Repower 71 OMC to 4.3/5.7 MPI Merc?

[jstcrzyengh]

Hello All,<br /><br />I am tired of the old OMC drive, but love the boat so I am going to bite the bullet and do a complete repower. The original engine is a 215hp 305 chevy V8. I am considering going to the 4.3 MPI which has 220hp. The boat is a 1971 IMP 21ft. I would like to cruise at 22-28knotts, WOT can be whatever past that. The current engine has no compression in 2-3 cylinders so I have no idea how fast it goes with the current engine. I am betting on a lighter engine, 22-28knott cruise speed, and I would like to be at 2-4mpg (hoping that isnt too hopeful) at cruise. <br /><br />Right now my biggest debate is the 260hp 5.0 MPI or the 220hp 4.3 MPI. I was looking at the 350 MAG MPI Horizon, but that seems to be suited for a big boat that needs to go FAST. This is a fishing boat and to be honest if I can shrink the engine box by going with the 4.3 that is an added bonus for me. Anyone with thoughts?<br /><br />James

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Forget the Mercruiser altogether, go with the Volvo 4.3GXi DP it will do way more than the old OMC ever did, and is still a V6, but it does have the Duo Prop which is really the best part. The Bravo III drive is also a dual prop, but (according to Merc reps) isn't that effecient till 50 MPH. So why have it. The Volvo Duoporps are great. Even at low rpms.

 

[jstcrzyengh]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Why do you say the Volvo over the Merc? What is the advantage to dual props. My plan was a Bravo III the whole time anyway, but I am going to guess that the duo props are more efficient? <br /><br />In your best estimate what am I looking at for a 21ft by 8.6ft boat as far as cruise speed, WOT, and fuel economy with that Volvo for those differnet categories? Also one thing I liked about the merc is the smartcraft. Instead of having to buy a fuelscan, and all of these other meters, I can just use the one. Does Volvo offer anything like this? Also any idea on price?

 

[KaGee]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

You are going to have a major transom rebuild if you try to convert to anything other than what you have.<br /><br />305's are a dime a dozen, just repower, or jump up to a 350.

 

[jstcrzyengh]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

I realize that the transom will have to be reglassed and recut. I already have some quotes on that. Anywhere from $800 - $1500. That is ok by me because if I ever try to sell the boat it will have either that Volvo GXI or the Merc in it and will raise the value of the boat to some degree. Also the smaller V6 gives me more space to work with since I will be building a new engine box for it.

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

The transome is going to have to be rebuilt no matter which one he goes with. The stringer drive he now has has a huge hole in the transom.<br />As far as the Merc vs. Volvo question, the engines are the same GM engines, the drives are the biggest difference.<br />The Volvo DP drive is more effecient at all speed ranges than the Merc Bravo III. As I said before, even the Merc reps admit the B3 isn't much help till you get to 50mph. <br />Another problem is the B3 and it's corrosion problem. The drive only comes with 2 huge SS props and most users complain about the increased corrosion. Merc keeps throwing new zincs at it, but it still doesn't cure the problem.

 

[Capt Ken]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Why spend all the money for dual prop anything. Being all he wants is a dependable fishing boat, go with the 4.3 MPI with the Alpha I drive. Its the least expensive game going and parts can be found on any street corner.

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Alpha drives are the dog style clutch for shifting, (CLUNK in gear) Bravo and Volvo both are cone clutch shifting.<br />Volvo Duoprop = quicker planing than single prop, straighter low speed steering (not the zig-zag of a single prop). On my present boat, I converted my Volvo 280 drive to Duoprop, you couldn't pay me to go back to a single prop.<br />The V6 with a single prop would be marginal at best for a 21' boat. If wanting to stick with a single prop, the 5.0 or 5.7 would be a better choice.

 

[jimmythekid]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

i hate v6's<br /> v8's dont weight that much more, and power, longevity, resale, is probly better. fuel milage might not even change or get worse with the v6. Seems like a v6 would requre more throttle, negating any savings from minus 2 cylinders might give if any. Whatever drive, stick with a V8 you will be glad you did.

 

[jstcrzyengh]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Well, here is how I look at it. The OMC stringer driveline right now probably weighs twice what the 4.3 does. Though if I am wrong please let me know. I have also heard different reports that the 305 in the boat is either 215hp or 245hp. The 215hp makes more sense from a carb'd 305 that is a non roller (1971). The Volvo is 225hp, 10 hp gain, or possibly a 20hp loss. The weight difference MORE than makes up for 20hp. Also the MPI hp and Torque curve is MUCH Stronger on the 4.3 than the old 305. If I lose 8 knots off of my top end speed because moving down to the 4.3 it is WAY worth it to me. I attached a link to some of the pictures. The first picture is the current 305, as you can see the 4.3 would take up a lot less space at least length wise. Second picture shows you the area I am trying to "shrink" basically. Their are "side steps" that hold the fuel tanks RIGHT next to the engine and I want to remove these and move the fuel tank forward for better weight distribution. The third picture is the "side steps" that I mentioned in closer detail. I also showed the helm and how close it is to the back area. Since it needs a new helm chair anyway I am going to go with a flush mount for the chair and than a pull out. So for local trips I can just go with out it, and on long trips I can pull it and throw it down below. Allowing three guys after it is all said and done to fish comfortably. Also excuse the dirty boat, it has since been detailed <br /><br /> http://www.projectfireball.com/IMP/testpage.html

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

First of all, the V6 is only 60 pounds lighter than a V8, and is only 5" approx. shorter. Since all the omc housings and drive are older, they were still aluminum so the big weight savings you were hopeing to make aren't really there.<br />While you may loose your estimate of 8 kts at top end, don't forget that loss also decreases the low end at the same time.<br />While your hopes of a lot less weight and a lot more room aren't there, you are going from probably the 245hp V8 with 4bbl carb to a V6 MPI, that has less HP and torque by a small amount. The use of the DP with bring that lower HP up to what you have now, or maybe a bit of an increase in top speed and getting on step.<br />If you go to a single prop carbed V6 you may find you can't even get on step with a few friends and their equipment on board.

 

[whywhyzed]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Originally posted by jstcrzyengh:<br /> I realize that the transom will have to be reglassed and recut. I already have some quotes on that. Anywhere from $800 - $1500.

wow... that sounds really LOW... I wonder if once they start cutting, the price will double or triple on a hull that old...

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

We have transoms replaced all the time up here for $1200 to $1800. That is new 1-1/2" thick plywood all the way across and glassed in.

 

[jstcrzyengh]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Well, again only losing 20hp I am not too worried about it, but you have seen the pictures I assume you see the space I am working with and trying to consolidate. Would it be worth it to move up to the 260hp 305. Is there a volvo DP equivalent? 90% of my trips will be limited to 20 miles offshore, so not a whole lot. Also we do a LOT of trolling here in So Cal, and instead of having to buy a kicker I like the idea of having the smaller engine to make it easier. However do you think with equipment and a full load of guys that the volvo would be underpowered? I would like to cruise at 23-30 knots or so.<br /><br />James

 

[WillyBWright]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

Way too much consideration for a 1971 boat. No 1971 is worth all this fuss! You have problems you don't even know about. Chances of that boat not having considerable rot in the transom and stringers is ZERO!<br /><br />It never fails. A customer comes in for a repair with a boat they've developed an attachment to, and the boat is just too old to justify it. With preciuos few exceptions, just about every customer that opted for a newer boat was 100% happier that they did rather than waste the money on a boat on borrowed time. WHY SPEND THOUSANDS ON A BOAT THAT's WORTH HUNDREDS??? As far as making the boat more valuable? POPPYCOCK! First you have to find somebody that wants a 34 year old boat in the first place, let alone one that's going to pay an arm and a leg more than it's worth in the Bluebook (yours is too old to be included in any Bluebook). Ain't gonna happen! It's nice to have an attachment to a faithful craft. Trust me, that feeling of nostalgia goes away the second you slide behind the wheel of your new boat and turn the key.<br /><br />I'll give you something of an example...a customer traded in an old POS aluminum boat with a really nice newer 90Hp Merc on it. The motor alone was worth about $3000. The boat motor and trailer were worth $2000. See what I'm saying? The newer motor didn't make the package any more valuable. The package made the motor LESS valuable.

 

[jstcrzyengh]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

There is no rot, had it checked prior to purchase. Also since I would be replacing the transom anyway it really wouldnt matter much if it was, in the transom that is. However to address your points... $10-$15k on a boat I like with a BRAND NEW drivetrain, or $60-$150k on a boat I can't afford... Choice is VERY clear. Unless you want to grant me $100k for my own Parker, striper, Davis, or Radon?

 

[WillyBWright]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

$60K to $150K for a 21 footer? Where are you shopping??? No matter how much $$,$$$ you put into that boat, it'll never be worth more than a grand or two to anybody but you. You could put that same $10K to $15K on a good used boat at least twenty-five years newer than your current one. The choice IS very clear. You're wasting your money on that old boat. <br /><br />Another major problem you risk encountering is with insurance settlements. Say you run over a rock pile and trash a $4000 outdrive. Do you think your insurance company is going to give you $4000 to repair a $2000 boat? NOPE NOPE NOPE! The biggest misconception regarding insurance is "I insured my boat for $10,000". It doesn't matter. No insurance company will ever give anybody more than bluebook value for any boat in any condition at any time or in any place. All you do is pay a higher premium.<br /><br />You might find this hard to believe, but the least expensive part a new package is probably the hull. In most cases, the power source is by far the most expensive part. Often more than the boat and trailer combined.

 

[rodbolt]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

hello<br /> the question of wheter to sink 15K or so into a hull that old is one I cant answer without seeing it.<br /> whoever does the transom needs to add the new stringers that will be required and also set up the engine bed and do the transom cutout.<br /> they should have the omc/volvo fixture to do both.<br /> after that the package is a bolt in.<br />my opinion of the alpha drive is this, unless I had a sale for it you could not give me one. however I make good money on them so I hope they keep making them. the v6 with the xdp composite drive seems to work well. the v8 with the dp-e drive works well. I have 4 crabbers here running the v6xdp on 25 parkers and they work 11 months a year. no pots allowed in the water for febuary. <br /> so is the old boat worthit? beats me. a new nice package in the 19-24 ft range can be had new for 19-28K. check the boat shows.<br /> keep us posted on this project.<br /> the main problem I have here is no one can do the transom/stringer and get it right the first time and I gave up glass work years ago. nasty stuff .

 

[Don S]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

I personally don't care what year someones boat is, if they want to repower and put in a brand new engine package so they can trust the boat to go long trips and the hull is in good shape, no problem.<br />But from the sounds of your posts jst, it sounds like you are wanting someone to say you can put a 4.3L carbed engine with a Mercruiser Alpha drive to save room and weight (neither will happen)and then expect it to run 25 to 30 mph getting 3-5 mpg, well, thats plane crazy.<br />The V6 MPI Volvo DP will give you your 25-30 mph, but 1.5mpg will be more in line with milage.<br />If you go to a Volvo 5.7GXi DP, then you can use a lot less throttle, stay at 25 or 30 and get 3 or 4 mpg.<br />There is just no way in the world you are going to use the smallest engine, smallest drive and get the milage you want. You may get 30 mph max, but you would always be running wide open and getting very minimal milage. It would also be very hard on the engine.<br />As far as insurance goes, I had a customer that had a repower done to his boat. He also updated his insurance about haveing the new package. About a year and a half later he hit a log, poked a hole in the drive and broke the transom shield in the process. His insurance paid for it. But that is something you would have to look at on your own policy.

 

[rodbolt]

Re: Repower 71 OMC to 4.3/5.7 MPI Merc?

I am with DonS.<br /> if your hull is rated for it and done properly a 5.7/dp package is the only way to go. just cause you have the extra ponies and torque does not mean ya have to use it. nothing like an underpwered rig.<br /> I am currently chasing a ghost on a 33 grady with a pair of F225 yams. boat has every option grady offered,up to and including a bow thruster, and curtains on the flybridge as well as 25ft outriggers. the complaint is at 4800 rpm and he hits a wave in the ocean the engines drop 600 rpm and he has to get on the throttle to get it back. the boat is severly underpowered and he will never ever be happy. at 4200 it does not want to stay on a plane. not much I can do to help him