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  1. #1
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    Tuning a 260 for optimal fuel mileage

    I'm looking for tech help to tune a 260 to get the optimal balance between top end speed (rpms) and fuel mileage. The boat is an Allison XTB 21 with a newly built 2.5 Merc 260 on a 15' mid, sporty with 1.87 gears, and currently running a 5 blade 29" wheel. It has a Merc ECU with an A-48 chip. Don't know what the dial number is set on, yet. Got a friend bringing a four button Merc tuner to check that this weekend.

    I like to ride on trips on the river at 40-50 mph, and return without running out of gas. My fuel tank right now is only 18 gallons, but I'm having an extra 13 gallon tank made that will go in the live well in the center rear, just in front of the big center rear compartment that houses the main fuel tank. That will give me 31 gallons capacity.

    The 260 is just getting broken in, and I made a run with it about two weeks ago to see how the mileage was. I went 32 miles downstream to a sandbar, where I hung out for several hours. I began with very nearly a full 18 gallons, maybe 17 1/2, and had a 190 lb. passenger and a Yeti Roadie, full. I ran about 40 mph on average, with a couple short "spurts" up to 65-70. I put in 2 more gallons that I had in a jug and still ran out of gas about 7 miles shy of the starting point, and had to be towed by a friend. I figure the mileage I got at very close to 3 mpg, maybe 2.9.

    Is it feasible to tune this motor to get 3.5 - or better, and still retain a strong top end speed? I have a friend with a V21 and a 280 that gets 4 mpg at 40-50 mph. Understanding that the 280s have variable timing, and a 260 doesn't, is it possible to get a 260 anywhere close to that 4 mpg?
    Last edited by AlliStan; 07-19-2023 at 09:10 PM.

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  3. #2
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    allison 21 is a decent size boat,so will be rough to improve MPG..pretty sure factory set dial number on 100?all you can do is play with gearcase hieght and maybe pitch up?five blades might be a bit much?thinking good four blade would be more ideal?try to concentrate on moving wieght aft to help with less wetted surface.wouldnt hurt to flow injectors as maybey you have some rich ones?
    "Whacker Smacker" #561 IHBA M/E div1 2007& 2006 first looser champion
    2007 Sportsmen Driver of the Year

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  5. #3
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    Thanks, Stoker. I realize the boat is heavy and I may be chasing a dream, but we're going to check the ECU number and see what's what. Michael Dixon told me that changing the number by "1' digit changes fuel consumption nearly 10%. I'm fortunate in that Mike Clemens, of Outback Outboards, built this 260 for me and flow tested the injectors before putting them in. He has lunch with Dixon and me on Saturdays, when he can, and he has the four button tuner to check it, hopefully this Saturday.

    The prop works so well on the XTB that I'm hesitant to change it. I've been through several ....... a 3 blade chopper, a Lightning ET, and a 4 blade Hoss, all about 28s. But, nothing is as stable as that 5 blade 29 (Dixon says it was originally a 28 but Caldwell worked it to a 29). XTBs can be a bit hard to drive. Some of my Allison buds say they're the hardest of all Allisons. The side steer imbalances the boat, and I had to balance it for my weight by moving the batteries to the left side, etc. Prop shaft height is even with the pad, and with at least two of the five blades in the water all the time it is a much more stable driver than with any other prop I've ever tried, and it has a better hole shot.

    I'll post on this thread what we find the dial number to be, and what we try if we change it. I'm beginning to think part of it is just that the 260s are thirsty motors. My son's River Rocket has a 260 and his drinks gas too, even being a much lighter boat than my XTB.

    For the sake of "argument" let's say I was at 2.9 mpg with the 190 lb. passenger. How much would you expect the mileage to increase without that passenger?

    Thanks for the reply! Stan
    Last edited by AlliStan; 07-20-2023 at 06:31 AM.

  6. #4
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    Time for a Brucato and a steam wheel so you can dial in up for hard runs and dial it back for cruising
    Hydrostream dreamin

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  8. #5
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    I like brucato. Not for his application. The a48 is badass. Kevin who commented on here first (stoker2001) is currently running a 260 I built for him while Clemons re-plates his 280. He can chime in on here more, but with that horn intake and a48 he is getting better gas mileage than he has ever gotten in the last few years running 280 or 300 drag with a slide and 849849 ecu. Mostly the horn I think helps the fuel mileage and you probably have one as well. I put 5 petal sportjet reeds in Kevin's motor too, that helps. 260s are thirsty motors, but you should be able to get 3.5 to 4 mpg. 2.8-3.2 is poor like rich ass drag racing mpg where you have it richened up to not blow the motor on hard revs. The a84 you can set at 99 and still be safe. Brucato is a good way to kill a motor built by one of the best LOL...especially if you are trying to get better mpg. They run real damn good when you lean them out. You'll say "Man this thing pulls hard and I'm getting way better mileage" ...till you're 20 miles down the river and it dies and you think your starter is bad cause the motor won't crank. Little do you know its burned up #2 and #5. I wonder how I know
    Not too much experience here with allisons, but I know tunnels like my mirage are not really affected by weight as far as gas mileage. Like not even noticeable if you leave all your fat friends at the dock.

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  10. #6
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    Thanks for that information 25two.stroke. That's very helpful. Just to doublecheck something you wrote, didn't you mean that "The a48 you can set at 99 and still be safe."? You said a84, but I think it was a typo.(?)

    BTW, I do run a horn. You nailed it, brother.

    Stan

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    A48 yes
    sorry, I write on here when I'm on the phone with customers who don't stop talking about their broken sterndrives. I get it a really boring phone call and start reading and responding on here...and I make some typos.

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  13. #8
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    run a couple of miles, stop & pull some plugs. youll know exactly where your at. forget the numbers, boxes, all that gingerbread. and you dont have to get your wallet out..

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  15. #9
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    Interesting info about the 260 brain. Maybe an adjustable fuel pressure regulator would be the trick?
    Hydrostream dreamin

  16. #10
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    Quote Originally Posted by LakeFever View Post
    Interesting info about the 260 brain. Maybe an adjustable fuel pressure regulator would be the trick?
    The problem with adjusting the fuel pressure is it’s not linear across the entire map. It will effect some areas more than others
    Erik Kiser

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  18. #11
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    ^ Above is exactly true. It affects the lower rpm more than the upper end. And if you double your fuel pressure, for example, you will not double your fuel flow volume. I think a stock reg with that a48 turned down to 99 will be as fuel efficient as he can get. You can only go so far on a heavy boat...they are not ever gonna be fuel efficient. A set of sportjet reeds would be the next step after if you want to keep throwing money at it trying to get fuel economy. It might get you a tad better.

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  20. #12
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    I would leave it Alone

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  22. #13
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    Well, back to square one. The fuel number was already set on 99, as checked with a DDT. Clemmons ordered me a set of EGT probes and a reader for them. I'm going to leave it as is for now, then when I get it rigged to read EGTs we'll play with lowering the number a little more. Dixon used to have a DCB with two 260s on it that was running cool with the numbers on 97 or 98. It's all for fun, but I love a challenge. OBTW, it already has a set of sportjet reeds in it.

    I'll run it tomorrow and check the mileage again. This time I'll be 230 pounds lighter. More to come ..........

  23. #14
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    Thanks,
    Keith

  24. #15
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    You’re going to end up burning a piston trying to make that engine into something it’s not. There is zero advantage to running a 260 lean other than a few dollars saved on fuel. Gonna seem like a bargain when you consider how many thousands a new plating job and pistons is gonna cost you. Sounds like you need a 250 Pro XS
    Erik Kiser

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