Mercruiser 5.0L or 5.7L

[mr 88]

Re: Mercruiser 5.0L or 5.7L

You are not going to see a change in fuel consumption by going to a 5.7. It takes X amount of energy to move your boat,the engines will consume Y amounts of fuel to meet your goal. The Y will be the same for either engine.The 5.7 will allow you to cruise at a lower rpm,like 50-100 r's compared to the 5.0 BUT it will still use the same amount of fuel,bigger cylinders require more fuel.I would be trying to figure out where to secure fuel cells in/on the hull for your offshore runs. You could put in anything between 7.4 and 3.0,your still going to use the same amount of fuel traveling at cruising speed. The answer to your original question is, no

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

The only answer to this is to leave earlier and run at 2600-3000 rpm just above planing speed,staying out of the secondaries for sure. As I stated in the above post. No matter what you put in it the fuel consumption is about the same.

I'm not trying to go faster by going to the 5.7. I just want to lower my rpm's when cruising to my fishing holes. I cant pitch up anymore as the 24's rev out at 4800 loaded. I travel 2.5-3 hrs to get to my fishing grounds. Main objective is to save fuel so i don't have to carry extra fuel on board

 

[moosehunter74]

Re: Mercruiser 5.0L or 5.7L

The only answer to this is to leave earlier and run at 2600-3000 rpm just above planing speed,staying out of the secondaries for sure. No matter what you put in it the fuel consumption is about the same.It takes X amount of fuel to produce one hp.If your setup requires you to use say 180 of your 210hp and now you put a 275hp in it, it is still going to need 180 hp of that 275 to move it and that 180 is going to need the same amount of fuel to hit that number,it will work less as far as strain on the actual engine goes.

So your telling me that if two identical boat one with a 5.0 and the other 5.7 will burn the same fuel? Makes no sense. If one motor is working harder then the other one is going to burn more fuel. It's like towing a travel trailer, the bigger motor will not work as hard as the smaller motor towing so it should burn less. Am I wrong with this theory?

 

[thumpar]

Re: Mercruiser 5.0L or 5.7L

The only time a 5.7 would use more than a 5.0 in an identical bout is if you use the hammer down. Until the point that they are at the same speed it will be the same consumption.

 

[84EdH]

Re: Mercruiser 5.0L or 5.7L

It makes sense to me. 5.7 can maintain speed at lower rpm because it makes more hp at the lower rpm. Trade off is that 5.7 has bigger cylinders and it eats more fuel to feed those bigger cylinders.

The bigger travel trailer engine does not need the rpm the little engine does to tow so it works less hard and would last longer but would not burn less fuel. The work to push your 2000# plus boat 200 miles is the same wether you use either engine.

 

[Walt T]

Re: Mercruiser 5.0L or 5.7L

The 'stroker' motor is a 5.7 bored and with a 400 crank. Its the same external dimensions as a 5.0 and 5.7. A 5.7 will fit if a 5.0 is in there now. But no matter what you do any fuel savings will be almost insignificant. If that's all you're looking for, save your money. The outlay for the engine change will never ever be made up for by fuel cost savings.

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

Yes you are wrong. The 5.7 will have more torque at a lower speed not hp.Lets say you are using 180 hp of your 200 available to run the speed you want to . When you put in a bigger engine you still need 180 hp to get the same results.It takes X amount of gas to generate one hp and X is the same in every 4 stroke engine. So to produce that 180 you will need the same amount of fuel,doesn't matter if you have a 4 bamger or 7.4 it will use the same amount of fuel to get your needed 180hp.

So your telling me that if two identical boat one with a 5.0 and the other 5.7 will burn the same fuel? Makes no sense. If one motor is working harder then the other one is going to burn more fuel. It's like towing a travel trailer, the bigger motor will not work as hard as the smaller motor towing so it should burn less. Am I wrong with this theory?

 

[Maclin]

Re: Mercruiser 5.0L or 5.7L

Saying this a different way for the OP, You can only lower the RPM's by prop pitch change or drive ratio change. Just upping the power will not change RPM's at any given speed by itself. Regarding fuel consumption, if RPM's do not change then fuel consumption will be about the same between 5.0 and 5.7 as the 5.7 will not need as much throttle to keep the same RPM's. Same goes for speed, if pitching up and lowering RPM's to keep same speed then fuel consumption will be about the same.

 

[thumpar]

Re: Mercruiser 5.0L or 5.7L

The 'stroker' motor is a 5.7 bored and with a 400 crank. Its the same external dimensions as a 5.0 and 5.7. A 5.7 will fit if a 5.0 is in there now. But no matter what you do any fuel savings will be almost insignificant. If that's all you're looking for, save your money. The outlay for the engine change will never ever be made up for by fuel cost savings.

A stroker like the 383 is not bored. It is only a crank change for more stoke, hence the name stroker.

 

[Silvertip]

Re: Mercruiser 5.0L or 5.7L

A stroker like the 383 is not bored. It is only a crank change for more stoke, hence the name stroker.

I take issue with that statement. Anyone that actually builds a 383 would very likely go through the block to clean up the bores with a .030 to .060 bore. That engine is still a stroker. The stroker part of it is the longer stroke whether it comes from a 400 crank or a stock crank that has an altered stroke. Because the engine is also bored does not mean it suddenly is not a stroker. Boring a small block anywhere in the range of .010 to .060 is pretty standard rebuild practice on these engines and doesn't suddenly change the terminology associated with stroker.

 

[Fastatv]

Re: Mercruiser 5.0L or 5.7L

Walt may be old school...as I am. Back about a million years or so ago :lol:, to achieve the 383 CI, a 30 thousandths overbore was required ( which made the 350 CI become a 355 CI ) along with the 400 CI crank installed, then became a 383 CI. In today's market, they may have new aftermarket blocks that are actually 355 CI when new.

 

[Scott Danforth]

Re: Mercruiser 5.0L or 5.7L

I would also do a 377/383 stroker

regarding fuel economy Most SBC's will burn between .40 and .50 lbs/hp hr. if both your 5.0 and 5.7 have the exact same gearing and same props, the fuel economy between the two would be very close at cruise. However put higher pitch props on the 5.7 or larger motor that makes more torque at a lower RPM, and the 5.7 would get better fuel economy than the 5.0 for the same hull at the cruise speed. Where the larger motor uses more fuel, is when you decide to take the right hand, and mash the throttle forward and go faster with the bigger motor than you did with the smaller motor. going faster creates more hull load, which requires more prop load, which in turn requires (burns) more fuel.

 

[moosehunter74]

Re: Mercruiser 5.0L or 5.7L

I would also do a 377/383 stroker

regarding fuel economy Most SBC's will burn between .40 and .50 lbs/hp hr. if both your 5.0 and 5.7 have the exact same gearing and same props, the fuel economy between the two would be very close at cruise. However put higher pitch props on the 5.7 or larger motor that makes more torque at a lower RPM, and the 5.7 would get better fuel economy than the 5.0 for the same hull at the cruise speed. Where the larger motor uses more fuel, is when you decide to take the right hand, and mash the throttle forward and go faster with the bigger motor than you did with the smaller motor. going faster creates more hull load, which requires more prop load, which in turn requires (burns) more fuel.

That's what I've trying to achieve but everyone says no. By putting a 5.7 and propping up I should be able to back off on trottle, cruise at a lower rpm at the same speed as the 5.0

 

[Walt T]

Re: Mercruiser 5.0L or 5.7L

I actually bored mine 40 over so it's a 389

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

Were telling you no,because that is the correct answer.If there was a magic answer then all the boat and car mfgs. would be using it.Why do you think the car people put so much into aerodynamics ? Because they can't do any more with the tranny and rear gear ratios.Every little 4 banger **** box gets about the same mileage if the gears etc are close,THEY can't do anything more with the internal engine to increase mileage. I will say this again it takes X amount a fuel to produce a single HP and that fuel goes into basically the same combustion chamber,they all operate under the same principal. You may be able to cruise at a lower rpm but it is still going to take the same amount of hp/fuel to meet that number.The displacement is bigger on the 5.7 so even though the rpms are less you still need the same amount of fuel to reach the needed hp. YES the engine is turning over slower but you have bigger cylinders to feed and that's when it evens out.Putting a higher pitched prop will only make the engine work [suck more fuel ]to reach your desired cruising speed. The 5.7 vs 5.0 or anything else is not going to give you any better mileage with all things being equal,even when perfectly propped to your hull.Like I said I would be working on where to put extra fuel tanks than trying to squeeze 10 more miles out of a 5 hour trip.

That's what I've trying to achieve but everyone says no. By putting a 5.7 and propping up I should be able to back off on trottle, cruise at a lower rpm at the same speed as the 5.0

 

[Walt T]

Re: Mercruiser 5.0L or 5.7L

Moose, if that were possible all boats would be doing it and the 5.0 would have gone the way of dinosaurs long ago. Unless you're using a small bowrider, swapping to a 5.7 will not by itself get you into a pitch higher with your boat. It is very difficult to get a boat motor to turn a higher pitch prop, especially B3's and Dp's as they have real grip on the water. For a boat your size it's gonna take 75 hp gain minimum AT CRUISING RPM to achieve that. Most HP ratings are at 5000 rpm so while that sounds nice it's useless. Torque output at 2500-3500 rpm is what you're most interested in and the only way to get torque is to increase stroke. Increasing diameter helps also as the ladies say, but stroke is the main producer of torque. This is why the big semi's on the road overwhelmingly use inline 6 cylinder motors and why locomotives use those massive inline engines. Inline 6 motors pretty much all have a relatively long stroke compared to V type engines. The longer stroke allows the combustion inside the cylinder to drive down the piston for a longer period of time therefore producing more torque. Torque is a measure of power produced over TIME which is why it's different than HP. This is why those big diesels have 400-500 HP ratings at 3000 rpm which isn't much different than say a Mustang or a Corvette or maybe my sleeper S-10 but those 'Vettes and 'Stangs can't pull an 80,000 lb trailer up over I-70 through the Rockies. The diesels put out 1500+ lbs-ft at sometimes HALF that max rpm which tells you that: 1- There is a whole lot more energy in a gallon of diesel than there is in gasoline, and 2- That diesel fuel is allowed to work on those pistons a lot longer than any other engine made.
Remember the old CAT 3208 diesels and the old Detroit 8.2 V-8s? They were pretty good for inner city bus use and small trucks but completely useless out on the highway. No torque because of the V-8 design.
I worked for Tidewater Marine when I was young, that's where I cut my teeth on the massive Detroit and Mann diesels which were exclusively inline 6-8-12 and 16s. Piston and rod assemblies on some of these were 6-7 feet long.

Anyway my point is, longer stroke will get you a higher pitch prop at the same low rpm, higher HP ratings will not. Therefore you will need modifications to a 5.7 to get there. Don't get me wrong, I'm actually all for it. Sure it will cost you a small fortune if you have to pay a shop to do the work but in my mind, the satisfaction of cruising 5 mph faster and hearing that engine sounding like it's not working at all is great. Of course the added bonus of top speed being higher is always nice. Not that you need it, but it's nice to know. I love hammering my boat and just feeling that leap forward even when I'm already at 3/4 throttle. That all by itself makes it worth it.

 

[Cptkid570]

Re: Mercruiser 5.0L or 5.7L

I'm wondering if you would get more fuel efficiency by putting your money towards a fuel injection system rather than going with a different engine. You all are way more technical to me.. in keeping it simple for me, I would think that a larger engine would use more gas. I've never really heard that going with a bigger engine would save gas. And, as I said, that is just myself being simple minded and not very techinical. I don't think I would swap out a good running engine. With as much as you use your boat, just give it time and you will need a new engine...at that time, you can toy with the idea of a 5.7 liter.

Maybe put your money towards a different prop that has more bite.. I'm sure prop shops would let you test some out.

Is the drive ratio proper for an 8 cylinder engine? I have a 1985 5.7 liter alpha and I believe it has a 1:50 gear ratio.

 

[HT32BSX115]

Re: Mercruiser 5.0L or 5.7L

I'm wondering if you would get more fuel efficiency by putting your money towards a fuel injection system rather than going with a different engine. You all are way more technical to me.. in keeping it simple for me, I would think that a larger engine would use more gas. I've never really heard that going with a bigger engine would save gas. And, as I said, that is just myself being simple minded and not very techinical. I don't think I would swap out a good running engine. With as much as you use your boat, just give it time and you will need a new engine...at that time, you can toy with the idea of a 5.7 liter.

Maybe put your money towards a different prop that has more bite.. I'm sure prop shops would let you test some out.

Is the drive ratio proper for an 8 cylinder engine? I have a 1985 5.7 liter alpha and I believe it has a 1:50 gear ratio.

EFI probably wouldn't help much.......But, he's probably getting far better efficiency with the Bravo III than he would with ANY single prop drive.

Someone earlier mentioned that the slip is 26% and it appears that everyone brushed right by that.

Your slip is most likely NOT 26% IF the boat isn't overloaded/waterlogged, doesn't have a dirty bottom or your NOT dragging the trailer!

You probably DO have an inaccurate TACH, Speedometer, wrong drive ratio (says 2.20:1 on the outside and someone rebuilt it with different gears) OR the props are NOT 24p.

Since the drive is probably NOT mismarked and the props are CLEARLY marked 24p, your TACH or Speedo (or BOTH) are suspect.

you said the props were powdercoated? Did someone also re-pitch them?

Measure your speed with GPS and get another tach.

Mine (21' Four Winns Liberator , 4300lbs) runs 3000 RPM, 35 MPH with a 1.81:1 drive and 26p Props. (approx 15% slip)

Somewhere around 15% slip normal for a lot of big planing boats running a Bravo III. The lighter it is, the lower the slip



Regards,


Rick

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

This post along with your first post which more or less said you have a 2150 Bl which I guess is a 21 ' CC or Walk around cuddy and you cruise at 3800/28mph. So your going 75 miles off shore and back [ in a single engine small boat IMHO for possible conditions ] and want the best of both worlds in speed and fuel mileage ? Not going to happen with your targeted cruise speed and more importantly the RPMs you are running.Every boat in your size range etc with a SB in it cruised the most efficiently in the 28-3000 rpm range a bit after that you start to crack the secondaries and there goes the fuel mileage.Time for a bigger boat with a larger gas tank or maybe convert to diesel :}

I'm not trying to go faster by going to the 5.7. I just want to lower my rpm's when cruising to my fishing holes. I cant pitch up anymore as the 24's rev out at 4800 loaded. I travel 2.5-3 hrs to get to my fishing grounds. Main objective is to save fuel so i don't have to carry extra fuel on board

 

[moosehunter74]

Re: Mercruiser 5.0L or 5.7L

EFI probably wouldn't help much.......But, he's probably getting far better efficiency with the Bravo III than he would with ANY single prop drive.

Someone earlier mentioned that the slip is 26% and it appears that everyone brushed right by that.

Your slip is most likely NOT 26% IF the boat isn't overloaded/waterlogged, doesn't have a dirty bottom or your NOT dragging the trailer!

You probably DO have an inaccurate TACH, Speedometer, wrong drive ratio (says 2.20:1 on the outside and someone rebuilt it with different gears) OR the props are NOT 24p.

Since the drive is probably NOT mismarked and the props are CLEARLY marked 24p, your TACH or Speedo (or BOTH) are suspect.

you said the props were powdercoated? Did someone also re-pitch them?

Measure your speed with GPS and get another tach.

Mine (21' Four Winns Liberator , 4300lbs) runs 3000 RPM, 35 MPH with a 1.81:1 drive and 26p Props. (approx 15% slip)

Somewhere around 15% slip normal for a lot of big planing boats running a Bravo III. The lighter it is, the lower the slip



Regards,


Rick

Hi Rick, I bought this package off a friend of my that the haul was cracked. He gave me all the paper work when he bought it new as a package. The drive says its 2.20:1 and my props say 24 pitch. I had 22 pitch that came with the drive. I took it for a test run and opened it up and I hit 5300 rpm wot. So I bought (used) 24 pitch which dropped my rpm down to 4800 rpm wot. Like I said earlier my props were matte finish but it's all worn off and now it's all shinny. Would the loss of the matte finish off the props cause the slip to be up at 26%? All my readings are off my chart plotter and GPS. Also it's a 2 barrel carb that's on my 5.0L. The boat is a bayliner 2159 trophy Alaskan bulkhead