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Gas mileage outboard vs. io

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Gas mileage outboard vs. io

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Old 03-18-2002, 02:50 PM
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Question Gas mileage outboard vs. io

I'm curious about gas mileage of io's in relation to outboards. I currently have a 4,000 lbs. boat with a 330 454 and I'm looking at a 5,000 lbs. boat with twin yamaha 250's. Six of one, half-dozen of the other or what? Any thoughts, experience, opinions, or junk on yamaha outboards, would be appreciated
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Old 03-18-2002, 03:50 PM
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Hard to help without more info on the two boats but I would bet that twin outboards would use a lot more fuel than the single I/O especially considering the extra weight.
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Old 03-18-2002, 03:53 PM
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SuperDave From what I don't know my Brouthers yamaha all ways does him right eventhough he does not treat it like should be.found the only proublem was fuel left to stand would varnesh then carb proublems.so in my mind all out boards should be run often or run dry- drain carbs when long sit up might follow,and spray often wd 40 up under flyweel ie electronics.My thoughts on outboards great no bilge proublems, more deck area, great speed.if you are on set backs great.if you had jack plates you could get the best of both worlds great mid range and top end!My thoughts on inboard great speed, cost to repower seemed to be less for me when I was boat shopping but I wanted new power eather way.I love the torque band of a 4 stroke and the sound, like a Harley off in the distance you can feel it's power.You asked so I'am in the middle off the road again!James
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Old 03-18-2002, 04:02 PM
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It's a mid 90's nova 23' spider vs. a mid 90's 30'scarab center console. The nova wasn't designed with fish guts in mind. The nova is 8.5' wide and I'm guessing the scarab is 8'. The scarab weighs 1000 lbs. more and has an extra 170 hp., but I remember hearing some crap about volumetric efficiency, I think it's basically that a larger motor or more hp won't have to work as hard to get the same results and therefore in some cases may conserve gas, or maybe not.
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Old 03-19-2002, 09:54 AM
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Old 03-19-2002, 01:32 PM
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Having had both an IO and OB (although two totally different boats), I believe the gas usage with a IO is much better hands down. I believe the efficiency of a 4 stroke is greater than the 2, and the fact that you have to mix oil with the gas cannot help with an OB. It seems like I was always gassing up the OB (17 C'mate w/ 115 in line 6 Merc) - the new boat (98 22 Baja Hammer, 7.4 MPI) never seems to need gas. My only concern is reliability - I am hoping that the IO is as good as my OB was. I guess my concern centers around basically an engine designed for an automobile that has been "marinized", vs. the other way around for an OB. No problems yet though, knock on wood.
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Old 03-19-2002, 04:52 PM
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I had a 26' cat last year with twin 2.5 offshores. This year I have the same hull with a blown 510. I can't really tell the difference in gas usage. If you stay in the throttle alot in either boat it uses gas rahter quickly. I can burn 100 gallons a day in either boat. At least with the I/O you are not paying for pre-mix oil.
 
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Old 03-19-2002, 10:01 PM
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I believe 2 strokers use about 50% more fuel to do the same amount of work
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Old 03-19-2002, 11:45 PM
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Actually the number for an old style 2 stroke is more like 30% more fuel than a four stroke. I can back this up as a friend used to have a boat very similar to mine, size, weight, and horsepower. His was I/O, mine is O/B-my mileage was about 25% less than his but the boat was about 10 mph faster due to the higher rpm capability of the outboards.
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Old 03-20-2002, 01:16 AM
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As a general rule, an outboard (2 cycle) will use 10% of it's rated HP in gallons per hour at WOT, so a 225 for example, would use 22.50 GPH at WOT. A 4 cycle inboard will use app. 1/2 pound of gas per 1 HP, so a 225 4 cycle inboard would use 112.50 pounds of gas per hour at WOT which is app. 16 GPH.

For example, we're currently dynoing one of the new 500 EFI's with a Whipplecharger. At anywhere from 2,800 RPM all the way to 5,600 RPM she's burning app .5 to .55 at Brake specific fuel rate, hence whether it's supercharged, injected or carburated the 1/2 pound per HP holds pretty steady. Obviously the higher the HP the greater the consumption while the constant rate of consumption per HP stays in fact, the same.

One other note, the HP has increased about 10 HP per 10 minutes of dyno run time. (This is a new engine, -0- run time, factory specs and parts for a 500 EFI) other than a Whipple blower and Merlin 310 cc heads. We're running more dyno time in the morning testing some other ECU's. Right now the engine has 20 minutes on it. We're betting that at 40 minutes hard dyno time she'll have fully sealed and achieved max HP. Now, if we could keep gaining 10 HP per 10 minutes, I'd keep her on the dyno say, 3 weeks at WOT

Take care,

Steve
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