Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

[golf101]

I'm looking at a regal 2500 and think either of these engines will do, with the 8.1 maybe even being a little too much power (I know that sounds odd, but I buried the speedometer on a test drive yesterday at 65 mph on a choppy lake and had only barely trimmed the drive - 52-55mph WOT is fine for my needs). I'm trying to get a feel for the relative fuel efficiency of the two engines and not having a lot of luck. I expect that the boat weight and hull shape will impact these numbers, but wondering if there is a head to head comparison of the two somewhere I can look at for comparison purposes.

 

[golf101]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

Bubba1235, thanks for the response. Just for clarification, my question is that assuming the two engines were put in the same boat, what would the difference in fuel usage be. Anyone aware of any test results like this?

 

[Don S]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

But in general, I would gustimate about a 10% to 15% increase in fuel usage with the big block if everything else where equal.

He answered that, but I think his 10% to 15% is a bit conservative, the big blocks like gas.............. A lot.

 

[rodbolt]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

its also gonna depend on how you wish to run.
trust me its hard to run a light hull with an 8.1L HO and talk fuel economy.
back the throttle down to 2800 or so and your speed will be about the same as a 5.7L in the same hull at about 3800 to 4000 RPM.
I have about 9 customers with 25ft southwesters that crab daily for a living.
one has an MPI 5.7 with an SX DP, cruise at about 28 MPH is about 2900,same hull with an SX and a 5.7 carb turns about 3100 to 3200 for the same speed, the rest have 4.3L motors and have to turn close to 4000 RPM to keep up.
fuel burn is about equal on all that try to run 28 to 30 mph.
the 5.7 may be slightly better on fuel but will have to turn more RPM for the same cruise speed as the 8.1.
that and with the 8.1 you will have to get a bug pick for your teeth .

 

[HT32BSX115]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

Howdy,


I think Trailer Boats Magazine has done tests like this using BB vs SB in same size/type boats a couple of times.


Aside from the slight weight differences (BB/Bravo about the same as adding 1 person to the back of the SB/Bravo boat)

496 Bravo III weighs ~1200lbs
350mag Bravo III weighs ~1000lbs.


If you put both engines in the same boat and put the extra person in the SB boat so both weigh the same the fuel burn at the approx the same speed will be approx the same....

Both engine will be producing about the same HP so they'll produce about the same fuel burn.

The difference, is your right hand......when you got it, you're gonna use it!! I know I would!




Regards,


Rick

 

[bruceb58]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

I would get the bigger engine no matter what the fuel economy numbers are. When you are fully loaded you will appreciate having the big block. I have a 7.4L in a 24' boat and would never trade lower.

 

[KJSmitty]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

I currently have a Cobalt 220 with the 5.7 GXi DP. Weighs in at about 4500lbs. Looks like the Regal 2500 would be approx 4600 with the Small Block and 4800 with the Big Block.

With the weight of the 2500 I would think the 5.7 would be quite the slouch out of the hole. Cruise would be fine however if overall speed was not a factor (Mine runs about 52-53 WOT - plenty).
I don't have any data to back it up but also don't think average cruise would show a significant fuel consumption difference between the two. In my boat, even a 10-20% difference would only equate to 1 or 2 gallons more a weekend.

Overall, depending on the use, I just don't/wouldn't let "fuel economy" come into question with a pleasure/runabout boat. It's like those folks that buy an expensive sports sedan then complain about the cost of premium over regular fuel.... Test drive both if you can and make your decision off of the performance desire and how you will use the boat etc. Think about pulling skiers, full tank of gas/wet, loaded and unloaded with passengers etc...

In my case, I was initially only shocked by the sticker difference between the two in the Cobalt. Yet even at 4500 lbs, If I had to do it over I would get the 8.1.. Rarely do many of us use the top end speed, but with pleasure/runabout style play and a pretty heavy boat like the 2500, the low end torque/power of the 8.1 over the 5.7 could be the difference between being content with your boats performance and "loving" it.

Best of luck on your shopping!

 

[golf101]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

Thanks for all the comments. I'm going to go with the 8.1. Test drove it, and the boat was like a rocket. Don't really need that level of power in general (buried the speedometer at 60 with the drive barely trimmed), but with a loaded boat I'm sure the extra torque will be worth it.

 

[khgardne]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

I am looking on line at a 37 formula boat with 8.1 Penta volvos. I am curious what kind of consumption I would see per hour. on this size boat, and these twin engines.
thanks, and is there a better model with diesels I should be considering?

 

[45Auto]

Re: Fuel economy of Volvo Penta 8.1 DP versus 5.7GXi DP

I am curious what kind of consumption I would see per hour. on this size boat, and these twin engines.

Probably about the same as one or the other of these Formula 37's with twin 496's that was tested by Boattest.com. The Super Sport is considerably lighter than the PC: