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Fuel comparison of M6 VS LT4?


djheywood

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The title says it all.  Does anyone happen to have any data that compares the same boat with the LT4 VS M6?  I’m curious if the fuel consumption is like 10% worse, 30% worse, 50% worse etc.  Both surfing, idling and cruising.  The LT4 is tempting for power and speed but I’m hesitant without knowing the fuel burn rate.

This stuff should be on their website...

thanks

brian 

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I have a friend owns a 2019 24 MXZ with the LT4 and he says it is a gas guzzler, but I have the M6 and I think it is too. They both like gas!

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Looking at moving from a 22 LSV M5 to a 23 MXZ M6.  Any concrete numbers comparing the two, sorry about the hijack.  I can see how going from naturally aspirated to super charged and a big jump in HP and torque would impact fuel consumption but would not seem that the M6 would be that thirsty.  Don't really have a good gph number on the M5, we burn 24-28 gallons a day but have not kept up with the hours per day.

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In my MXZ we burn through about 60%+ on a full day of surfing / cruising / tubing.  (57 gallons x 60% = 34 gallons)

We are gonna be stepping up to a 25 for space reasons.  PM me if you want to talk about a gently used 2020 w 80 hours.  :-)

 

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21 minutes ago, djheywood said:

In my MXZ we burn through about 60%+ on a full day of surfing

What is "a full day of surfing"??  Some people, that is 3 hours of run time, to others it is 10 hours of run time.

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Sorry, for us it is probably 3 hours of surfing, an hour or two of tubing and the rest of the day idling, putting around cruising.  If we surfed for 8 + hours I am confident I could burn through a tank in one day easy.

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all good points above.  Just to be clear, I didn't start the thread because I was worried about the cost of the fuel.  It was more about if I can enjoy a full day on the lake with the LT4 without getting the dreaded low fuel warning.  I can definitely do this on the M6, even with my measly 57 gallon tank, but i'm beginning to think that if you put the LT4 in a 23 foot boat you could probably easily burn it up in one day.

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It seems that guys who have the engine in the M240 are reporting crazy low RPM numbers so I’d guess it will be very similar to your M6. Maybe even better. 25 will have a bigger fuel tank too I believe. 
 

I think you should get the LT4. Because, well... because it’s the LT4!

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6 minutes ago, justgary said:

If you look at this from a physics standpoint, a particular hull with a particular load running at a particular speed needs a specific amount of thrust.  More thrust and it goes faster, less thrust and it goes slower.  If you change the engine, you still need exactly that much thrust to go the same speed.  Thrust is produced through a combination of torque, RPM, gear ratio, prop pitch, and prop slip.  Torque is produced by igniting a specific volume of air with a specific volume of fuel in each cylinder as the engine rotates. 

The volume of air (and thus the fuel) is regulated by the throttle butterfly.  At idle, the butterfly is nearly closed, so not much air gets into the engine.  At wide open throttle, the intake manifold is basically at atmospheric pressure, and the engine can pump its full displacement of air.  A supercharger provides higher-than-atmospheric pressure to the intake, so the engine can pump more than its displacement of air (effectively increasing the engine displacement, and thus the potential torque at a given RPM).

The engine is a low-pass filter, meaning that it cannot produce more and more torque as RPM increases.  This occurs for several reasons, but the primary one is that it takes a finite time to pump air, mix the fuel, and ignite it inside the cylinders.  The result is that the engine has a peak torque at a specific RPM, and is less efficient at producing torque at a higher RPM.  It continues to produce more horsepower at higher RPM because HP = torque * RPM, but torque is really what you are after.  It is worth noting here that RPM alone is not an indication of torque produced, since one can rev his engine in neutral to very high RPM, yet the butterfly is barely open.  Torque is more accurately estimated by absolute manifold pressure times RPM, since that tells you how much air/fuel you are actually pumping through the cylinders.

In the case we are examining, we have a big boat laden with literally tons of ballast.  We aren't going to get to our desired speed with a 4 HP outboard since it doesn't produce enough thrust.  Clearly, we need a bigger motor.  Assuming we have a motor that is at least big enough, what can we do to optimize the fuel burn rate?  If your motor is barely big enough, you have to run wide open throttle and use the correct gear ratio and prop pitch to achieve your desired speed.  If you don't, you can't reach that speed.  But what if your motor is *bigger*?  Now you have an opportunity to optimize throttle, gearing, and pitch to hit your desired speed.

Your "easy" button for optimizing is by changing your prop pitch since transmission makers generally don't offer gear sets in different ratios.  The goal would be to get the motor to produce an appropriate amount of torque on the lower side of its torque curve, before it begins to flatten and then fall off due to the inefficiency of high RPM.  When you get done, you will find that each combination of throttle/gearing/pitch produces the exact same thrust for you to go that exact speed.

It is possible that some of you are using so much ballast that you can't get the RPM down with a different prop (see "barely big enough" above).  In that case, you are definitely underpowered.  Do not assume you are underpowered if you have not tried different props, since the prop you have was probably not chosen for optimizing fuel burn in your activity.  Assuming you have more motor than that, you should be able to find a sweet spot for your primary activity.  Once you do that, you can't really improve much on your economy.

The point of all of this?  If your engine is big enough, changing to another engine that is big enough won't change your fuel consumption very much if you have optimized your prop for your activity.  After all, the boat needs a specific amount of thrust to get to that speed, and that thrust is produced by a specific amount of torque, which is produced by a specific amount of air/fuel mixture pumping through your engine (with a specific gear/prop combination).  

If you want better fuel economy, take a look at your prop first, not your engine.

There are plenty of cars that come with different engine options and get different MPG numbers from those engines. An F-150 can be a 2.7 eco, 5.0 coyote or a 3.5 eco, and all will have different mpg numbers, given all other variables being the same.

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8 minutes ago, boardjnky4 said:

There are plenty of cars that come with different engine options and get different MPG numbers from those engines. An F-150 can be a 2.7 eco, 5.0 coyote or a 3.5 eco, and all will have different mpg numbers, given all other variables being the same.

Yes, over the same driving course consisting of a variety of speeds and load conditions.  In each of those cases, if you had a specific speed and load to optimize for, you would probably find that they are very similar by the time you get the gearing and wheel diameter optimized.

I see different ratings for F-150s with the same engine.  This illustrates how hard it is to compare engines when the same engine gets different mileage in a different trim package.  I simplified the physics, but I'll stand by the fact that it takes a specific amount of power to move your boat at a specific speed.

I will accept that you may not like your optimal prop because the engine might redline at 23 MPH when it isn't loaded.  That is the problem with optimizing, and why everything has tradeoffs to make in order to find a suitable solution.

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Also if your boat ballasted up the way you like it..at the peak torque rpm, that will give you the speed you want, will yield the best fuel economy. Impossible to make all that perfect so as @justgary says make sure your prop is optimized for what you want to do. 

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35 minutes ago, justgary said:

Yes, over the same driving course consisting of a variety of speeds and load conditions.  In each of those cases, if you had a specific speed and load to optimize for, you would probably find that they are very similar by the time you get the gearing and wheel diameter optimized.

I see different ratings for F-150s with the same engine.  This illustrates how hard it is to compare engines when the same engine gets different mileage in a different trim package.  I simplified the physics, but I'll stand by the fact that it takes a specific amount of power to move your boat at a specific speed.

I will accept that you may not like your optimal prop because the engine might redline at 23 MPH when it isn't loaded.  That is the problem with optimizing, and why everything has tradeoffs to make in order to find a suitable solution.

Absolutely agree with the bolded. We want to be able to surf efficiently at 11mph, Wakeboard at 22.5 efficiently and cruise the lake efficiently at 30mph. Impossible to do both.

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1 hour ago, boardjnky4 said:

There are plenty of cars that come with different engine options and get different MPG numbers from those engines. An F-150 can be a 2.7 eco, 5.0 coyote or a 3.5 eco, and all will have different mpg numbers, given all other variables being the same.

Importantly, there are many cases where the bigger engine produces better economy (ie, BMW 3L has better city mpg than the 2.5L) This translates to saying that a bigger engine with a lower pitch (but 15" diameter prop) likely gets better fuel economy than the smaller engine (with a 14.5" diameter prop) at certain speeds.
 

 

15 minutes ago, boardjnky4 said:

We want to be able to surf efficiently at 11mph, Wakeboard at 22.5 efficiently and cruise the lake efficiently at 30mph. Impossible to do both.

  This is definitely true, with fixed gears and props there is a search for a middle ground; More than likely, you won't get 11mph and 30 mph as efficiently as 11mph and 25mph;  AND everybody should try to surf at 12mph plus if they have the ballast (and with front ballast) because the boat is closer to plane.. if they're worried about efficiency and wave sizes, 

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So in a 5 hour day in the m240, with the lt4, we used about a half a tank, mostly surfing, some wakeboarding, very little idle time. 

In my 25 lsv with m6, we burn just over 8 gph surfing, also with little idle time. That 87 gallon tank lasts us about 3-4 days on average. I definitely feel like i am constantly needing to fill it.

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3 hours ago, mikeo said:

If you want to see the actual fuel flow rate, you can put a Scangauge in and look at the actual numbers:
https://www.wakegarage.com/projects-archive/miscellaneous-projects/accurate-fuel-flow-gauge-r110/

In addition to the tank levels, you can see GPH data.

my only problem with the ScanGauge is that its not 100% accurate.  Without a physical flow sensor in the fuel system, the gauge is only getting "calculated" flow rates based on the multitude of fuel/flow charts in the ECM programming.  None are dead on balls.  They all have play in them.  Otherwise, we wouldn't need closed loop metering.

I wonder...has anyone put a WB O2 sensor on one of our engines to see what kind of A/F ration we are running?  That might be enlightening....

Edited by Texan32
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1 hour ago, Texan32 said:

my only problem with the ScanGauge is that its not 100% accurate.  Without a physical flow sensor in the fuel system, the gauge is only getting "calculated" flow rates based on the multitude of fuel/flow charts in the ECM programming.  None are dead on balls.  They all have play in them.  Otherwise, we wouldn't need closed loop metering.

I wonder...has anyone put a WB O2 sensor on one of our engines to see what kind of A/F ration we are running?  That might be enlightening....

If you have a clogged injector or similar you'll have some error, but I've been within .01 (one hundredth) of a gallon filling up when I use the same pump and fill on the trailer. The whole reason I went down the ECM/Scangauge path was to get an accurate tank reading since the floats in the tank only read in 1/8 or 12.5% accuracy for a motionless volume. A physical flow sensor doesn't work with the newer injected systems that have a fuel return line, of those that also measure the fuel lines (supply and return) I couldn't find one that was approved for marine use. (there were a bunch of on-road/race options)

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14 hours ago, mikeo said:

If you have a clogged injector or similar you'll have some error, but I've been within .01 (one hundredth) of a gallon filling up when I use the same pump and fill on the trailer. The whole reason I went down the ECM/Scangauge path was to get an accurate tank reading since the floats in the tank only read in 1/8 or 12.5% accuracy for a motionless volume. A physical flow sensor doesn't work with the newer injected systems that have a fuel return line, of those that also measure the fuel lines (supply and return) I couldn't find one that was approved for marine use. (there were a bunch of on-road/race options)

thats pretty awesome that you are getting that much accuracy for what you were looking for.  My point was that the scangauge can only be as accurate as how good the engine mapping and how well the engine is performing (to still efficiently utilize the tune).

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9 hours ago, Texan32 said:

thats pretty awesome that you are getting that much accuracy for what you were looking for.  My point was that the scangauge can only be as accurate as how good the engine mapping and how well the engine is performing (to still efficiently utilize the tune).

It is probably more a function of the fuel pressure regulator and clean injectors than ECM mapping.  The ECM simply integrates the injector times and calculates total gallons used by assuming a specific fuel pressure.  With a closed loop ECM (with O2 sensors), the ECM can probably even estimate fuel pressure deviation by watching how much more or less it has to inject compared to the programmed value.

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13 hours ago, justgary said:

It is probably more a function of the fuel pressure regulator and clean injectors than ECM mapping.  The ECM simply integrates the injector times and calculates total gallons used by assuming a specific fuel pressure.  With a closed loop ECM (with O2 sensors), the ECM can probably even estimate fuel pressure deviation by watching how much more or less it has to inject compared to the programmed value.

Im not sure how much ECM tuning has changed.  But i was pretty deep in GM tuning in the early 2000's.  The fuel pressure was not one of the systems the ECM monitored for fueling.  And because its not a monitored input for adjustment, i can deff see that causing variations.  The big ones we worked with were the (base) Static Injector flow rate and the MAF table.  We would also tweak the PE mode (let it kick in a little sooner for blower engines) and of course the open loop tables.  But i had a WB O2 on my car so i actually knew what my A/F ratio was for that.  I have tuned numerous stock vehicles where the LTFTs were in the double digits.  Thats not what i would call good factory tuning.  Are you getting the correct A/F ratio in closed loop?  Sure.  But the ECM is working its butt off trying to compensate to get there.

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On 9/3/2020 at 7:04 AM, Texan32 said:

Im not sure how much ECM tuning has changed.  But i was pretty deep in GM tuning in the early 2000's.  The fuel pressure was not one of the systems the ECM monitored for fueling.  And because its not a monitored input for adjustment, i can deff see that causing variations.  The big ones we worked with were the (base) Static Injector flow rate and the MAF table.  We would also tweak the PE mode (let it kick in a little sooner for blower engines) and of course the open loop tables.  But i had a WB O2 on my car so i actually knew what my A/F ratio was for that.  I have tuned numerous stock vehicles where the LTFTs were in the double digits.  Thats not what i would call good factory tuning.  Are you getting the correct A/F ratio in closed loop?  Sure.  But the ECM is working its butt off trying to compensate to get there.

Fuel pressure is monitored now.

O2 sensors are also wideband.

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  • 3 months later...

We bought a 25LSV with the LT4 in early November, so we don't have a ton of hours on it.  However in the 16 hours we have thrown down, it was getting a workout, lots of surfing and a bit of touring.  We were pushing it to break in the motor on the touring part, varying between 28 and 47MPH, so gas consumption was up there.  Bottom line is, I'm thrilled with it's burn rate.  It's averaging 5 to 5.5 GPH.  Which is basically what I was getting out of my old 383 Hammerhead, but there's a lot more boat to push around, and the power upgrade is astounding.  I never would have believed a boat this size would hole shot so hard.

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