Gear ratio change

211libwtfo

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I have discussed on here and mainly betwix my own ears trying to get more zing zang out of my 88 liberator 7.5l king cobra. And I ran into the previous owner who had a 4 blade 25p prop on and said it would eat up most BAJA and others he also mentioned that every time at fast speeds and when the prop would get out of water it would shell the gears. So he replaced with the beefy set. What I?d like to do is change the gear ratio to gain hole shot and top end? An alpha 1 I run with is faster on both of those with a 2? longer boat with a 454. All types criticisms welcome and thanks in advance.
 

Scott Danforth

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What I’d like to do is change the gear ratio to gain hole shot and top end? An alpha 1 I run with is faster on both of those with a 2’ longer boat with a 454. All types criticisms welcome and thanks in advance.

cant have it both ways, you can improve your hole shot at the sacrifice of top end OR you can improve your top end at the sacrifice of the hole shot.

as for an alpha drive in front of a BBC, they dont last long, especially if the drive comes out of the water.
 

Lou C

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If you have a 460 King Cobra keep in mind that that is the one with the standard Cobra upper gear housing and the bigger V8 outboard lower gear housing. I don't think too much is still available for them. Just go easy on it because if it blows up I don't think you can even find OE standard gears for them. Then you'd have to decide what to do as far as options, convert to a Volvo SX or convert the whole works over to a Merc. Either way a lot of $$$$
​I don't think a standard Cobra would be strong enough, they are equivalent to an Alpha, limited to 300 hp.
 

HT32BSX115

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What I’d like to do is change the gear ratio to gain hole shot and top end?
It's really not going to happen. If it has the OEM gear set (1.43:1 if I remember correctly) There are no "beefier" set of gears available for that drive and going to an Alpha would be a reduction in reliability (and you'd need to mate an Alpha with a 454 or other Mercruiser engine and transom mount)

You're NOT going to get higher speed unless you turn the engine above 4600.

My Brother ran mine with the 460 and King Cobra at 5000 RPM using a 20p (19 repitched/cupped) stainless 3-blade prop.

It would do a tad over 60 mph (when it finally got up on a plane) at 5000 or so and had "better" hole-shot than before (It was always a dog outta the hole even when new......my brother bought this boat brand-new) He even installed Bennett Tabs hoping to improve the hole shot.....(it didn't help very much.......if at all......

If you want it to do better "out of the hole", you're going to need to install either a Volvo Duo-Prop drive (with expensive modification) on the existing 460, or (for about the same price) switch to a Mercruiser 454/496 and a Bravo III.
 

211libwtfo

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Yea thanks. I want to do something just not sure which way to go. Would it be better to get a different boat with what I?m looking to achieve but if I go ahead and make the conversion then I wouldn?t be starting over as in reliability....I really like the liberator and it is a solid boat...would like it to be slightly deeper V. To many decisions.
 

Bondo

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Ayuh,.... yer Lib has 2 problems,...
1st is the motor, which parts can't be bought for,....
The 2nd is the drive, which parts are pretty tough to buy parts for,....
 

Lou C

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If the right deal comes along, try doing the same thing that Rick above did with his....ie buy an insurance salvage boat and pull out a Merc 454/B-III package. Perfect for the boat.
​I still have all the OE Cobra stuff in mine because the parts I need except the exhaust, I've been able to get. When the oddball bat wing exhaust went NLA some years back I did some research and found that the center riser style used on the later model Cobras and joint venture Volvos would fit. I had to buy the Volvo style 90* elbow to mate the 4" riser hose to the 3.5" Y pipe and it all fit perfect. When the time comes I'm going to start looking for a Volvo SX single prop drive in the right gear ratio for my 4.3 and pick up the Volvo pivot housing which is needed to install an SX on a Cobra transom mount. That and a Volvo style raw water pump.

​Instead of rebuilding my old Q-Jet again, I was able to pick up all the OE parts needed (ie adaptor, fuel line, throttle bracket) to install a brand new Holley 4160 that is calibrated for the OMC/Volvo V6. OMC used the Q-Jet up to '89 but then changed to the Holley and it was used by them and Volvo till the end of carbed engines. This is the kind of research you have to do to keep an old boat going, unless you want to change everything.
 

211libwtfo

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In passing a season ago or so I mentioned to the trustyest of boat mechanics around, about if my omc took a crap did he have a line on old stock or any contingency plan, and his nontialonte response was to put the ?adapter? plate on.?.?.?
And mentioned putting a bravo on it
 

Scott Danforth

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best advice i have would be following Rick's path with your Libby. sell the OMG and Ford stuff to someone desparately trying to keep the obsolete stuff running. buy a late model BBC/dual prop setup.

Rick came out ahead dollar wise. it took him a while to sell the stuff.

you have already stated you love the boat..... so fix the issues with the obsolete drive and motor
 

211libwtfo

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Yes I do believe you?re right it?s a good boat nice look and with just a lil search there are plenty of salvage boats between Missouri Illinois Wisconsin Iowa and Minnesota...
So I jus need to find out which boats have the combo I?m looking for.
 

Scott Danforth

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just watch out for flood salvage boats.... drives ok, electrics not
 

HT32BSX115

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In passing a season ago or so I mentioned to the trustyest of boat mechanics around, about if my omc took a crap did he have a line on old stock or any contingency plan, and his nontialonte response was to put the “adapter” plate on.?.?.?
And mentioned putting a bravo on it
Yeah....Essentially, that "Mechanic" had no idea what he was talking about!!

There is NO adapter plate to swap out any OMC Cobra.to a Mercruiser Bravo .......... but you DO have to swap everything! to include the the engine and transom mount.

Several years ago, (the Late) DonS did a "Conversion" by adding a Volvo DPS to an OMC Dog-clutch gimbal mount using some sort of Volvo kit (which may no longer be available)

His thread describing it follows:

http://forums.iboats.com/forum/engi...6483-omc-cobra-to-volvo-penta-dp-s-conversion

The problem is you keep an engine that has un-obtanium manifolds (and back then un-obtanium coupler)

Now, I believe you can actually get couplers, but manifolds are still a problem. Not unobtainable but a LOT of money for the suitable replacements.

Going to a 454/496 Volvo DP package would certainly be every bit as good (some say better!) as using a Mercruiser similar setup but neither are all that easy to find and the engine and drive packages are literally worth MORE sitting on the floor of your shop than the entire boat would be worth with them installed in it!! It is what it is.....

If you can find a good deal on a package, it's not that hard to do the swap, it was sort of fun, and the performance improvement made it worth it to me.
 

211libwtfo

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Ah right on. I?m always thoroughly impressed of the knowledge and willingness to help on this site. I?m definitely gonna keep an eye out for this combo. Which one is better, easier to find, or has good part support. Volvo or bravoIII.
 

HT32BSX115

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Which one is better, easier to find, or has good part support. Volvo or bravoIII.

In 2006, originally looked for a Volvo BBC and Duo-Prop combination. Never found one.

Then we had a big windstorm and a wind damaged 98 24ft Bayliner (454/BIII) showed up on Craigslist in Seattle. At the time I was looking at a 2004 300 (prop) hp 5.7 MPI Mercruiser Bravo I Closed cooled with about 400 hrs of salt water use. A Merc dealer up north had it sitting on the floor ready to install

The owner wanted $7500 and turned down my $6000 offer for it. I was about to offer $6500 when the 97 454/BIII appeared (I offered $5000 and got it)

The boat would have likely performed OK (at least as good as with the 460) with the 300(propshaft) HP 5.7MPI+B1 I was "that-close" to buying it and didn't know the BIII would be a better choice.......(in fact, I was apprehensive about getting a BIII because I thought I would lose a LOT of top speed)

Right after I bought the package, a newer 425hp 496/BIII (closed cooled, 80hrs since new) showed up on Craigslist for $8000.

Looking back, I should have bought it because I could have probably easily sold the 97 454/BIII for about $7000+

I saw these deals over about a 6-8 month period.

But even though I saw similar deals, I never saw a Volvo package for sale used and I looked for quite some time.

Bottom line, for every Volvo powered boat out there, there's probably 10 times more Mercruiser boats (and parts etc) . So the chances of finding a Mercruiser package seems more likely.

Knowing what I know now, I would be ok with a 300HP 5.7 or a 320HP 6.2L engine but I would definitely want a Bravo III (or Duo-Prop if Volvo) . A Bravo I might be ok but you already know what the hole-shot feels like with a single prop drive.

If you find a 300+ Prop HP Volvo Duo-prop and the price is right don't hesitate to get it.

By the way, the original 454/Alpha that was installed in the 1987 Four Winns Liberator was rated at 330 crankshaft HP (I think 88 with the 454 was a Bravo) . The OMC 460 was rated at 340 (crankshaft) hp.

Not sure what year it all changed from crankshaft HP to propshaft HP but my 1997 454 is rated at 300 PROPSHAFT HP

I suspect that both the older 454 Alpha and 460 Cobra were around 300 Prop HP plus or minus a few.

Good luck "hunting"!

Rick
 

211libwtfo

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Thanks a lot again. I?m on the hunt!!!! Definitely hoping for the DP....
 

HT32BSX115

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Thanks a lot again. I?m on the hunt!!!! Definitely hoping for the DP....

Let me also add that If I had a "choice" of both, I would probably still go for the Mercruiser. ONLY because (since you're obviously going to install it and maintain it yourself) you will probably find that the used parts availability for Mercruiser is MUCH better than for Volvo.

Case in point......Just did a search for "Boat parts" on eBay.

Mercruiser 71,800

Volvo 25,000

I have done this every so often for several years and the results are approx the same every time I do it.
It doesn't mean Mercruiser is "Better", It just means there's MORE Mercruiser parts out there for scrounging than Volvo. (new would be the "same" for either since you just order what you need from a dealer)

Also, what ever you do. If you see a Volvo XDP drive in any package. RUN AWAY

Just do a search on the XDP. Bad ongoing problems including a class action etc......
 
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