MPG vs HP Compromise

[jtcawrse]

Hello All,

I have a 2001 M1088A1 with the CAT 3126 engine.
I'm building it into an overland vehicle for traveling the world.
I've replaced the fuel tank with 151 gallons.
I've already had Bryce change the gearing.
I expect to be well under the max GVWR for this truck.

I know there is a tradeoff between horsepower and miles per gallon.
I believe the M1088 has the engine set at 330 HP but this has not yet been verified by a CAT computer.
I expect to get around 8 MPG, also not verified yet.

The question I have is, does anyone have any real world stats about the different available HP for this engine and the miles per gallon?

Would I notice a difference in increased mileage and longer life of the engine if I reduced the horsepower?

Would I miss the horsepower in highway driving, especially on hills?

Any thoughts or experiences appreciated.
Thank you in advance for your thoughtful responses.

 

[simp5782]

Hello All,

I have a 2001 M1088A1 with the CAT 3126 engine.
I'm building it into an overland vehicle for traveling the world.
I've replaced the fuel tank with 151 gallons.
I've already had Bryce change the gearing.
I expect to be well under the max GVWR for this truck.

I know there is a tradeoff between horsepower and miles per gallon.
I believe the M1088 has the engine set at 330 HP but this has not yet been verified by a CAT computer.
I expect to get around 8 MPG, also not verified yet.

The question I have is, does anyone have any real world stats about the different available HP for this engine and the miles per gallon?

Would I notice a difference in increased mileage and longer life of the engine if I reduced the horsepower?

Would I miss the horsepower in highway driving, especially on hills?

Any thoughts or experiences appreciated.
Thank you in advance for your thoughtful responses.

Install a rail pressure gauge and drive off of that.

More hp you are going to burn more fuel period. I've got a truck set at 515hp and 1800ftlbs and it gets 3.5mpg loaded at 77k at 80mph .Another Identical truck stock with 400hp 1350ft lbs at 77k gets 4.5 running 80mph.

Speed is money how fast can you afford to go.

Of course hitting any down shift points at the proper curve is going to help alot

 

[Ronmar]

I think when you are dealing with this much weight, rolling drag and wind resistance, finding MPG sweet spots is not easy. I think you have probably done the best thing in that regard by re-gearing. Getting the sustained load closer to the peak torque RPM is probably the best thing you can do for it.

At that point, the engine is going to use whatever HP it requires, to meet the loads above, at highway speed. You could tune it up for 600 but it is still going to use only what it needs to sustain that load.

So regardless of the tune, the real controller of the MPG will always be your foot…

 

[simp5782]

I think when you are dealing with this much weight, rolling drag and wind resistance, finding MPG sweet spots is not easy. I think you have probably done the best thing in that regard by re-gearing. Getting the sustained load closer to the peak torque RPM is probably the best thing you can do for it.

At that point, the engine is going to use whatever HP it requires, to meet the loads above, at highway speed. You could tune it up for 600 but it is still going to use only what it needs to sustain that load.

So regardless of the tune, the real controller of the MPG will always be your foot…

Why I use a gauge to measure injector pressure and drive off it. You will see times it's pulling at 200psi but letting off the throttle to 120psi will Maintain the same speed.

 

[Ronmar]

Yep, HEUI oil pressure can indicate how hard you are working it…

 

[Third From Texas]

My old 3116 truck had 230hp and got 7mpg. It SUCKED just going over the causeway bridge here to the beach (it would drop to 45mph on the slight incline).

My M1079 w/C7 has 330hp and gets 5mpg (but it's a heavier truck). It does 58mph w/o slowing while going up anything I've driven across three states.


All measured via fill ups and actual mileage (I used to be pretty mythodical with keeping the records until I realized the C7 mileage simply doesn't falter no matter what. I do plan on getting the real-time C7 data when I redo the cab and dash (I'll have a display interfaced).

 

[Keith Knight]

I bought my 2003 M1078A1 5 years and have driven it over 5,000 miles. 2,000 miles at top speed of 58MPH and RPMs screaming at you with the original gears and got 3.5-4 MPG.

I changed the ring and pinions out added a rear locker and had CAT reprogram it to 330 HP. Driven it with this configuration for 3,000 miles and now with the cruise control set at 60MPH I get 6-6.5MPG and the engine sounds much happier. Top speed is now 74MPH but who needs to drive it that fast!
Lower RPM’s with more power hits a sweet spot.

 

[coachgeo]

you will get more MPG improvements by doing aerodynamics than anything else..... such as incorporating a cab roof air dam to smoothly guide air up over the habitat roof. Maybe rear wings like used at ass end of big rigs. same with removable air guides like what hangs below big rig's trailer? Guess you could do removable front; under bumper , air guide to smooth the slice into the air that big brick driving down the road is doing???

pic for attention.... never heard anything about this truck besides the original post of the pics.... years ago

 

[Third From Texas]

I'll test that theory someday...

 

[Ronmar]

Yea air drag is a big factor, you double the air velocity, you quadruple the drag… for practical purposes, you are going to have to do some pretty extensive modifications to make these rough contoured bricks any more aerodynamic… The kicker for an over lander is the need for protective structures and solar panels ect. It is nearly a lost cause,mand simply driving slower is probably far more practical…

Gearing is equally as big an issue. Peak torque for the 3126 is 1440 RPM, it is 1560 RPM for the 3116. The closer you get to peak torque/volumetric efficiency, provided the engine can make enough HP to perform the work required at that RPM, the more efficient you should be.

If my math is correct, 3.07’s turn a 2600 RPM cruise into a 2049 RPM cruise. 2.87’s turn a 2600 RPM cruise into a 1911 RPM cruise. It is probably not going to crawl as well but with the excellent torque converter in the Allison, this probably isn’t as big an issue...

 

[Tow4]

My M915A1 gets 5.5 to 6 mpg pulling my M373 trailer at about 60 mph. Full disclosure, I did run the A/C and that may have hurt the mileage a little. My M929 got similar mileage. It's a toy, and I don't care what the mpg is. If it was a daily driver I would.

If you bought this truck and are concerned about the fuel mileage, you bought the wrong truck. You buy a truck like this to do a job. If you are going to work it then you factor that in. If it's a toy, then don't worry about it.

 

[Third From Texas]

When anyone walks up and asks I like to tell them "if I cared about fuel economy I wouldn't have bought an army truck"

 

[coachgeo]

When anyone walks up and asks I like to tell them "if I cared about fuel economy I wouldn't have bought an army truck"

Makes sense that attitudes flex after big truck purchase based on size of pocket and price of fuel. Hard to enjoy it if you can't afford to drive it "like you once could"

 

[coachgeo]

I'll test that theory someday...

View attachment 867653

what truck is that one off of?

 

[Third From Texas]

what truck is that one off of?

Guy said it was off a Freightliner. It's sectional and I've got it in pieces. With the lower sections and the rear off it sits pretty well on the S&S cab roof. My "plan" is to incorporate it into the roof rack (both as air deflector and storage cubby). But I'm still a couple years away from doing the exterior and repaint stuff.

 

[coachgeo]

btw here is a kit that fits our cabs supposidely (Man L2000 is said to be basically same cab)

 

[Green Mountain Boys]

When anyone walks up and asks I like to tell them "if I cared about fuel economy I wouldn't have bought an army truck"

When I get the same question, and they have an attitude or tone in their voice like I am responsible for climate change, I say "Pound for pound I get better fuel mileage than your (insert vehicle name)".
That usually ends the conversation right there because they have to stop and think a little.
I use my truck for work so I usually have a load on it.

 

[Third From Texas]

When I get the same question, and they have an attitude or tone in their voice like I am responsible for climate change, I say "Pound for pound I get better fuel mileage than your (insert vehicle name)".
That usually ends the conversation right there because they have to stop and think a little.
I use my truck for work so I usually have a load on it.

I'm gonna have this printed up some day. Used to be able to buy them online but they stopped selling them.

Yeah, make 'em think...

 

[Keith Knight]

They truly don’t need the air foil. Before I decided to put my mini split A/C unit in the middle behind the cab just above the roof line. I was concerned going 60 mph that the wind blowing into it would cause problems manly because the directions say not to allow more than a 10 mph wind to blow into it.
Sooooo…..I purchased a wind meter got between the cab and habitat, had my wife drive 60 mph and max wind speed registered was only 7 mph. Because of our flat front air goes up and over the habitat.

 

[Awesomeness]

you will get more MPG improvements by doing aerodynamics than anything else.....

That is a misleading or confusing way to phrase or think about the problem. The aerodynamic losses make up the smallest contribution, and so even if you could make them totally go away, it would only have a very small effect. However, playing with aerodynamics is way easier to do, compared to changing the efficiency of an engine, especially for laymen.

You can look up various studies on the topic, and you will find that the vast majority of wasted energy happens at the engine. I don't remember the exact numbers, but I'll throw out something close (within 5-10%?). The engine wastes 70% of the fuel's power (in other words, for every gallon of fuel you burn, you only get 30% of that energy out as the crankshaft rotating). A further 20% total is wasted by the drivetrain (e.g. transmission friction, axles, etc.). And the last 10% is wasted by the aerodynamics and rolling resistance of the truck.

What this is telling you, is that if you get 5MPG, even if you could make aerodynamics totally go away, you could only gain 10%. In reality, adding big airfoils between the cab and cargo box, or rear "cones", bumps the total MPG by like 1-2%. Fleets do it because that 2% over a million miles driven per year adds up to big savings. One FMTV overlander driving 2000 miles per year, at 5MPG, makes a 4 gallon ($20) difference. (PER YEAR!)