that prop, and I want to try a 21". I believe that if the drive was a 1.65 ratio (78% of a 2.00), a 17" pitch prop would result in the same output as the 21" I WANT to put on, and a 15" prop would result in exactly the same performance I have now.

Actually, I think it would be real cool to test this theory, just to see...
smile

#449355 - 01/05/09 01:56 PM Re: Volvo-Penta SX Ratio Problem [Re: Ignatz]  
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PrecisionMarine Offline
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PrecisionMarine  Offline
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I realize yours is an older post, but I happened on it and wanted to reply.
The Volvo SX is an exceptionally robust drive, and as you've found, available in 8 different ratios (1.43, 1.51, 1.60, 1.66, 1.79, 1.89, 1.97, 2.18) Both the upper and lower units are reduction gearsets- the upper is offered with two differing ratios, the lower in five ratios.
Your 4.3L GXi most likely was originally equipped with a 1.97:1 drive. The 1.43:1 unit was indeed used with higher HP engines such as the 5.7L and 7.4L.
If you have carefully and accurately measured your current drive as having a 1.43:1 ratio, then both the upper and lower were swapped- the upper for 1.79 and 1.97 are common; the 1.43 is different.
For reasons that are a bit too involved for this post (engine torque and HP curves, propshaft RPM, boat weight and design, anticipated prop diameter and pitch) the drive ratio needs to fall within a narrow range.
Yes, you can 'punt' and change only your prop. But to expect best overall performance you should return to your original drive ratio. The shop who arbitrarily changed components did you no favors, and likely charged you dearly to do so.
Best wishes for an easy resolution!

#454587 - 03/09/09 07:07 AM Re: Volvo-Penta SX Ratio Problem [Re: PrecisionMarine]  
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Ignatz Offline
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Ignatz  Offline
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CT
Since the dealer only removed the lower half I only checked the ratio in the lower unit. My original complete unit was 1.79. At the time I didn't realize or factor in the ratio in the upper unit. Because I noticed a problem with speed/rpm and knowing the dealer only touched the lower unit is why I focused my attention on that. After measuring the 1.43 at the bottom and suspecting a ratio swap maybe I jumped the gun since finding a lower ratio is what I was expecting.

As an update to this, after re-installing the lower unit the boat returned back to normal. I have no idea what happened, but it would seem that when the dealer installed it something was obviously wrong. I can't imagine what was wrong, but it's good now.


2005 Chaparral 190 SSI - Volvo 4.3 GXi-SX
2005 Jeep Grand Cherokee - V6
#454637 - 03/09/09 03:51 PM Re: Volvo-Penta SX Ratio Problem [Re: Ignatz]  
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Ignatz Offline
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Ignatz  Offline
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CT
I was originally looking for exactly what the different ratio combinations were. Not what the part numbers where, but the actual ratios. After some googling I put the pieces together. Here's what I found if anyone's interested.


Upper (SX and DP-S):

3857007 BRP HSG AY,GEAR 22:23 (1.43, 1.51, 1.60, 1.66, 1.68, 1.78, 1.95)
3857010 BRP HSG AY,GEAR 21:26 (1.79, 1.89, 1.97, 2.18, 2.32)

Lower (SX):

3855692 BRP G'CASE AY,SX 19:26 (1.43)
3855693 BRP G'CASE AY,SX 18:26 (1.51 and 1.79)
3855694 BRP G'CASE AY,SX 17:26 (1.60 and 1.89 )
3855695 BRP G'CASE AY,SX 17:27 (1.66 and 1.97 )
3855696 BRP G'CASE AY,SX 17:30 (2.18 only )

To get the final ratio value, multiply both ratios together. So for my 1.79 I have 21:26 and 18:26. So the final ratio is (26*26)/(21*18) = 1.79

So my original estimate of manually checking the lower and guessing 1.43 since I knew that was a VP ratio (ignoring the upper at the time) at least the number I found make sense now, 26/18 = 1.44.


2005 Chaparral 190 SSI - Volvo 4.3 GXi-SX
2005 Jeep Grand Cherokee - V6

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