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Fuel consumption - merc. 502?

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Old 08-09-2002, 10:09 AM
  #31
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wow!!!! i cant even figure mine out in gph because basically wherever i go takes less than an hour to get there....cruise at 60 mph....i just figure i get 1mpg and go from there.......125 gal tank will get me there and back....
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Old 08-09-2002, 10:30 AM
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simialr Straight line consumption curve for a 502 in a displacement hull, note the displacement hull actually uses less fuel, gph, @ 3000rpm in this comparision because the planing hull was not quite on plane in the above example yet and so under its heaviest load @ about 3000 rpm
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Old 08-09-2002, 12:39 PM
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ursus,
What is the difference in fuel consumption between a 502 mag, 502 EFI or 502 MPI? There must be somewhat of a difference??
I would think the MPI would be allot more fuel efficient, right??
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Old 08-09-2002, 01:17 PM
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crap, ursus

can't argue with those charts.

Of course you are right about the "drop" in GPH for a planing hull - it does not occur. I was certainly letting the MPG stuff filter through on that coment.

As far as two similar motors using the same GPH at the same RPM with the same load - sure they use the same, but the key here was the "same load" comment. Rarely will you get a truly "same load" situation between two vastly differing hull types.

Your two hulls that are reflected in your graphs apparently DO keep fairly even parity between the load/rpm scenario.

I've owned houseboats with 454 carb motors and runabouts with 454 carb motors of the same flavor. I've used Floscan meters on them both as well and, like some others here have kept a fuel journal with Tripmeter, gals used, engine hours, and comments on all of my fuel fill ups (well, all of them on the houseboats and cruisers, and lots but not all of the playboat fillups). My numbers show my old houseboat being more evil to a tank of fuel @ 3000 rpm than my old playboat was @ 3000.

Not even trying to start some crap, cause we all (at least I) look forward to your input on OSO.

I just still think my old houseboat had the throttle plates open farther than my runabout did @ 3000 and that this indicates a more heavily loaded motor @ 3000 and as such, more GPH fuel consumption.

Both boats were propped for 4600 WOT, both boats used 24/25 GPH wot, and there was a 2 to 3 GPH difference at 3000 (11 vs 14).

No floscan on the Formula, but the fuel usage booklet I got when I bought the boat jives with everybody else's numbers on the 502's.

********************

From what I'm seeing, the carb 502 Mag and the 502 Mag EFI share similar usage curves.

Do you the numbers for us on that comparison?
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Old 08-09-2002, 02:01 PM
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Default I may have missed this but here it goes anyway

The hull speed is about

1.3 x sqrt (waterline length), measured in knots

so a 40 footer displacement can cruise nearly effortlessly to about 9 mph. Then the fuel is wasted with little improvement in speed.
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Old 08-09-2002, 02:07 PM
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sorry if my post seemed confrontational was not meant to be somtimes I get carried away,, see my signature , I was just trying to point out gph was dependent on load and rpm

as you pointed out the most efficient @ 3000 will be the runabout or sport that planes at 2000-2500 rpm (sorry I couldn't find a graph for a light sport boat that planes @ 2500)

next the most efficient will be a displacement hull which is designed to still be cutting water at those speeds, (the 2nd graph)

the worst will be a planer that doesn't plane till 3200 rpm because at 3000 it is plowing a ton of water and the engine is really loaded, ( the 1st graph)

sorry no numbers on carb vs efi, I have heard ancedotally that efi is more efficient and I belive my cruise mileage improved when I repowered with a 502efi versus a 330hp carb but a lot of variables there, dif prop etc. ( my total fuel usage has increased tho as I am on the throttles more for some reason

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Old 08-09-2002, 11:02 PM
  #37
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ursus

no offense taken nor intended, just wanted to make sure I wasn't being perceived in the wrong tone, also.

I have a friend with a cruiser that will not plane fully until 3600rpm (can we all say "underpowered" together). It does then actually reach a nice top speed, but he has no way to cruise long distances economically (half into the 4 barrels at a decent plane). His fuel usage at anything above 12 mph (either GPH or MPG or any way you want to look at it) is dismal for a 29' cruiser.

I'd still like to find some info to compare fuel usage on a bigblock Mag motor carb vs multiport efi (502 or 454).
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