27 replies to this topic

[wolfe10]

Peak Torque of 1050 ft-lb is reached at 1400 RPM, it would be recommend for
best overall performance and fuel mileage, to run the engine at approx.
1700 RPM.

On my next trip at the end of this month I will try running at 1700 rpm to see if I get better fuel mileage. I can maintain these rpm at lower speeds by locking out 6th gear if needed. I also realize that tire pressure, breaking and terrain come into play here. Thanks for the info that you guys provided. It got me curious. I'll let you know if I improved my fuel mileage.

Road Dog

Road Dog,

Read VERY carefully the statement they give you. They did NOT, repeat NOT address what RPM to run the engine for best MPG. They gave an almost double talk answer of BEST OVERALL PERFORMANCE and fuel mileage.

You will get better MPG at closer to 1500 RPM if on relatively flat terrain (where it does not downshift on every upgrade). In rolling hills/mountains, you will need to run a little faster to get closer to "Best Overall Performance."

1500 and "economy mode" on the Allison will likely give you best MPG.

Brett Wolfe

[Road-Dog]

Brett......Thanks for the info. I do use the economy mode when I'm not in the mountains. I did notice on my last trip when I was running 55 and locked out 6th gear that it had real good power. I don't recall what rpm I was actually running. I'm getting old. LOL
Thanks again for the info.

Road Dog

[Labrador47]

I agree with Tom. Your criuse speed has a lot to do with your fuel economy. The wind is beyond your control.

We are running a Country Coach powered with a Cummins ISX 600-1850 lb.ft. torque and weighing in at 51,000+ on the 'CAT' scales at the travel center with full fuel and water.

We tow a Saturn Vue most of the time. I found that running at 68-70 mph on the highway was delivering me 6.5-6.8 mph on average.

A head wind is a curse! It can rob you of another 10-15% of your economy at almost any speed. It does seem that life is a head wind mose of the time.

We decided that we would slow down a little and attempt to save some money by cutting our criuse speed to 63-64 mph on those long runs.

Well, we are now enjoying 7.3-7.6 mpg now on a regular basis.

That big flat front on our coach really plows thru the air like a brick. The higher our speed the more wind resistance created.

We take a little bit longer to get to our destination but it is worth it. With diesel over $5.00/gal last year we learned a good lession. $.70c a mile just for fuel wasn't any fun. besides, in the scary event of a front tire blow out, controlling the wagon is a bit easier at 60 that at 70.

Bill Herring
F122708
2008 Country Coach Affinity
ISX600/Allison MH4000
Naples, Fl.
Toad-Saturn Vue

[deenad]

Peak torque on the ISC is 1400 rpm, on the ISL's it's 1300.

We have the Banks kit on our ISC 350 and have 1200 ft lbs from 1400 to 1700 rpm, I still try to run around 1600 rpm even so. I also find that, with the much wider torque band I run in "economy" mode most of the time.

[FieldEngineer]

Maximum fuel economy is achieved by operating your engine at the RPM where the BSFC, Brake Specific Fuel Consumption, is logged at its lowest. The engine manufacturer has this data. Also, in response to some who said that operating at RPM below torque peak damages the engine; this is only true if the engine will not acclerate smoothly if you step on the pedal. When you take off from a stop, you are operating at less than torque peak RPM and you also know that the engine acclerates without straining. Where operating at below peak torque (lugging) occurs, the engine will not accelerate under load....this is damaging specifically to the connecting rod bearing (upper half). Your auto trans should be set up to prevent this from occuring by downshifting accordingly.

Gary Spires
Retired Cummins Field Service Engineer

[AndyShane]

Wow. Great topic. I'll pay more attn to the peak torque rpm now, for sure.

I've noticed in general, the eastbound drivers are turning in significantly better mileage. Indeed, Caterpillar says in the guide Brett mentions that we can reap a 12% fuel savings from wind alone.

Plus, engines vary wildly: my C7 turns in the 7.2 avg of them all, which I can improve upon using the techniques you've been discussing. My buddy with his mammoth Libertyjust returned from a week of camping in Colorado, again beat 8.0 mpg for the trip, despite his heavier weight and 515 hp engine. Similarly, the couple from whom we bought our Fleetwood purchased a Newell and it got better mileage with more hp and a heavier bus.

[mpoore]

Hello, As a first timer and non mechanical person, I'm open to suggestions. I have a Damon Intruder, 36 ft. Workhorse chassis, 8.1L engine, 345 HP, Allison transmission. I generally leave it in overdrive mode all the time. Is this a good idea or should I shut off the OD when in city traffic. When in OD it shifts into high gear at around 55 MPH and downshifts at around 50 MPH. The longest trip so far (I've only had it for 2 weeks) was around 350 miles in hilly but not mountainous terrain (not loaded and no tow vehicle) and I got 7 mpg exactly.

Thanks for any suggestions.