Volvo Penta Gearing Calulation

Recently changed my 24' Walkaround from a Volvo Penta 1.6 single prop to a DuoProp. I changed just the lower to a DP 1.78 Gear Ratio. The specs are as follows....

Stock Numbers:

Single Prop SX 1.6-
WOT---4500 RPMs 36MPH.
14.5 x 19p SS Prop

The Boat was always a disaster with Top speed and seemed to never want to plane correctly. I built a high horsepower 302 Ford to replace the 180hp anchor that was in the boat from stock, Played with countless props and could never get the boat to a normal speed without tons of slip or over working the engine. So Fast Forward to Present.....

SX-M 1.6 Upper (stock)
DP-S 1.78 Lower
F3 Volvo Props.
WOT -- 4000rpms 37Mph

The boat is much better overall in acceleration and responsiveness. The Boat rides 1000% better and can easily raise the Bow up with slight trimming as when the single prop drive was on, it would just plow thru the water.
Ive read on another forum that the Ratio is calculated between the upper and the lower? Some say its just the Lower, some say just the Upper? A math equation on another forum states that i have actually went down to a 1.51 Ratio?!? Here is the Equation.

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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


I have a chance to get a 2.38 Upper locally and i would like to possibly explore putting this on and see if it helps. With that calulation and providing it mates up, it would bring me to 1.79 if the equation is correct.




Any Clarification would be helpful!

I’ll post to this in the morning.

Meanwhile… in one sentence you say that you changed the lower unit to a DP 1.78.
Yet in another sentence you refer to an SX and DP-S drive.
These are two entirely different style drives.

RicardoMarine said:

I’ll post to this in the morning.

Meanwhile… in one sentence you say that you changed the lower unit to a DP 1.78.
Yet in another sentence you refer to an SX drive.
These are two entirely different style drives.

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Thanks Ricardo, I hoped you would reply! I'm totally lost w this

Sorry for the confusion with the different Volvo types, as these were the Letters they were sold to me under. This is a picture for the sake not adding to the confusion.
On my Upper its refered to as "Joint Venture" SX-M on the plackard. The lower i purchased 2nd hand off a drive that had the plackard of 1.78.

Sorry for the confusion with the different Volvo types, as these were the Letters they were sold to me under. This is a picture for the sake not adding to the confusion.
Thanks..... that helped.

FYI.... DP typically refers to Duo Prop as in the AQ series family (main suspension fork/pivot tube geometry) such as the DP, DP-A, DP-B, DP-C, DP-E etc.
When I read DP 1.78, I assumed that you had an AQ series drive. I'm now clear that you have one of the Gimbal suspension drives.


On my Upper its refered to as "Joint Venture" SX-M on the plackard.
Yeah..... I think that in order for OMC to save face (during the bankruptcy and sale), Volvo Penta agreed to use the term Joint Venture.
With few exceptions, this new cone clutch design is all Volvo Penta. None of the internal OMC Cobra dog clutch drive parts will interchange with them.
When the Volvo Penta gimbal system drive came out (after Volvo Penta's purchase of OMC in late 1993) the 1994 and later gimbal system drives were the SX or DP-S, etc. Stern drive parts between the two (AQ series and gimbal system) will not interchange.

The lower i purchased 2nd hand off a drive that had the plackard of 1.78.
NOTE: the 1.78:1 is an over-all reduction. In other words, it is a combination of upper unit reduction and lower unit reduction.
My experience is primarily with the AQ series, so I am not sure how this is achieved with the SX or DP-S...... meaning that as far as swapping the SX style upper unit, I do not know what combinations will work in order to reduce the 1.78:1 to a 1.95:1.

With the AQ series, and with the exception to the E drive, all upper units (transmissions) are of the same reduction. Even at that, we have three transmission bearing sizes (from the early 270 to the later C drive) that prevent across-the-board swapping.

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Now, as for the differences in final reduction (i.e., ratio) and as you have noted in your previous post, the single prop reductions are different from that of the Duo Prop reductions.

Even though you have turned your Ford 302 into a higher hp engine, you will need the 1.95:1 reduction.
The small block engines (5.0L, 5.7L, 5.8L, etc) just don't have enough torque to spin a 1.78:1 and yet allow for a good propeller selection.
We don't start using the 1.78:1 until we get into a well built 6.2L (377), 6.3L (383) SBC or the BBC (454).


Bottom line, I just don't know enough about the later SX/DP-S drives to help you with the correct combinations.



.

RicardoMarine said:

Now, as for the differences in final reduction (i.e., ratio) and as you have noted in your previous post, the single prop reductions are different from that of the Duo Prop reductions.

Even though you have turned your Ford 302 into a higher hp engine, you will need the 1.95:1 reduction.
The small block engines (5.0L, 5.7L, 5.8L, etc) just don't have enough torque to spin a 1.78:1 and yet allow for a good propeller selection.
We don't start using the 1.78:1 until we get into a well built 6.2L (377), 6.3L (383) SBC or the BBC (454).


Bottom line, I just don't know enough about the later SX/DP-S drives
to help you with the correct combinations.



.

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Thanks Ricardo, but is it safe to say that that the formula is correct? The engine I build is a small block Ford with added stroke (not too much) achieving numbers of 400hp and 350ft lbs of torque at 5000rpms . This has been verified with an engine dyno, and the exact spec I would have to look for as it's been about 4 yrs.
So essentially using an Upper from the 2nd column of the equation, and the 1.78 Duo lower , will give me 1.78? Seems anything from the first column in the upper category will make me go to 1.51. Assuming they all mate up , which has been partially verified at 18 spline on both units. As long as you dont see any catastrophic failure eminent, I'll do it for science and report back.

Thanks again Ricardo , I'm pretty new to this forum but always come across your post to reference when I'm having any issues with Volvo and your spot on!

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jbyrne23 said:

Thanks Ricardo, but is it safe to say that that the formula is correct?
It's best if you do the math for yourself.

The engine I build is a small block Ford with added stroke (not too much) achieving numbers of 400hp and 350ft lbs of torque at 5000rpm . This has been verified with an engine dyno, and the exact spec I would have to look for as it's been about 4 yrs.
That sounds more like an Automotive build.
Use caution..... marine engines are most comfortable at a high RPM of 3,600 to 3,800 for cruising.
Those hp and torque numbers @ 3,600 to 3,800 rpm will not be the same.

Also, the ignition advance is quite different from that of Automotive.
Too much spark lead too early will cause ignition induced detonation. Very destructive!


So essentially using an Upper from the 2nd column of the equation, and the 1.78 Duo lower , will give me 1.78? Seems anything from the first column in the upper category will make me go to 1.51. Assuming they all mate up , which has been partially verified at 18 spline on both units. As long as you dont see any catastrophic failure eminent, I'll do it for science and report back.
The issue is that most small block engines will not work with the 1.78:1 over-all reduction.
That ratio is intended for the BB engines.

If this was a little light weight 18 foot Go-Fast hull, your engine/drive combination may work.

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RicardoMarine said:

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Not to beat this to death, but how is it possible that the boat came factory with a 5.0FL which appears to be rated at less than 200hp , came with a 1.6 GR? I understand the cam duration torque has a lot to do with what's going on, but from day 1 the boat was a total slug. I am thinking and moving in the right direction going from 1.6 to 1.78 in more rotations at the prop to engine rotation (1 RPM engine = 1.78 Prop)?
The boat currently is going 37 MPH as opposed to the 24MPH it did when I got it, so I'm happy with anything going forward in this project. I would just like to get the RPM up more on the motor.

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jbyrne23 said:

Not to beat this to death, but how is it possible that the boat came factory with a 5.0FL which appears to be rated at less than 200hp , came with a 1.6 GR?
The s/p drive w/ a small V-8 typically requires the 1.6:1 reduction.
The Duo Prop drive w/ a small V-8 typically requires the 1.95:1 reduction.

I understand the cam duration torque has a lot to do with what's going on,
Engine displacement, cam profile, C/R, combustion chamber design, fuel delivery system, ignition advance all play a very important role.

I am thinking and moving in the right direction going from 1.6 to 1.78 in more rotations at the prop to engine rotation (1 RPM engine = 1.78 Prop)?
If you are suggesting that the 1.78:1 will spin the props faster than the 1.60:1 will, your math is incorrect!
The 1.6:1 will turn the propeller faster (per engine RPM) than what the 1.78:1 will (per engine RPM).
However, you must understand that the dynamics between the s/p drive and the DP drive are considerably different regarding reduction.


The boat currently is going 37 MPH as opposed to the 24MPH it did when I got it, so I'm happy with anything going forward in this project.
Understood. The Duo Prop definitely increases performance!
On my twin engine SND F/B boat, I went from s/p's to DPs, and the improvement was excellent.
I've installed many DPs for customers, and all have been very satisfied.

I would just like to get the RPM up more on the engine.
Your OEM specs should give you what's called a WOT RPM..... (wide open throttle rpm).
This is a "test" RPM that the engine should be capable of achieving in order to verify drive ratio and prop selection.
It is NOT an RPM that is to be sustained for any duration, unless you want to shorten the life of your engine!

I would think that the 1.78:1 would over-burden this small V-8.
But like said earlier, the 1.78:1 may work (with the small V-8) on a very light weight hull.

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