Tuning a 260 for optimal fuel mileage

**BUZZIN' DOZEN** · 07-23-2023, 07:01 PM

In my opinion, there is not a better fuel curve than the A48 for a relativity stock 260 for the lake/river.
Like erik said, if you want to rebuild it, FAFO.

**LakeFever** · 07-23-2023, 07:35 PM

Originally Posted by patchesII

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

This is the winning perspective. You can take a tiger out of the jungle

**AlliStan** · 07-24-2023, 05:54 AM

Thanks for the cautions, fellas. But, I have to ask, how will I burn #2 or #5 when I only lower the number from 99 to 98 and watch the EGT readouts, carefully testing it? Clemmons rigs an electronic digital readout meter to the probes, and it mounts to the dash with hook and loop. It retains the highest temp the probes attain, and can be reset. He has one on his own personal boat.

Respectfully, I'm working with two guys who have forgotten more about these motors than I'll ever learn, and they're both good friends. Michael Dixon was a very successful ODBA racer, and modified engines for his own purposes, in ways you couldn't imagine. We talk daily, and live two miles apart. Mike Clemmons builds my motors and stays covered up with work because he is so good. His shop is 25 minutes away. He also runs his own 2.5L on his Triad V21. Both have my best interest at heart. Together, we're not going to do anything to hurt this 260.

Thanks for the information and concern, but that"FAFO" comment is uncalled for and not applicable here.

**tunnelmike** · 07-24-2023, 06:54 AM

Running a motor for miles at a time is completely different than making drag passes.

**AlliStan** · 07-24-2023, 08:02 AM

Originally Posted by tunnelmike

Running a motor for miles at a time is completely different than making drag passes.

True, obviously. But, twin 260s on a DCB is not exactly a drag boat either, eh?

**rjdubiel** · 07-24-2023, 08:15 AM

2 and 5 always run hot, not sure why, but they do. The best builders still have issues with trying to get things dialed in and it all changes with different weight in the boat, different props, all the stuff you know already. Just remember you are asking for opinions and getting them. So some you may want to consider and some you may not, but you will get opinions from all over! The good guys, the dudes that know some but not much like me, and the ones that don't know chit! haha.

Good luck with the tuning and have fun! Tuning is a process and I am still learning as well. Good post though to read though for others with 260's and looking for options.

**Stoker boy** · 07-24-2023, 09:23 AM

Allistan, if you have all this professional help I’m curious why you’re on this board asking the questions.
I personally know some of the guys here and they have great knowledge on these type motors.

First of all, you have a high revving 153 ci. high-performance motor. They seem to work best on lighter boats. Almost all of the time people ask how to improve performance on a high-performance motor, not how to get better fuel economy. The Mercury 260 high-performance does not have a reputation for longevity.

I’ve been playing with these motors for 30 years, you can have EGT’s with alarms, gauges, lights.
You can preset the EGT temperature and make the motor go rich when it reaches that temperature.
I’m sure it’s probably helped me save the motor a few times. Even with all that, I still had a 55 gallon drum full of broken motors and parts. They don’t always blow up because they’re too lean(and yes, that will stick a piston) sometimes the side mounted ring locator pin would sink, the ring would spin and catch the port. Top pinned seem to work much better.

If I want to make a longer trip, I carry more fuel.

**Dave Strong** · 07-24-2023, 10:26 AM

Originally Posted by patchesII

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

Bingo, you want fuel economy get a 225 Pro Max.

Dave

**AlliStan** · 07-24-2023, 11:47 AM

Stoker, thanks for the lengthy reply. I appreciate it and will try to answer your question. I enjoy discussing these boats and motors and like to hear other's ideas and opinions. I got a bit put off when I was told I was "F"ing around". I'm appreciative of everyone's opinion except that one. Your opinion is appreciated. It may well be that there is no other solution than hauling more fuel. 25two.stroke said it was his opinion that 3.5 mpg was attainable. That would make me happy, but it may be impossible in my case.

**Stoker boy** · 07-24-2023, 12:22 PM

AlliStan,
3.5 miles per gallon may be attainable with some boats. The truth is, I really don’t know anything about fuel economy because it’s never been an issue for me. I have always built my motors for high performance. Many years ago a mercury dealer told me the specs on a 260 were 29 gallons per hour at wide-open throttle. So many factors for miles per gallon. Water condition, wind condition, boat set up, hull design, weight, prop selection, gear case. My 2.5s are nowhere close to stock and the largest prop I have ever run is a 24 pitch. I don’t know what my miles per gallon are but I know they’re terrible. Later this year I’m going to try a 29 or 30 I’m thinking that will give me terrible acceleration but less RPM at cruising speed and I’m pretty sure I’ll get more miles per gallon.

If I were in your position and I wanted miles per gallon, I would probably try different props before I would mess with the fuel curve.

**AlliStan** · 07-24-2023, 02:30 PM

Thanks, Stoker. Good information.

**25two.stroke** · 07-25-2023, 09:51 AM

I saw it mentioned so I will comment. 97 and 98 as a setting is realistic. I know some ecus came from mercury set at 97. But...it is slipping my mind as to which chip it was and what application. Leaner is a dangerous game. Richer is a complicated issue as it can sometimes make egt show hot cause fuel is still burning across the probes from the bottom cylinders. EGT shows hot but you're actually rich. Lean it out and you find a sweet spot...then you bump it one number leaner to see if you can make it better and boom that motor is done. Faster than you can read the egt and shut it down the piston is melted. Youre already set at 99 and 99 is safe. That is a known fact. Anything beyond that is an educated guess. I would prefer to have the motor on O2 sensors if I was trying to lean it out for fuel economy, but thats not normally possible. You are trying to achieve something that is not supposed to be achieved, I'll be honest about that. 260s are not "fuel efficient." But theres no harm in trying if you have the right data. Different boats load the motor up different so you are running in different areas of the fuel map. Where you are cruising on the RPM-VAC map puts your 260 on a different number in the fuel chart than my motor with our boats running the same speed.

Prop slip % is another good thing to consider.

**AlliStan** · 07-25-2023, 04:36 PM

Thanks again for the additional reply, 25two.stroke. Excellent info.

**Charli** · 09-21-2023, 05:03 PM

Not sure how I found this thread, but..... I have a 21 Daytona with twin 260's. 94's with LL blocks. I have Brucato ACU's and Simon Alternators. I had a 14 gallon reserve tank made that sits out of the way, between the tunnels in the back. Both my main tanks are 25 gallons each side, so 64 gallons full. We did a little over 100 miles a few weekends ago at Havasu. Ran hard sometimes and other times we cruised. I put 5 gallons of reserve in each tank and when we got to the dock I had quarter tanks. 15x32 Mazcos

I was happy I didn't run out, lol. Long story short, they love to drink fuel, and if I were you I would just add more reserve.

**265mercman** · 09-21-2023, 06:45 PM

i run a cut 225 promax with 260 front and horn.. on hydrostream comp vking weight is 1200 lbs with 10 gallons of fuel.. i run a a-6 tuned curve on brucato at 60 mph turning a 30p et.. i run 70 miles round trip on 16 gallons.. that my tank size and it gets me back on the trailer... that is even a few hard pulls over 100mph on it ... averages 4.3 mpg so cant say they want do it but i think with you boat probally a lot heavier you could get atleast 3-3.5 mpg