mercrusier 5.7

cpt ron jc

Petty Officer 2nd Class
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Aug 25, 2010
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i need a new engine my boat a 25 larson san marino has a 5.7 big block can i down size on engine to save fuel if so what engine would still move the boat well.
 

thumpar

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A 5.7l is a small block. Going smaller in a boat that size could actually cause more fuel usage because you will have to work the engine so hard.
 

Bondo

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Ayuh,..... +1 to AllDodge, +2 to Thumpar,.... Yer more apt to increase yer fuel mileage by goin' to a HT383 crate motor,....

But it all comes down to the mathamagic of btu's, 'n ton's moved through the water, at what speeds,....
 

achris

More fish than mountain goat
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There are 3 proven ways to reduce fuel consumption.

1. Sell the boat, invest the money...
2. Use less enthusiasm on the right hand....
3. Ensure the engine is running at peak efficiency. That means keeping the maintenance up to date, and the engine tuned, using good quality consumables (oils, filters, greases ect)... BTW, cheapest isn't the best quality.

Smaller engines NEVER mean less fuel for a given speed. As Bondo said, it comes down to simple maths. A boat of x weight travelling at y speed will require z horsepower. Z horsepower is created by burn x gallons per hour. Those are numbers that DO NOT CHANGE.... Adding a smaller engine will only require that engine to work so much harder to produce z horsepower. Let's say you need 120 horsepower to push your boat at 23 knots. What do your think would be better, a 140 horsepower engine running at 86% of full power, or a 260 horsepower engine running at less than 50%? And I know which one will last longer too.... BTW, a 5.7 in a 25 foot boat is about as small as you should be thinking. Most 25s want at least 300-350 horsepower, and are usually paired up... ie twin engines in the 170+hp range.

Another example is my car... A Landcruiser currently with a 6.5litre V8 turbo diesel, originally with a 4.2 litre straight 6 diesel. I now use LESS fuel when I'm towing my boat than the original engine used without towing...

Just saying...

Chris......
 
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slag

Chief Petty Officer
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471
There are 3 proven ways to reduce fuel consumption.

1. Sell the boat, invest the money...
2. Use less enthusiasm on the right hand....
3. Ensure the engine is running at peak efficiency. That means keeping the maintenance up to date, and the engine tuned, using good quality consumables (oils, filters, greases ect)... BTW, cheapest isn't the best quality.

Smaller engines NEVER mean less fuel for a given speed. As Bondo said, it comes down to simple maths. A boat of x weight travelling at y speed will require z horsepower. Z horsepower is created by burn x gallons per hour. Those are numbers that DO NOT CHANGE.... Adding a smaller engine will only require that engine to work so much harder to produce z horsepower. Let's say you need 120 horsepower to push your boat at 23 knots. What do your think would be better, a 140 horsepower engine running at 86% of full power, or a 260 horsepower engine running at less than 50%? And I know which one will last longer too.... BTW, a 5.7 in a 25 foot boat is about as small as you should be thinking. Most 25s want at least 300-350 horsepower, and are usually paired up... ie twin engines in the 170+hp range.

Another example is my car... A Landcruiser currently with a 6.5litre V8 turbo diesel, originally with a 4.2 litre straight 6 diesel. I now use LESS fuel when I'm towing my boat than the original engine used without towing...

Just saying...

Chris......


"Smaller engines NEVER mean less fuel for a given speed."


If all engines had the same efficiency, then maybe that would be true, but some engines are just more efficient than others and can get better mileage than their larger brethren even when pushing the same load.

1978 5.7 liter chevy 350 made 220 hp @ 5200 rpm.
2014 4.3 liter mercruiser makes 220 hp @ 4800 rpm

Which is more efficient and which is burning more fuel?
 

alldodge

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"Smaller engines NEVER mean less fuel for a given speed."


If all engines had the same efficiency, then maybe that would be true, but some engines are just more efficient than others and can get better mileage than their larger brethren even when pushing the same load.

1978 5.7 liter chevy 350 made 220 hp @ 5200 rpm.
2014 4.3 liter mercruiser makes 220 hp @ 4800 rpm

Which is more efficient and which is burning more fuel?

HP is not the hole equation, torque is what will pick it up out of the water. While both have the same HP, the torque difference is the biggy. Formula -1 race car produces more then 1000HP @10K RPM, but I wouldn't use it in a boat, there is simply not much torque
 
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slag

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We're talking about fuel efficiency. Look up the numbers. I did. The old saying "There's no replacement for displacement" is not as true as it used to be. My wife's explorer has the 3.5 ecoboost in it. It does more work and gets better mileage than the 5.4 in my old 97 F150. Bigger is not aways better.


The larger engines make most of their torque down low. The engines in a boat are revved relatively high which is great for smaller engines that have the grunt to push the boat.
 

thumpar

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We're talking about fuel efficiency. Look up the numbers. I did. The old saying "There's no replacement for displacement" is not as true as it used to be. My wife's explorer has the 3.5 ecoboost in it. It does more work and gets better mileage than the 5.4 in my old 97 F150. Bigger is not aways better.


The larger engines make most of their torque down low. The engines in a boat are revved relatively high which is great for smaller engines that have the grunt to push the boat.
Road vehicle fuel efficiency has nothing to do with boat fuel efficiency. They are totally different. It is like trying to compare a car motor to a tractor motor.
 

slag

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Road vehicle fuel efficiency has nothing to do with boat fuel efficiency. They are totally different. It is like trying to compare a car motor to a tractor motor.


Boating, driving, doesnt matter. We are talking about engine and fuel efficiency. Don't obfuscate the topic. My example was just that, an example.
 

thumpar

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Boating, driving, doesnt matter. We are talking about engine and fuel efficiency. Don't obfuscate the topic. My example was just that, an example.
Take a car and put only one gear in it. Choose your engine and try it out.
 

JaCrispy

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Regardless of the fuel efficiency debate, the OP said he has a 5.7 "big block". Is it a 5.7 or is it a big block?
 

pachanga27

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Dec 3, 2010
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My .02 cents worth. I looked up the weight of your boat on the NADA guidelines site - I don't know the year of your boat - but I just picked 1991 as the year - the NADA site lists the 25 model as a mid-cabin and says it weighs 4900 pounds dry - and that boat in 1991 could have the 5.7 gas with 330 HP or a diesel with 185 HP. Just on my experience - that boat weighing 5000 plus pounds at least with a full tank of gas, water, passengers - IMHO - anything smaller than the 5.7 will not work... That is a lot of weight to get up on plane quickly... and quickly being - you are on plane in a time that is comfortable for your passengers and the bow dropping so you can see where you are going and not running over something or someone you do not see crossing front of you in crowded water. The 4.3 is darn good motor (again IMHO - I have a 4.3 in a 20 foot Sea Ray Seville) but I do not see it being the motor of choice in a 25 foot 4900 pound cruiser. I have a Sea Ray Pachanga 27 with twin 5.7s and I believe that when I set the tabs and the outdrives are trimmed right - those motors are making excellent use of every gallon of gas. I also fall back on the boat designer whom designed the hull, the boat and made the decision as to which motor was decided to be the best for the boat and in your case... the 5.7L is the one he chose.
 
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cpt ron jc

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thanks for all the input, interesting....i will stay with was in the boat from the start 5.7....
 

Tail_Gunner

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Ayuh,..... +1 to AllDodge, +2 to Thumpar,.... Yer more apt to increase yer fuel mileage by goin' to a HT383 crate motor,....

But it all comes down to the mathamagic of btu's, 'n ton's moved through the water, at what speeds,....


Take heed here what Bondo is trying to tell you the 383 will get better milage and will run the same speeds...lower rpm is the answer. That comes from tourqe. Now dont everybody go off the deep end here....In this situation we have a lot of weight and tourqe always wins the day.

I have two boat 19' 4.3 hopped up to about 300hp and a 22' 454 doing about 330 hp..2850lbs and 3500 bls......The 4.3 boat will run away from the 454 with one or two people...now put 4/5 people and the 454 runs away from the 4.3...it loses 1 mph 330hp yes but about 485lbs ft of tourqe compared to the 275 lbs that the 4.3 has.

As the sig below says.....it how far you move the wall
 

Walt T

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Hey Slag, Don't be using them big words like "Obfuscate" you're gonna confuse Bondo. He might think it's a dirty word and blacklist you from every forum on the planet.Never underestimate Bondo power.

I stuck a 389 stroker in my boat and the fuel "economy" is about the same. 2-3 mpg.
 

tpenfield

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My 2 cents. . . I did read an article in a boating magazine quite some time ago where they ran actual performance tests on same boat with SBC (5.7) and BBC (7.4) engines. The fuel usage for average cruise and all-round consumption (efficiency) was only marginally better for the small block engines versus the big block . . . Like only about 5-10% better. In an automobile scenario, you will tend to see a much bigger difference of fuel economy versus the boats, because automobiles typically need/use less of the available horsepower. Boats are using a much higher percentage of the available horsepower of their engines, and that is why you do not see a big difference in economy between engine sizes in boats. So, the trade-off in a boat application is better low-end torque from a bigger engine versus a small fuel savings from a smaller engine. Most folks prefer the better power at a small increase in fuel consumption Of course at WOT the big blocks are consuming a much greater amount of fuel than thei small block alternatives, and that is where the main difference lies -in the higher throttle and speed ranges. But for normal cruising speeds, not a big savings between the two engine sizes. /End of 2 cents. :)
 

Tail_Gunner

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My 2 cents. . . I did read an article in a boating magazine quite some time ago where they ran actual performance tests on same boat with SBC (5.7) and BBC (7.4) engines. The fuel usage for average cruise and all-round consumption (efficiency) was only marginally better for the small block engines versus the big block . . . Like only about 5-10% better. In an automobile scenario, you will tend to see a much bigger difference of fuel economy versus the boats, because automobiles typically need/use less of the available horsepower. Boats are using a much higher percentage of the available horsepower of their engines, and that is why you do not see a big difference in economy between engine sizes in boats. So, the trade-off in a boat application is better low-end torque from a bigger engine versus a small fuel savings from a smaller engine. Most folks prefer the better power at a small increase in fuel consumption Of course at WOT the big blocks are consuming a much greater amount of fuel than thei small block alternatives, and that is where the main difference lies -in the higher throttle and speed ranges. But for normal cruising speeds, not a big savings between the two engine sizes. /End of 2 cents. :)

Agreed to some extent but 350 vs 383 in a 5000lb boat seem's like a absolute no brainer. While I don't have personal experience I do know truck's and power to weight ratio is a big thing....maybe tourqe to weight would be be better said...Where's QC at a time like this...it's one of his pet peave's...as it should be dealing with Cat.
 
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Silvertip

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Sep 22, 2003
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The smaller motor will also very likely be geared differently and run a very different prop than the bigger motor. Small motors cannot spin big props efficiently without changing gear ration. The Eco Boost comparison to naturally aspirated is also not a valid comparison. I've been hearing some not so flattering stories from Eco Boost folks who do not like high load, high boost fuel economy numbers. Lastly, as was pointed out, a boat doesn't have a multi-speed transmission. The prop must get you out of the hole, cruise efficiently, and allow the engine to reach the manufacturers recommended WOT rpm. If the prop is wrong at WOT it is wrong across the entire power band. Fact is, any engine will burn roughly 10% of its rated HP at wide open throttle regardless of of displacement.
 
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