Gas Usuage--EFI vs Regular?

[DayCruiser]

I figure I use 2.5 gallons of gas an hour putt putting along at 2000 rpms.
1990/89 engine Don tells me, 3.0, 130 hp 4cyl Mercruiser i/o with a merc carb. This is not scientific because I try not to think of gas usage while boating--real bummer lol
How much improvement if it were a EFI? estimate? Everybody tells me the newer boats get better fuel economy. I think i would mortgage the house before I stopped my addiction to boating so thats not the issue

 

[Firestar]

Re: Gas Usuage--EFI vs Regular?

I have a 1996 21 Celebrity 5.7 MPI with a 23P. I was out for 5-6 hrs combined 20% idling along at 1500-1700, 70% cruising at 2500-2700, and wide open 10%. I used 20 gals. I had 4 adults and 1 teen on board and started out with a full tank 50 gals.

 

[tommays]

Re: Gas Usuage--EFI vs Regular?

The 3.0 is as thrifty as it gets and it is hard to use less than 4 gallons and hour and get anywere in fact at 2000 RPM your getting a much lower MPG

I have a fuel flow connected to my GPS on my 3.0 and you start getting the best fuel use above 23 MPH with small amounts of trim makeing a big difference


at 3.50 a gallon the fuel flow mewter is quick pay back


Tommays

 

[DayCruiser]

Re: Gas Usuage--EFI vs Regular?

Oh ok so you get around 3.5 gph. Mine is 18 ft with me and 2 other people usually

 

[DayCruiser]

Re: Gas Usuage--EFI vs Regular?

The 3.0 is as thrifty as it gets and it is hard to use less than 4 gallons and hour and get anywere in fact at 2000 RPM your getting a much lower MPG

I have a fuel flow connected to my GPS on my 3.0 and you start getting the best fuel use above 23 MPH with small amounts of trim makeing a big difference


at 3.50 a gallon the fuel flow mewter is quick pay back


Tommays

Ya 3.0 put putter lol
How much is a "fuel flow"? How is it hooked up?

 

[Firestar]

Re: Gas Usuage--EFI vs Regular?

I could use one of those too!

 

[Ro-Tator]

Re: Gas Usuage--EFI vs Regular?

Does anyone have specs on how many gph an mpi 350 uses, compared to a straight-up carbed 350?

I don't mean to hijack. I'm wondering the same thing as the threadstarter

 

[QC]

Re: Gas Usuage--EFI vs Regular?

You can find a lot of this by poking around on Boat Test dot com. Remember a key thing though . . . a modern carbed 350 makes 250 at WOT RPM with a full load i.e propped correctly. An MPI 350 Mag makes 300 at the prop. Fuel is horsepower and visa versa. Yes, the MPI will use that fuel more efficiently. But it will burn more at WOT as it is making 300 bhp vs. 250 . . . So the question is very hard to answer. A more valid, but still messed up comparison might be a 250 Carbed 5.7 vs. a 260 MPI 5.0 . . .

Also, at different RPM along the propeller load curve (actually pretty straight) different hulls and props will consume different GPH for the same engine depending on where the WOT RPM fell. If WOT was at 5000 for an MPI 350 it would use less fuel at 4600 than the same setup would if propped for 4600 at WOT. The latter would be using almost as much if not as much as the other example at 5000. So in turn they will also use vastly different amounts at 4400 because one is closer to full load . . .

The point is be very careful how you compare these numbers. I know Ro-Tator is collecting lots of data, but it is still real easy to mess this up BTW, I've sold engines my entire 29 year career. Mostly diesel, but again fuel = horsepower, so the 1st question is how much horsepower is required at the point you are concerned about. A 300 MPI (or a 250 carbed for that matter) may only be making 147.3 bhp at the cruise speed you are concerned with . . . and they are probably within 10% of eachother depending on a bunch of other variables. I just gave myself a headache and I don't get headaches

 

[Ro-Tator]

Re: Gas Usuage--EFI vs Regular?

^ I enjoyed reading your post.
I'd be interested to know how the MPI compares to a carbed engine of equal size, when it comes to efficiency at a given rpm. It seems to me, that the MPI equipped small block would produce more bhp than the carbed engine, if both were at the same rpm. Like you say though, power is fuel. It'd be interesting to see a chart, showing the two engines, their rpm from 800-wot, the horsepower generated, and fuel consumed per hour.

I was gonna' keep typing, but I think I'll stop. It's late, and today hasn't been a very good one for me, as far as thinking goes.

 

[QC]

Re: Gas Usuage--EFI vs Regular?

I'd be interested to know how the MPI compares to a carbed engine of equal size, when it comes to efficiency at a given rpm. It seems to me, that the MPI equipped small block would produce more bhp than the carbed engine, if both were at the same rpm.

Yeah those charts for gasoline engines are pretty elusive. However, I need to get something cleared up first. When it comes to marine engines they don't "make" power, the propeller requires power . . . twisted way to think about it, but it's important to understand. I mentioned above that propeller load curves are actually quite flat. Take a look at this chart. It is from a 150 bhp Cummins diesel, but the concept is the same. In all three graphs the bottom line is a theoretical propeller's requirement vs. RPM. The line above is the engines "available" power and torque and fuel use at full load. The available horsepower is only used if you mash the throttle all the way. But if you bump your RPM 100 at a time, it pretty closely follows the lower propeller "demand" curve. So look at the top one of the three. That's the horsepower curve. At 2500 RPM the prop needs less than 50 bhp to keep it turning, yet the engine is capable of making well over 100 bhp (top line). Soooo unless there is prop curve data on the charts you want to see, it is meaningless. It doesn't matter to you that the engine can make that 100+ at 2500 when you are considering cruise efficiency. Yes, you might need all of that to get her on plane, but once there, what do you do? You pull the handle back until she is humming along at the "required" less than 50 to keep her cruising.

BTW, the second graph illustrates very well why I always say that torque is not important to marine engines. Yes, a higher torque engine will plane quicker, but it won't go any faster unless it has higher horsepower at WOT RPM. Yes, that would be more torque too, but only at WOT RPM and the number everyone always discuss is "peak" torque which is always at a much lower RPM . . .

 

[Ro-Tator]

Re: Gas Usuage--EFI vs Regular?

your explanations make a lot of sense. The other factor is the hull of the boat. some require huge amounts of throttle to get onto a plane. If one were to remain at the squatting speed in such a boat, they'd use more fuel than they would while running at planing/cruising speed.

In my case, I'm trying to compare the 350 and 454 carbed engines. Like you say, it's hard when there are so many variables. I think I might post again in the thread I made a few days ago. I don't wanna' hijack this one too much