305, 350, and 454 Efficiency estimates (?)

[Ro-Tator]

I'm new to I/O engines, but have a lot of boating/boat rebuilding experience.
Obviously, every hull design is different when it comes to efficiency.
However, I'm trying to get a rough idea of how much fuel these three engines use when at a nice cruising speed. I also realize that different propellers go well with different hulls.

To be a little less vague, let's say the engines in question are stock, and being run in a 24 foot v-hull slickcraft.

How many miles per gallon can I expect from the 305 (with 4-bbl carb), 350, or 454? I'm talkin' real rough estimates.
*GM engines with Mercruiser outdrives, btw**

Any and all posts will be helpful. If you have a different style of boat, feel free to just say "I have X boat, X engine, and get __ efficiency."

I did a few searches, and some lurking already, btw.

Thanks, guys

Oh- Don: I'm talkin' '80s engines, to give you an idea of the vintage.

 

[Zackman]

Re: 305, 350, and 454 Efficiency estimates (?)

You can find information on the mercruiser website http://www.mercurymarine.com/engines/inboards/sterndrives/mpi5l_test.php

It looks like in their tests for carburated engnes they don't post fuel efficiency, but do on the MPI engines. Should give you a good feel for fuel efficiency. Select on each motor on the menu near the top of the page and then select test data.

Of course they don't have the 454, but the 496 should give you a good idea.

 

[neikrk6]

Re: 305, 350, and 454 Efficiency estimates (?)

Last weekend I ran 72 miles (GPS) on 25 gallons of fuel. 21' open bow Conquest with 5.7L, 260hp with 19" prop. 6 adults and 3 coolers. Our normal cruising speed was 25-30 mph.

 

[Haut Medoc]

Re: 305, 350, and 454 Efficiency estimates (?)

3 mpg or so.......​

 

[Mischief Managed]

Re: 305, 350, and 454 Efficiency estimates (?)

It's not really the engine that determines the fuel economy, it's the hull and the prop selection. All three of those engines will probably deliver fuel consumption rates within 10% of eachother for a given speed, assuming proips are selected well. The bigger engines will allow for a higher cruising speed at a given RPM.

 

[Ro-Tator]

Re: 305, 350, and 454 Efficiency estimates (?)

Thanks for the great replies.

This is exactly the type of info I was lookin' for.
The more info, the merrier.

If I don't come back to the thread to post for a while, it's just because my computer is having lots of problems.

Thanks again

 

[paulrfrancisco]

Re: 305, 350, and 454 Efficiency estimates (?)

As my signature shows I have a 30', 9500# dry, with twin 300hp 454's... I just recently cruised round trip 74 miles plus about 5 miles to the gas dock and burned 119 gals... thats about .66 mpg.

 

[Ro-Tator]

Re: 305, 350, and 454 Efficiency estimates (?)

Is that .66 mpg for both motors, or each?
I thought a stock 454 made more than 300 hp.

Thanks for postin' the info. I appreciate it.

 

[QC]

Re: 305, 350, and 454 Efficiency estimates (?)

Ro-Tator,

paulfrancisco's numbers will not help you (no offense paul), but it is for both.

Haut Medoc is giving you some good stuff as well as Mischief Managed. Fuel efficiency should not be your deciding factor as they will be very close. Despite a lot of assumptions to the contrary, at an efficient hull speed (just when she comes over onto a clean plane) the 5.0 would probably give slightly better economy than the 5.7 which would give slightly better than the 7.4, all within 10% as MM noted. Here is a link to a test that will help you some: Big Block vs. Small Block test

It is not perfect as gear ratios matter, as do props as you have mentioned, but it is good data non the less . . .

I have a 23 ft Carrera with a 6.2 MPI and it gets better than 3 all day, and I usually run at around 4000 RPM and 40 MPH. MPI's are better though, so I would count on 2.8 - 3.2 for you overall depending on how fast you run and which combo you get.

 

[Ro-Tator]

Re: 305, 350, and 454 Efficiency estimates (?)

Thanks for all the great info.
I understand that paul's setup is very different from the one I will have. However, his efficiency gives me an idea on how much fuel one uses when really askin' the big block do do a lot of work.

When I have a few minutes to spare this evening, I'll check-out that link you posted. I'd be more than happy with 2.8-3.2 mpg.

As for preferences, I like running at the nice easy speed, where the hull has just gotten up on a plane, and the engine is running easy. however, I'm also a power addict and will "blow it out" once in a while (so to speak). When it comes time to really give the sucker some throttle, I don't care what kinda' mileage I get, because it'll only be for a short period of time before I return to cruising speed.

Thanks again for the info.

 

[bekosh]

Re: 305, 350, and 454 Efficiency estimates (?)

I have actually been tracking my fuel this year for the new boat.
It's a 1991 Wellcraft 216XL 300hp 7.4l(454) 4bl carb with a Bravo1 and a 21p Mirage stainless prop.

We've run 510 miles and burned 210 gallons of gas for an average of 2.42mpg. (Avg price $3.09 gal for $650 and counting.)

Mostly we run at about 3000rpm/35-38mph, but we will 'open her up' occasionaly for a 55-57mph run.

Our trips have been anywere from 24 mile round trips to the sandbar, all the way to a 110 mile tank emptying trip from Fond du Lac to Nipple beach on the Wolf river near Weyauwega. (See "My Google Map" in my sig for the places I've been this year.)

 

[paulrfrancisco]

Re: 305, 350, and 454 Efficiency estimates (?)

Is that .66 mpg for both motors, or each?
I thought a stock 454 made more than 300 hp.

Thanks for postin' the info. I appreciate it.

that was for both motors

 

[Ro-Tator]

Re: 305, 350, and 454 Efficiency estimates (?)

That doesn't sound too bad, bekosh. It sure does add-up when you take a long trip though. Yeow. I appreciate all the info about your boat.

paul, thanks for confirming the info regarding the dual engines.

I'm enjoying reading the info that you guys are posting. I haven't been out in my boat much, due to my work schedule, so it's good to read about boats and engines.