Mercruiser 5.0L or 5.7L

[dan t.]

Re: Mercruiser 5.0L or 5.7L

To MR.88, sounds to me that your BSFC is exactly that, BS, it might work on paper, reality is a little different,you can prove mathamaticly that a bumble be cant fly but they seem to do a pretty good job of it.. In over 40 years as a mechanic I have learned a thing or two. One of the first things was that just because you were an engineer with a degree doesnt mean you know how things work in the real world. We have to repair and redesign things that were designed by "engineers" on a regular basis. Another thing I learned is that invariably a larger engine running at 25-50 % throttle will always burn less fuel than a smaller engine running at full or close to full throttle. This stuped argument could go on for ever so I am out of here, I have better things to do.

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

Pulled from another post on this site...1hp=1.414 btu/sec. There is aprox. 111,800 btu's in a gallon of E 10. Post was about repowering as well and it went on to say the same thing as I have stated. Basically a 300 hp V8 and a 4 cylinder 140hp engine will use the same gas,going the same speed on the same hull. The main reason fuel mileage increases on a boat[besides being propped correctly and losing a lot of ballast]] is with how the fuel is delivered. Carb vs EFI for example and the reason for that is there is less waste in the fuel delivery,NOT in the combustion chamber as it still needs 1.414 btu's to make a hp. Dan T ,don't care if you are a mechanic for 100 years,you are wrong,period. You have redesigned exactly what that was drawn up by a engineer ? Re-routing some wires under the hood? The engine,transmission,brakes,the ECM ? This is the real world right here.

 

[Cptkid570]

Re: Mercruiser 5.0L or 5.7L

I used to have a Ford Explorer with a V8, my company swapped me to the same exact explorer, but with a V6, same year, make model, options, everything. Except one with a V8 and the other with a V6. Drove them both in the exact same conditions. The V8 got better gas mileage. I could tell that the V6 had to work harder too.

Look at this:
Fuel Economy of 2010 Ford Explorer 4WD

In this though, the explorere with the V8 has a 6 speed transmission while the one with the V6 has a 5 speed tranny. But, as you can see, the theory is correct.

Going back to what the post is actually about, I don't think that the RPMs in the 350 will decrease that much from the 305 to really make a difference in fuel economy. Especially not enough to replace a good running 305. Now, when the 305 wears out and is time to be replaced, I'd probably replace it with the 350.

In a 21' boat, if fuel economy is really what you want, wouldn't a 4 stroke outboard be better on gas?

 

[mr 88]

Re: Mercruiser 5.0L or 5.7L

Cpt K are you 100% positive the transmission and rear end were geared the same and again not really under lab conditions as far as your own personal observations. Generally speaking V6's and V8's are geared different for workload/towing capabilities in trucks and SUV's.

 

[thumpar]

Re: Mercruiser 5.0L or 5.7L

You can't really compare car fuel usage to a boat. Boats are usually run a certain RPM for a long period of time. In a vehicle you are constantly changing the RPM with lots of idle time for city driving. They only real way you could come pair a vehicle fuel usage would to have the exact same vehicle geared correctly for the V6 vs V8 (boats use the correct one for each) then drive them down the highway at the same speed with no stopping.

 

[Bondo]

Re: Mercruiser 5.0L or 5.7L

You can't really compare car fuel usage to a boat. Boats are usually run a certain RPM for a long period of time. In a vehicle you are constantly changing the RPM with lots of idle time for city driving. They only real way you could come pair a vehicle fuel usage would to have the exact same vehicle geared correctly for the V6 vs V8 (boats use the correct one for each) then drive them down the highway at the same speed with no stopping.

Ayuh,.... 'n every inch of that highway, Uphill,.....

 

[achris]

Re: Mercruiser 5.0L or 5.7L

Well, that was a long and painful read! Somebody should get the pair of you and bang your heads together.. :facepalm:

So, what do we got? One guy saying a bigger engine will use less fuel for the same speed in the same boat. That makes sense to me. I had an inline 6 '165' and changed to a 4.3LX... Used less fuel UNTIL I pushed past the previous top speed. Also my car, was a 4.2l inline 6, changed to a 6.5 V8 (both diesels)... the V8 uses less fuel for the same cruising speed. Now, where is gets hinky is I changed from a 4.3 4bbl carb to a 4.3MPI, and I use less fuel... and using the same 4.3MPI with prop changes made .... no difference... For the same boat speed with the same engine with different prop pitches made absolutely no difference to the amount of fuel consumed...

Conclusion... Different engines have different fuel consumption characteristics, but in general, a large (more torque) engine will burn SLIGHTLY less fuel for the same speed.

How much the extra power/torques will change the revs/prop pitch is easy to calculate too... Likely the 350 will need to go to 26" props. A change from 24 to 26 is a increase of around 8.3%. Therefore one would expect a decrease of around 8.3% in revs to achieve the same speed (it will be slightly different to that due to water not being solid)... so, 3700 less 8.3% is....3400rpm... Will you notice a difference in the fuel consumption? unlikely.... Trim and load on the boat and weather conditions are likely to have more of an effect...

If you go to a 350, also go to MPI.... THAT will make a difference...

Chris.........

 

[Maclin]

Re: Mercruiser 5.0L or 5.7L

I like to compare the Boat take-off duty cycle to a Tractor Pull take-off duty cycle....


Tractor Pull...:
Weights on the sled start out low, then as the tow vehicle pulls and the sled moves, the weights move forward in a linear fashion to put more and more load on the tow vehicle.

Boat take-off...:
Boat starts off with a small hill of water between it and planing, like the weights already at the front of the sled at full drag, then as it climbs the hill it is like the sled weights moving down the sled for less and less drag. Then when the boat planes there is the least drag possible, still feels like pulling a sled as far as what the engine sees but not quite as heavy as it was at take-off.

 

[aerobat]

Re: Mercruiser 5.0L or 5.7L

it has been discussed before many times but some things still keep on being a myth : an engine has the best BSFC at WOT and peak torque rpm.

so saying a harder working engine uses more fuel is simply wrong and the opposite is true - for sure gents .

but in a boat keeping WOT at peak torque is not really possible with correct prop setting so things get different .

in this case practical ( assuming the 5.0 and 5.7 have the same BSFC ) - when you switch from a 5.0 to a 5.7 using the same drive and prop you will run the same rpm for a given speed and the 5,7 will use a little MORE fuel than the 5.0 because its works at a lower load than the 5.0
when you prop higher you will reach with the 5.7 a given speed at lower rpm,s than with the 5.0 and this may result in nearly the same or even slightly LOWER fuel consumption .

in any case the difference will be marginal and you will reach much more with optimum trimming , optimum weight distibution and finding the real sweet spot of your boat.

when performance is not critical you may pitch higher your current engine trading a little hole shot performance and maybe 1-2 mph topspeed for lower cruise rpm,s and a little improved economy.

in any case do not expect a significant change in fuel consumption between this two engines.

cheers

 

[Fun Times]

Re: Mercruiser 5.0L or 5.7L

Alright guys, Unfortunately this conversation just seems to be going back and forth of who and what is right vs wrong while most know that fuel consumption could go either way depending on boat, etc. etc. etc. X 10 and in the end we may not really know until moosehunter74 tries it out for himself in his boat the way he operates his boat etc.

Since this conversation seems to be heading into the argumentative stages with one another it has become apparent that this type of disagreement is not the type of conversations the host of iboats would like to see on their boating forums as they would prefer conversations to stay as peaceful, respectful, non cursing, knowledgeable and caring for one another as much as possible at all times while maintaining an G rated atmosphere.

So at this time we are going to ask that we let this conversation die down in hopes of avoiding a thread closure so that maybe sometime in the near future if moosehunter74 decides to go further with his ideas he would sill have the chance to come back to his open thread and provide us all an out come of how his project turned out which is always welcomed here.

Thank you guys in advance for understanding the sympathy of the situation at hand, it is very much appreciated.

Happy holidays to those who celebrate!
iboats Moderation Team

 

[moosehunter74]

Ok, I have finally done it. I picked up a 350 crate engine and doing the swap out for my 305. I'll keep you posted with pics and results

 

[moosehunter74]

I installed motor and here are the results.
305 cruise speed 27 mpr (gps) @ 3700 rpm
270 ltr tank and got 180 (gps) miles
Bravo 3 with 24 pitch props

350 cruise speed 30 mpr (gps) @ 3600 rpm
270 ltr tank and get 210 (gps) miles
Bravo 3 with 26 pitch props

 

[HT32BSX115]

I installed motor and here are the results.
305 cruise speed 27 mpr (gps) @ 3700 rpm
270 ltr tank and got 180 (gps) miles
Bravo 3 with 24 pitch props

350 cruise speed 30 mpr (gps) @ 3600 rpm
270 ltr tank and get 210 (gps) miles
Bravo 3 with 26 pitch props

:thumb::thumb: I love it when a plan come together!!

One thing no one mentioned is that a newer engine will likely have better (more) compression. One way to boost the BSFC is to increase compression.

Also, your slip still seems a tad high using http://www.go-fast.com/Prop_Slip_Calculator.htm
(remember that these prop-slip calculators are just a tool and are not always exact)

With 2.20:1 , 26p props and 15% slip would give you about 34mph at 3600 rpm (we're only talking a 10% difference) it's possible that your tach is still a tad off, but it looks like your performance is right where you want it!!

Also, make sure you tweak the drive trim (in or out) for max speed at a your desired cruising RPM. I get significant differences in speed playing with the trim and moving people around in the boat to get it level etc...

Nice job!! post a picture!!

Regards,

Rick

 

[thumpar]

Great info.

 

[moosehunter74]

Just came back from a trip this past weekend. Got some #'s if someone is interested. I put on 198 miles on my gps and I got 1/4 tank left still with 3 people on board and a cooler full of fish on way back. This is the result with my New 350 HO motor. My old 305 could only get 180 miles out of a tank.

 

[harleyman1975]

Cool! more power equals less fuel consumption!