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Old 05-29-2012, 07:47 AM   #1
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Wink Engine formula? Riddle me this ...

Oh, great Knowledge Base in the sky ...

here's a question or three. While looking at all these Motorhomes, engine size has not been addressed much. As someone whose father raced Corvettes, I tend to think bigger numbers are better. I know someone said in one of these threads you need the power to pull up the hill, but someone else reminded bigger also means bigger $bucks$.

So, real life, scenario ... I'm looking at 2 MH.

MH "A" is 39'; GVW 32,400; GCW 42,400; Cummins ISB 6.7L 360HP
MH "B" is 40'8"; GVW 32,400; GCW 47,400; Cummins ISL 8.9L 450HP

Loaded with a toad I figure I would max out MH "A" right at the 42-43K
same stuff leaves me 5k under max on MH "B"

All other things being equal ...
am I burning more fuel on MH "A" running it at max load
or does the smaller engine save me fuel?

or vice-versa ... am I wasting fuel with MH "B" on the bigger engine,
or does the bigger engine running 5k under max load run more efficiently and save me fuel?

Any way to tell about what mpg I might get on each engine?

And finally - if the MHs are equal in all other respects (they are definitely NOT, but we are hypothesizing here! ) and the only difference is the engine, anyone know about how much more the big engine would up the cost?

LOL! There's a riddle for you!

Jus' askin'.
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Old 05-29-2012, 09:30 AM   #2
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What years are the MHs because in 2010 the 6.7 went to SCR system in compliance with EPA. The 6.7 Cummins in 2010 uses DEF fluid and Cummins raised the timing. I have a 2011 Entegra with this system, and tow a 4,800 jeep without any problem. I have averaged a high of 10.4 mg w/o tow and 9.2 with the toad.
Most everyone will agree, I'm sure, that your floorplan considerations are more important than HP.
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Old 05-29-2012, 09:56 AM   #3
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Kathryn,

The Cummins ISB is the economy champ but in my opinion, a loaded weight of over 40k is too heavy for that engine. The transmissions may be different also. The ISL will have the Allison 3000 and the ISB might be a 2000 series. The 3000 is the standard by which all others are measured. The ISB will struggle with that kind of weight and, I suspect, will return about the same mpg as the ISL with the same load. The driving experience will be much better with the ISL.

If you drive conservatively both engines are capable of 8 mpg + or -.
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Old 05-29-2012, 10:25 AM   #4
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I agree totally with Steve's analysis
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Old 05-29-2012, 10:37 AM   #5
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The unofficial "formula" is 1 HP per 100 lbs of weight. Since you'll be "maxing out" MH A, I would say you'll be under that 1 HP per 100 lbs of weight.

Running around at maximum weight will place more stress on all your components (or at least the "weakest" component in the chain). The larger the margin you have, the longer all your components will last. Bear in mind that is comparing two MHs of the same age, mileage and condition at the time of purchase.

From my personal experience, my 450 HP engine does well in pulling my rig which is typically loaded to 38,000 lbs GVW and is 42 ft. We tow a 4,000 lbs Wrangler which puts our GCW at 42,000 lbs. Bear in mind too though that brute "pulling" power is better seen in terms of torque than HP.
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Old 05-29-2012, 11:31 AM   #6
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The old "how fast do you want to go, how much do you want to pay?" Comes into play too.

Going from an isb to an isc will add 25k to the cost.
Going to an isl will do that again.

That larger engine will come with a beefier chassis, suspension, drivetrain, all wieghing and costing more.

And yes, a 6.7 liter isb under full load, cannot burn as much fuel as an 8.9 liter isl under full load.
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Old 05-30-2012, 03:23 AM   #7
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Quote:
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The old "how fast do you want to go, how much do you want to pay?" Comes into play too.

Going from an isb to an isc will add 25k to the cost.
Going to an isl will do that again.

That larger engine will come with a beefier chassis, suspension, drivetrain, all wieghing and costing more.

And yes, a 6.7 liter isb under full load, cannot burn as much fuel as an 8.9 liter isl under full load.
Don't really want to go that fast, but I do know that equipment running at its upper limits wears out a lot quicker than stuff running under lighter loads.

Okay then - so just the change up in the engine model alone will cost/is worth an extra $50k. So when I am considering the differences - I can pretty well deduct that 50k from the higher price (as a fair cost for engine upgrade), and the margin that is LEFT is roughly the cost of the OTHER differences between the two!

I know I might be making it sound complicated, but in comparing the two felt it was not fair to include the normal cost of the engine upgrade. Now I can decide if the other upgrades tip the scale for the big bucks! LOL.
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Old 05-30-2012, 07:43 AM   #8
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Keep in mind that torque is king when moving large, heavy objects (such as motorhomes). Horsepower isn't that important unless you are measuring acceleration from a standing start (or merging up an on-ramp). The ISL will have 1,250 ft-lbs of torque whilke the smaller displacement ISB will have quite a bit less. This means that the ISB will spend more time in the higher revs as it downshifts to make it up grades while the ISL will chug along at lower RPM. The faster an engine revs, the more air (and fuel) it pumps through it so running a smaller engine harder at higher RPM isn't going to gain you any fuel economy benefit over a larger engine that's working lazily at a lower RPM.
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Old 05-30-2012, 07:49 AM   #9
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Keep in mind that torque is king when moving large, heavy objects (such as motorhomes). Horsepower isn't that important unless you are measuring acceleration from a standing start (or merging up an on-ramp). The ISL will have 1,250 ft-lbs of torque whilke the smaller displacement ISB will have quite a bit less. This means that the ISB will spend more time in the higher revs as it downshifts to make it up grades while the ISL will chug along at lower RPM. The faster an engine revs, the more air (and fuel) it pumps through it so running a smaller engine harder at higher RPM isn't going to gain you any fuel economy benefit over a larger engine that's working lazily at a lower RPM.

Got it! Good explanation for those of us less mechanically inclined!
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Old 05-30-2012, 11:33 AM   #10
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A 5.9L ISB pushing 40,000 lbs would do well at the Utah Salt Flats, emphasis on Flats.

I would think uphill would be a big performance challenge.

I have an 860 ft lb C8.3L 300hp that works hard going uphill with 36,000 lbs.

Or, maybe the newer 5.9L ISB has a lot more torque than they used to have?
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Old 05-30-2012, 12:00 PM   #11
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Quote:
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Kathryn,

The Cummins ISB is the economy champ but in my opinion, a loaded weight of over 40k is too heavy for that engine. The transmissions may be different also. The ISL will have the Allison 3000 and the ISB might be a 2000 series. The 3000 is the standard by which all others are measured. The ISB will struggle with that kind of weight and, I suspect, will return about the same mpg as the ISL with the same load. The driving experience will be much better with the ISL.

If you drive conservatively both engines are capable of 8 mpg + or -.
Whoa Nelly. I tow a 4400 lb toad, 03 Tracker, with a 5.7 ISB rated at 300 HP and we do NOT struggle at all. I am assuming the ISB in question is properly sized for the MH you are looking at.

There are few mountain passes that we drop into 3rd, but mostly 4th. Temp seldom varies except on very hot days.

Our ISB gave us 12 mpg towing the toad throughout the midwest and about 10 in the mountains.

I would not sell an ISB short.
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Old 05-30-2012, 12:15 PM   #12
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I've learned over the years that smaller isn't always better. If the engine works to hard it is worse than a bigger engine. Even with cars I had a 67 nova with a inline 6 cyl and swapped a 283 v8 in and it was much easier on gas and the engine.
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Old 05-30-2012, 01:05 PM   #13
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I'm at 100% on any tough grade, with a 360hp isc in a 33k coach.
It works fine, better than the v10 gasser i used to have, but still has a bit of a time on hard pulls.

But she always gets over the top, without that gasser screaming between my legs.

A 550hp isx sure would be nice on those hills...

The new 6.7 isb's are building a good reputation. Not sure i would discount them as quickly as in the past. That said, i did choose a 3 year old ISC Monaco over the new same price isb sportscoach the DW wanted.
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Old 05-30-2012, 01:07 PM   #14
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At first I thought the OP made a typo saying an ISB was powering a coach with a GCWR of 42,400. JMHO, but that's not enough engine for that much weight. I don't know what the torque is for that engine, but the 450 HP ISL has 1,250 ft/lbs of torque which is probably close to twice the ISB's.
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