Repower from 5.7 to 8.1 engines

[Vince_nj1]

Another buddy of mine has an '84 260 with twin 350's in it. It handles that weight very well also. Downside is the bilge is very, very tight and it doesn't perform quite as well as a single 502.

Was that a Sea Ray option? A 260 with twin V-8's?

 

[Tail Chaser 661]

Was that a Sea Ray option? A 260 with twin V-8's?

Actually its a single screw.
I edited the S out once.
But the S snuck back in engine.

 

[Tail Chaser 661]

TC,

I boated for almost 20 years out of Long Beach and have countless trips offshore mostly to Catalina and The Channel Islands. I had the afore mentioned Cobalt with a 260hp/Alpha and an '87 24' Sea Ray Sorrento with the same engine/drive package. The listed dry weight difference between your boat (5200) and my old 24 (3900) is 1300 lbs. Fairly substantial. My point is this: when I was in rough water going to or coming back from the islands I could not go as fast due to sea conditions. There were very few times I ever run wide open even up here on the glassy smooth waters of the Delta. I'm confused by your need for the additional top end speed. :huh:

Also you say you want to trailer your boat. Everyone has said that until they enjoy the ease and convenience of a slipped boat. Our Cobalt was in Alamitos Bay from the day it was new (1985-its still there by the way) and we had a bottom cleaning service twice a month so the zincs were changed regularly and it had a MerCathode system. That drive did not have a spot on it for 10 years and then the MerCathode system quit. There is nothing wrong with keeping a boat in salt water. Look at all the boats in Marina Del Rey and all the other marinas up and down both coasts.

I know you are getting lots of opinions here and you will make your own decision but to add one more....spend the money on the lady that has all that you need and sell the other one to some one looking for that lady.

One last thing. Our '89 340 has a v-berth that is wider than a king size bed. It comfortably sleeps me at 6'2", my wife and 2, 85lb dogs. The aft cabin berth is slightly narrower than a queen. It is 12' wide has a really great cabin layout and a huge cockpit. YachtWorld currently lists 7 Sundancers or Expresses in Southern Cal and and a total of 12 in California priced from $34,500 to $69,000.
http://www.yachtworld.com/core/list...&spid=104&slim=quick&No=0&Ns=PAll_sortPrice|1

I wish my boat only weighed 5200 lbs. Life would be simple.
When I bought it. I put in on a Cat scale on the way home.
9000 lbs sitting on a 3 axle trl full of fuel.

I'm not after top end speed.
I just mentioned that from the engine boat test reports.
The small block and big block got about the same mpg at there cruise speeds.
You open up the throttle on the big block. It will burn much more fuel.
Will also get you a much higher top speed also.
Just a little xtra to smoke your buddys on occasion:wow:


After more checking.
Looks like 26' Sea Rays With single smallblocks are using a 2.20 gear ratio min. Spining a 20" pitch duoprop. Sometimes at 3800 to 4000rpm to stay on plane.
The test used a 1.80 GR with a 24' pitch on the smallblock with a 26' boat.
So with a heavy Sea Ray. Them numbers are off 800 rpm.
26kts@4300 :smt018

Maybe 18kts@3500rpm:smt089

Thats about where Im at now.

 

[Tail Chaser 661]

The weight of the big block is not even noticeable. I have added aluminum headers (which saved a little) with IMCO's Sound Advantage and full closed cooling (which put some back). I even put 4 six volt batteries in the bilge at 80 pounds a piece, and it didn't make a difference. Oh, and the risers sit higher than the small blocks as well, so no water intrusion problems.

Another buddy of mine has an '84 260 with twin 350's in it. It handles that weight very well also. Downside is the bilge is very, very tight and it doesn't perform quite as well as a single 502.

The short of it is I (and friends and family) have built and run these boats with countless propulsion options. We all agree the single Big Block is the best way to go. There is no debate among us.

I don't want to ruffle any feathers, but a 383 won't touch a 502.

All this talk is giving me second thoughts about selling mine, which is inconvenient since I have a buyer.

All your info is priceless keokie.
Its from hands on, what works and doesn't.
And from four almost identical boats having different engine setups.:thumbsup:

What I have found researching lots of forums, test etc.
Boats weighing 7500 lbs don't work well with single 5.7 smallblocks.
And 26 boats in general. It seems maybe 1/4 of the owners are not happy with there small block.

Merc 8.1 375hp@4800rpm
It has 490ft/lbs @3200rpm

Merc 383ci 325hp@5000rpm
is 390 ft/lbs from 2800-3500 rpm

Merc 5.7 260hp@4400rpm
350 ft/lbs tq@3500rpm

Every 50ft/lbs of tq is good for a 2" prop pitch increase.
Which drops the rpms down 400 rpm per 2" pitch increase.
So yes, A 502ci would smoke them all. :grin:

I'm after the most reliable saltwater, Good performing package possible.

I still think I like a simple carburetor over EFI
Rumor says EFI gets up to 30% better mpg though. :huh:

 

[Tail Chaser 661]

Berth Control -- one point of correction. In the 80's models, the 30' had an 11' beam. You don't get 12' until the 340. (11'11")

Tail,

I have been reading this thread with interest and amusement. I have been down this road when I had a partner on an '85 23' Cobalt. Without going into a lot of detail he wanted the boat to be faster and against my arguments made the engine switch. This began a spiral down into a bottomless money pit. One change led to another and to another and the investment very quickly became many, many times what the boat would ever be worth. (I exited the partnership before I lost a lot of money) My partner claimed it was OK, that he loved that boat and would never even consider selling it.

About 3 years later he saw a good deal on a 2-year old 28' Chris Craft Corsair and fell in love. I don't need to continue. Everyone can guess the end of the story.

This was a very open saga as my partner owns a couple of very popular waterfront restaurants in Long Beach, CA and it was discussed a lot by many. During a discussion one day, a very seasoned old boater gave some very sage, if unheeded advice:

"I you have a wife or a woman you are not happy with , trying to change her will never work. You need to just get another one the way you want her in the first place."

Regardless of how much you love this boat or intend to keep it. Things change, life happens....

You sound pretty convinced this swap is what you want to do and I doubt anyone on this forum is going to change your mind. You need to just understand that you WILL lose money on this deal and most likely a pretty good amount. For every one of these swaps that go well the are 50 that are disasters.

Good Luck

I hear Ya, NorCal
To me, its just a tool for the job. I started out with a very strong capable/comfortable offshore cruiser.
I always like taking nice quality things, Then making them better.
Ive already been through every screw and wire on this boat.

Where this boat comes up short is....
I dont trust the Alpha 1. Boat came with a pitted overheated upper pinion gear. A Merc mechanic was suppose to change it out. 50 hrs later. Broke the gear:smt013

Then without doing my homework.
Bought a brand new Alpha 1 in 04.
I'm babying that new drive now.
I should have bought a bravo back then.

Small blocks are most reliable at 2800/3000 rpm.
I'm only doing maybe 16kts at that rpm. Very Boring.

So I need to change the engine and drive to achieve what I want.
That's, Maximum reliability with good performance.
Anything less, I might start thinking of another boat.

Buying another new/used boat. I could get a bad drive or engine.
Then I would really be money in the hole. Used boats always need work. Atleast I already know what this boat needs.
Its upgrades only to my specs. Not repairs.
Everything works on this boat. So there is no urgency. I'm just doing the homework early on my time frame.

Back in 1997 I bought new a Dodge Clubcab PU 4x4 with the 5.9 Cummins. With a 5 sp manual. Trk had 215hp with 440lbs tq.
About $8k later.
Ive got 380hp, 850 ft/lbs tq, 8 sp manual trans with other goodies. :grin:
The stock truck would have probably pulled my Sea Ray 35 mph on the big grades near my house.
I can pull them hills at 70 mph at 1/4 throttle with the upgrades the truck has now. Still get 20mpg empty.
Just improved the tool for the job.
Money well spent.:grin: Truck still runs perfect.
Then again, some people might say, go out and buy a new $50k Diesel PU :huh:
In the end, They all cost money and go from Point A to Point B.
O' Yea, The trouble with women is...
They are always changing.
No way to ever keep up with that :lol:

 

[Tail Chaser 661]

Norcal -
thanks for the correction, you are right about the beam.

Unfortunately for tail chaser, I think you are right about the motor swap too. I have talked to other people that swapped motors, and it seems to always cost more than they thought.

To give credit though Tail Chaser - you seem to have done way more research than most people, and you are planning on changing the drive, which is a spot where others have run into problems. It seems that the people that run into the problem are the "I can get a rebuilt 454 from my buddie's truck and sell mine and it will only cost a hundred bucks" crowd.

Here's a thought though. Your current motor gets the job done, it just has to scream to do it. If you are not rebuilding it anyway - you don't need the block or components - why not just let it scream until it or the drive dies? You may get five seasons out of it as is. If it ain't broke....

I do plan on putting more hrs on it before a repower.
I just wanted to know the exact best performing setup/options for this boat.
Boat probably has the orig engine, 740 hrs
One cyl down to 119 psi all others over 150psi.
I take my time and pamper this engine 2800/3200rpm
With an occasonal 3600rpm thrill.
Got 4200 rpm when the girl friend pissed me off :lol:
I traded that one in. Got to find me a upgrade now :grin:


To me. What would be a waste of money is.....
Blowing up this engine. Then for a quick fix. Or not knowing better.
Spending 5K min on a new small block.
Then still having poor performance due to the Alpha 1.
Worse yet, Is finding out my boat will not perform good with any Small Block. No matter what drive I use.
Its better to pay 17K and have a bitchin boat.
Then to pay 6k. Then think to myself while racing sailboats across the Channel. :smt089
Hey!... At Least I'm on the water. I wonder what my next boat should be. :huh:
I could go cheap. but its too dangerous being broke down at sea.
This boat turns sideways into the swell the moment you turn the engine off. I have shipping lanes to cross also.

This boat gets the best of everything.
I really made that decision after a certified Merc Mechanic did not change out a drive gear HE told me about.:smt013

 

[keokie]

Tailchaser, we are kindred spirits.

If you buy a newer bigger used boat, you will find plenty wrong with it, and it will be expensive to fix. And, will likely continue to break.

I put a lot of money in my 26, but I have used it for years. Further, I have never been on the wrong end of a tow rope. I have taken it to very remote places. Confidence in its reliability has been paramount.

I could have had a considerably larger boat for about the same money, but it would not perform nearly as well, nor would it offer the reliability.

As far as keeping a boat in a slip. It is a wonderful convenience, but it is also a far more expensive and much more difficult to keep the boat nice.

I think Tailchaser fully understands the trade offs with putting a lot of money in that old boat. I understood the trade offs as well and accepted them.

 

[NorCal Boater]

Tailchaser, we are kindred spirits.

If you buy a newer bigger used boat, you will find plenty wrong with it, and it will be expensive to fix. And, will likely continue to break.

I put a lot of money in my 26, but I have used it for years. Further, I have never been on the wrong end of a tow rope. I have taken it to very remote places. Confidence in its reliability has been paramount.

I could have had a considerably larger boat for about the same money, but it would not perform nearly as well, nor would it offer the reliability.

As far as keeping a boat in a slip. It is a wonderful convenience, but it is also a far more expensive and much more difficult to keep the boat nice.

I think Tailchaser fully understands the trade offs with putting a lot of money in that old boat. I understood the trade offs as well and accepted them.

My last post on this and then I'll leave it alone.

I bought my 1989 340 3 years ago this March. The boat had 440 hours on the engines then and 620 now. Since I bought the boat I have changed raw water impellors, tuned the engines, changed to synthetic oil and changed a raw water impellor on the genset. Mechanically, that's it. My biggest expense to date was the extended swim platform. That was choice, not need. I have updated the upholstery in the cabin....choice, not need. I replaced both mattresses....choice, not need. I replaced one bilge pump....need, not choice. Cost $120.

I don't understand the statements about buying a bigger newer used boat that will continue to break. Continue to break???? How do you qualify that statement? There are many, many boaters that have never been on the wrong side of a tow rope. I'm one of them.

TC....I hate to break this to you but you already have an old used boat. It's 24 years old. And you're about to put a s**tload of money into an old, used boat and since you said you've done a lot to get it the way you want it, I'm betting you've already put a lot of money in it.

I own a 20-year old boat that I love. It has more beam and space than 34 footers 10 years newer. But it's a 15,000 pound boat with gas big blocks and it burns some fuel. But I wouldn't spend the money for 8.1s because a friend got to the fishing hole or back to the marina 10 minutes before I did.

There are plenty of great boats out there that are 15-20 years old that have been meticulously maintained and therefore as dependable as yours Koeki.

Tail, it's your money and your boat. Good Luck whatever you decide. I'm out.

 

[Tail Chaser 661]

Re: Repower from 5.7 to 8.1 engine

After many hours/weeks of research.
My conclusion is.......

On a heavy 26' boat.
7500 lbs or more.
Not one performance/power issue ever with a big block engine.

You will get the same or better mpg with a big block EFI over a small block EFI engine.
2.0 MPG Seems to be the magic number.
Unless you run the Big Block WOT or Close to it.
That's with a 8.1 EFI engine

The 7.4 Carb engine has too many drive variables for consistent numbers on MPG.
Not a lot of MPG reports on the 7.4 engine either.
1.4 mpg with good performance. Seems to about right with the 7.4 engine.
That's what my 5.7 260 hp carb gets with poor performance.

My conclusion with all Small Block 5.7-6.2ci engines is....
You will always be turning 3500 to 4000 rpm to cruise and stay on plane.
Some heavy 26' boats work good with the small blocks.
There's also a uncomfortable amount 26' boats that perform very poorly with the Small blocks. Its a crap shoot.

I would like, and I could use a bigger boat.
But a non trailable boat sitting in a slip is out of the question.
Some Marinas, There is no vacancy also.

So my homework is complete. :smt038
Now I can have my life back again :lol:

Hmmmm....
I wonder what the MPG is of a Carb 502ci engine :smt017 :wow:

I think I may be giving the wrong impression that with the big engine I want a go fast boat.

NorCal Boater.....
Here's my logic...
SBC 20kts@3500rpm?
That's Max efficient engine cruise rpm with a 6.2engine bravo 3
BBC 20kts@2500rpm?
Engines not even working with the 8.1 engine bravo 3
My 5.7 260hp 18kts@3500rpm

Lets say i'm doing my 54 mile MDR to the east end of Catalina Island trip..

I then Encounter glassy conditions.
SBC I'm not going to dog the engine.
So I'm stuck at 18/20kts turning 3500rpm (Max TQ rpm on most SBC)
That's a 2 1/2 or 3 hr trip.

BBC Max efficient cruise RPM is 3200 RPM. (Max TQ rpm on 8.1 375 hp)
At 3200 rpm I would be seeing over 30 kts on the speedo.
1.75 hrs from MDR to east end of Cat.?

At them cruise speeds both new SBC and BBC gets the same MPG.
MY 5.7 Alpha would burn another 25 gals of fuel for the same trip.

Well.... I need to go work on my old boat now.
I have some Rod Storage holders to paint :huh: :grin:

 

[keokie]

Tailchaser, I can help you with that 502 Carb'd question. We did that originally in my dad's 260, then converted it to factory EFI. Fuel consumption is only about 5% better with EFI. However, EFI starts better, stores better, and idles smoother at varying altitudes.

NorCal, no disrespect, you have a beautiful boat, and it may be very reliable for you. But that is uncommon.

The problem is the complexity of old equipment, the expense of the replacement equipment and the difficulty to replace it.

I have a buddy with an old Cruisers with twin v-drives. Rudder seal is leaking on one side, starter is going bad on other side. No big deal, except that the motors have to be pulled.

If you keep and use an old boat, you will put big money in it.

When you install a complete new propulsion package you reset the clock.

 

[Tail Chaser 661]

Thanks a lot keokie.
Amazing all the engines you have tried with these boats :thumbsup:
This is where the problem going from small block to big block comes into play.

Nothing is interchangeable.
So buying a complete engine drive package together saves a lot of money and headaches.
Merc only sells a complete Big block package with EFI.
No carb versions.
Ive done enough work at this.
Merc 8.1 375 hp FWC Bravo 3 will be be a great offshore engine package.

I just thought of a couple of more questions keoki.

#1... With the Bravo 3.
Do you still need to use a little port trim tab.
To correct the boat Leaning/Listing to the left while cruising under a load like the Alpha 1 does?

#2.... Does the steering wheel stay straight and boat track straight if you let the steering wheel go while cruising very slow like 5 kts or so?

The alpha 1 will do 360 circles the moment you let the wheel go at super slow speeds.

Once again I am totally stoked how well YOU, your Family, and Friends had set up your early to mid 80's 26' Sundancers.

I really appreciate you sharing what you have learned about these boats.
Its a nightmare trying to get good knowledgeable consistant info out there.

Its kind of akward. After all my research I do on toys.
I'm used to being the one with lots of the answers :lol:

 

[keokie]

Tailchaser,

Glad I can help. I love to talk about boats.

As far as your questions. If your boat is level at rest, it will run level at speed with a BIII. The wheel will stay straight at low speeds. On plane, it will just barely turn (I believe to port) if you let go.

The boat still exhibits v-bottom wander at no wake speeds with a BIII, but handling around the dock, is greatly improved. Actually, handling throughout the entire RPM range is better.

In all, the big block/ BIII package makes the 260 feel like a different boat.

I was a little slow on the uptake with fuel injection, but after having it, I would not go back.

Incidentally, there are some significant differences between the 8.1 and the older 454/502 Mag EFI's. If you don't already know the differences, I can elaborate.

 

[Tail Chaser 661]

Thanks keoki,
The 454ci, Merc does not sell anymore.
310 Hp 440 ft/lbs tq. Weaker valves/springs also?

New EFI 8.1 375 hp 490ft/lbs tq@3200rpm.
New EFI 8.1 425 hp 490ft/lbst@4000rpm

Found a Dyno chart From there GM 8.1 Crate engines.
Tq was 510 ft/lbs Same RPM specs as both Merc 8.1's

Merc racing has the 525ci as there base engine now. 42K with there race drive. OUCH!
I have never found HP/TQ spec info on that engine.

The reason I'm so hung up on the tq specs is....
For every 50 ft/lbs of tq increase. I should be able to increase prop pitch by 2"
That should lower my cruise rpm by 400 rpm for ea. 2" pitch change for the same given MPH speed.
Cruising at low rpm will get the best mpg and durability with the big block.

The range with a 100 gal fuel tank is the next scary obstacle.
Hmmm.... Replace 30 gal water tank with a Aux fuel tank.
Install new smaller water tank under the V birth.:smt115
I hope not. :smt009 :smt001

Very cool your Bravo 3 stays straight at slow speeds.
So I guess a Duoprop also eliminates the boat leaning to port while cruising under power?
No need for a little extra port trim tab?

Does your Bravo 3 have the 1.8 Gear Ratio?

Thanks Again keoki :thumbsup:

 

[keokie]

The 454/502 Mags were quite different than a standard 454 or the newer 8.1's.

The 454/502's got all forged components and heavier hardware in the heads. Then obviously more aggressive cams and higher compression. Mercruiser built thses motors for the sportboat market with the assumption they would be run hard. The EFI versions had an EFI system developed by Mercury.

The 8.1's have an EFI system developed by GM. It is more precise and 8 channel (versus 2 channel in Merc's). The 8.1's also use roller cams. Unfortunately, they do not use forged components even in the HO. They also have a very poor excuse for a seawater pump, but that can be fixed.

In all, the 454/502 Mags are a bit more robust, but less refined than the 8.1's.

I have a slight preference for my 502 vs the 496's in my 310. The 502 pulls very hard all the way to the 5000 RPM redline, while my 496's begin to fall off around 4600. This may not be true for the 496 HO, as it has a different cam. The 496's seem to have a bit better midrange and are definitely smoother.

Finally, The data I have seen on the 496, including the 496 HO show peak toque at 503 ft/lbs (at about 3000RPM for the standard 496, and at about 4000RPM for the HO). The best source I found for the 502 said they were about 520 ft/lbs at around 3200.

 

[Tail Chaser 661]

Thanks keoki,
I guess my short term memory has faded.
I just rechecked the 2009 GM website again

8.1 392hp@4600 503TQ@3200

8.1 HO 419 HP@5000 503Tq@4000

You can see why the standard 8.1 falls off earlier.

Your 502ci Has some really sweet numbers
Massive low end tq for cruising.
And still rev out too 5000 like a small block.
You have a perfect engine there.

I definitely need to check out there new 525ci
Mercury racing says that is there most popular engine now.
I guess it replaced there 502ci

 

[swift1]

My vote is the stroker motor that fits. You will be glad you did, but won't realize it because you won't have all the greif of redo of stringers, and stuff not fitting, you may want to change a belt some day etc. You need room for the fresh water cooling kit which is a must to not rot out the new motor. Your prices are what I hear in Seattle, PLUS labor.

 

[Tail Chaser 661]

Too late,
Just wrote the check for the Merc 8.1 MAG 375 hp today.
I'm going for the lowest cruising RPM I can get.(Reliability)
That's diesel or big block power.

SB and BB both share the same motor mount location.
Replacing my 24 yr old fuel tank. Trimming two inches off the depth for more engine room. (same as small block)Then having the bottom of the tank extended 5" down into the bilge. That will put me back at the org 100 gals.
Merc 8.1 Mag comes with fresh water cooling and dry joint exhaust.
At this point, installation for SB and BB EFI engines will be the same.

I think a 383ci is a little stressed for heading straight out 100 miles offshore in a 8500lb single screw boat.
The 496ci was built as a workhorse engine.

 

[sprink56]

Here the repower for you:

http://www.gm.com/experience/techno...s/specialized/marine/2010_6200_LSA_Marine.pdf

The Marine Power Inc. folks tell me this is the new paradigm and replaces the 496/502 platform that is being phased out this year. They told me to figure under 700 lbs. in full trim with 530 HP & 540 lbs. ft. TQ. Look at the torque curve on this bad boy...over 400 lb. ft from 1100 RPM :thumbsup:

This is perfect for your boat.

 

[sprink56]

Too late,
Just wrote the check for the Merc 8.1 MAG 375 hp today.
I'm going for the lowest cruising RPM I can get.(Reliability)
That's diesel or big block power.

SB and BB both share the same motor mount location.
Replacing my 24 yr old fuel tank. Trimming two inches off the depth for more engine room. (same as small block)Then having the bottom of the tank extended 5" down into the bilge. That will put me back at the org 100 gals.
Merc 8.1 Mag comes with fresh water cooling and dry joint exhaust.
At this point, installation for SB and BB EFI engines will be the same.

I think a 383ci is a little stressed for heading straight out 100 miles offshore in a 8500lb single screw boat.
The 496ci was built as a workhorse engine.

Did you consider the LSA?

 

[Tail Chaser 661]

:grin: That's a pretty radical small block. I'll take two :lol:

I got spooked of the long term reliability of the Volvo D4 260 and 300 Hp diesels

Inner coolers, after coolers, superchargers and turbo chargers. Imagine the cost after the engine inhales a little salty air after few years :wow: Them 4 bangers weigh 200 lbs more then the big block. 3.2 MPG would have been nice.