When repowering, is going to larger engines going to give me better fuel consump

[dnsellers]

So here we go again. I have a Sea Ray Sundancer 280, about 32 foot overall. Its large and heavy, and still has factory v-6 4.3's. I was told if I bumped up to twin v-8s, my fuel consumption would go down (better gas milage) because they have to work less to get up on plane, keep on place, etc... now that makes sense except you also now have v-8s to power. Also, once your up on place you also have your momentum. They work less hard any way. What are my true benefits of upgrading?? Higher top end and cruising spped obviously. But I cruise from Virginia to South Carolina, and sometimes Florida. Fuel is going to be an issue. So my ending questions, with fuel consumption go up or down. How much of a top end/cruising speed difference can I expect. And yes, I would be re-doing the outdrives, not repropping. Thanks in advance everyone!!!<br /><br />Derek

 

[Don S]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

There is the possibility, but a few questions have to be answered. What do you have now? In detail, Volvo, Mercruiser, carbed, EFI, single prop, dual prop, what drive, and..... What YEAR?????<br />What's the weight of the boat (roughly)<br />What rpm and speed are you now running and what milage/GPM are you getting.<br />Do you have room in front of the engine for the longer V8? How about expansion room above the engine, can you go higher?

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Boat and current engines are a 1991, and they are currently 4.3's. One seized, the other i can't get running, which explains the upgrade. Cost wise is about the same for both the engines (4.3 or 5.7) They are carburated. And the 5.7 was a standard motor for this boat as well from the factory, so yes, plenty of rom for them. Boat weighs approx 8000 to 10,000 lbs

 

[Don S]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

What rpm and speed are you now running and what milage/GPM are you getting. <br />What top speed and normal cruise speed would you like?<br />It all works together ya know.<br />And what kind of drive do you now have????????

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

There in lies the problem. I have never run this boat. I bought a flawless boat with 2 non working motors and a previous owner who just didn t care anymore ( i got a great deal) Basically...lets say on a 9,000 pound cruiser, would it be wise to upgrade. Everything else aside. The 4.3s were the smallest set of motors you could get for this setup. Obviously the 2 350s will more her quicker and get up on plane quicker, but will i sacrafice fuel overall. Or the the fact that it gets it up quicker and has less strain pushing the boat thru the water actually have an adverse affect giving better fuel milage. This can be tough i guess.

 

[Don S]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Mercruiser, OMC, Volvo????<br />Outdrive, V drive, straight inboard.<br />I will venture to say that a 5.o or a 5.7 will have less load on it with outdrives than a 4.3, which could be equated to less fuel usage.

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

It has twin 4.3 merc's w/ alphas. Now see, your going where i was originally going. Yes, less load, but once up on plane, the load is usually lessened anyway, and at that point your running 16 cylinders as opposed to 12. This is a really tough one w/out some testing.

 

[QC]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

They're also heavier . . . so at the same speed, they should require a little more fuel not less, and as you have noted you have more reciprocating mass. However, my guess is at the same speed the 5.7s will get around the same fuel efficiency. If you go to TBI or MPI, as opposed to carbs, efficiency should improve slightly. Getting the boat on plane only lasts a few seconds, so you're not gonna save anything there really except for the 10 seconds earlier that you're on plane. I am guessing you don't use this as a ski boat, so once you're on plane you probably stay there for a while. Momentum is the same at the same speed, so that's not a factor either . . .<br /><br />You aren't gonna get an economic payback, if that's what you're after. So, this probably is more about top speed capability compared to the increased costs for the extra pieces you'll need to complete the repower i.e. Wiring, mounts, possible gear ratio, props etc. etc. It's only money. Go for it

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Thanks QC...that puts a bit more into perspective. I guess generally, what I would need to know is, say, after a 3 day vacation of "mainly cruising at cruising speed" comparing the 5.7 to the 4.3, would gas consumption be approx the same or better w/ the 4.3. Guys (and gals) I truly hate bringing this gas thing up again, but w/ fuel prices on the water around here @ 3.50 and planning several 400+ mile trips...trust me, it's a factor. So thanks again for everything<br /><br /><br />I need a better paying job.

 

[Reel Poor]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

I would recommend going with the 260hp 5.0 MPI engines in that boat. This keeps your Alpha drives safe from excessive hp. With this setup I would say you will improve performance considerably and fuel economy should increase slightly. With this set I would say you should cruise around 30/32 mph and get close to 2 mpg. Its just a WAG but its close.

 

[QC]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Yup, RP, I was gonna say the same. He had said carbs in the first post I think, so I backed off. Those would be perfect for you Derek. At the same speed, would get better economy than carbed 4.3s IMHO. Maybe as much as 10% better due to the MPI, but don't count on it.<br /><br />My point about the ROI is that you are probably in for $5K more for the MPI's vs. your carbed 4.3s. Let's assume you save 10%. With RP's estimation of 2 MPG and your weekend of 400 miles. 400/2 = 200 Gallons x $3.50 = $700 x 10% savings (optimistic) = $70 savings per trip. $5000/$70 = 71 trips for a payback . . . So IMHO it becomes about speed and initial cost, not about savings. Oh, these things are waaay more reliable than carbs if they are installed properly . . .

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Do you think it's reasonable for me to get, overall with both motors (leaving them the v-6s) 3 mpg? What amazes me is the affect even one MPG can have. Especially when you have a 150 gallon fuel tank.... literally the difference between a 300 mile range and 450 (obviously not exact but for example purposes. I'd like to get around 3 MPG total, not per.

 

[dnsellers]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Do you think it's reasonable for me to get, overall with both motors (leaving them the v-6s) 3 mpg? What amazes me is the affect even one MPG can have. Especially when you have a 150 gallon fuel tank.... literally the difference between a 300 mile range and 450 (obviously not exact but for example purposes. I'd like to get around 3 MPG total, not per.

 

[Reel Poor]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

I dont know about 3 mpg, that seems like a lot for a twin engine stern drive set up. I was kinda basing my estimate from experiance in my boat, 2000 Chaparral Sig 29 with twin Volvo Penta 250 hp 5.0 Gi's (throttle body injected) SX drives turning 19p props to 5k, crusing about 3300 to 3500 rpm at 34/35 mph. On an average day cruising I'll get about 1.6/1.75 mpg by Flow meters.

 

[QC]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

I stare at test data all the time. Done it since I was a kid. Most 25 footers never see 3 MPG. I think 2 MPG is as good as it will ever get with your boat.

 

[Mischief Managed]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

It takes X HP to maintain a specific speed in a given boat. To acheive X HP you need to move a specific amount of air through the engine and you need to mix a very specific amount of fuel with that air. So, having bigger engines will not give better fuel efficiency. Bigger grins are a given though. <br /><br />Carbs and FI are gonna have pretty much the same fuel consumption in a boat. The big difference in fuel consumption with a carb vs FI is during cold start up and at idle. In a car, this can make a noticeable difference in fuel consumption because once up to cruising speed, the car just sips fuel. A boat is swilling fuel at an absurd rate once up to cruising speed and that reduces the MPG effect of poor efficiency at low speeds to the point that it's pretty much irrelevant.

 

[mkast]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

http://www.iboats.com/sites/miboats...93.html?listing_page=listing_sum_index_1.html

 

[mkast]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

The 2005 Sundancer uses one 375 hp big block.<br />Sell the current power package.<br />Install one big *** engine with a Bravo III.

 

[paulrfrancisco]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

Or the the fact that it gets it up quicker and has less strain pushing the boat thru the water actually have an adverse affect giving better fuel milage.

Twin V8's equate to more weight astearn and may make it actually a little harder out of the hole.... although the engine should easily power through it... <br /><br />I think it will ultimately come down to the pleasure of driving it... Fuel economy may not be too different, but the boat will certainly perform better and there is no $ amount for that piece of it...

 

[QC]

Re: When repowering, is going to larger engines going to give me better fuel con

Re: When repowering, is going to larger engines going to give me better fuel con

mkast is correct. No question, single screw is more economical.