cpt ron jc
Petty Officer 2nd Class
- Joined
- Aug 25, 2010
- Messages
- 116
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.
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?
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.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.
Take a car and put only one gear in it. Choose your engine and try it out.Boating, driving, doesnt matter. We are talking about engine and fuel efficiency. Don't obfuscate the topic. My example was just that, an example.
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,....
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.