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Old 12-17-2014, 10:20 AM   #1
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Angry New Cummins ISB vs. ISC

I've been wondering why all the new entry level coaches feature an ISB and I find no ISC's. I also find the GVW's are also lower as well as the towing capacity. I thought at first that this might be something brought on by the coach manufacturers to keep costs down. When I went to the Cummins site, it looks like the ISC was retired in 2010. Is this true and was it forced on the industry by emissions standards?
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Old 12-17-2014, 11:21 AM   #2
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I don't think it was 2010, as when we were researching for our 2013, both the Phaeton and the Ventana had the ISC in their shorter (less than 42') rigs. I know that in 2014, both came with the ISL, but detuned down to 380hp and I was told that the ISC was no longer available.

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Old 12-17-2014, 11:34 AM   #3
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The ISB is generally suitable for coaches 40 feet or less. I have the 350HP ISB on my 09 Winnebago Journey. It has 750 ft-lbs of torque and works fine and provides 8-10 mpg on my 30K lb coach. It is definitely not a racing engine but is a good compromise between power and economy for most coaches in that range. I am not a diesel engine expert but am satisfied with the ISB.
The ISC was discontinued by Cummins a couple of years ago as you have found and replaced with an lower rated ISL (around 400HP) with the same torque as the 450 ISL.
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Old 12-17-2014, 03:14 PM   #4
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Cummins website still shows the ISC with it 1050 lb ft as an available engine. With heavy mh, torque is everything. I'm at 860 and would love a couple hundred more. ISB 6.7 is essentially the same engine I have in my Ram pickup. Works fine there but I'm about 7,700 lbs until I hook up the trailer.
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Old 12-17-2014, 03:21 PM   #5
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I just went through this with the ISC engine, Cummins still make one, it is the Chassis manufacture's that choose not to use the ISC engine........Spartan for one, Freightliner seems to still offer the ISC on their Chassis.........
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Old 12-17-2014, 03:39 PM   #6
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I just went through this with the ISC engine, Cummins still make one, it is the Chassis manufacture's that choose not to use the ISC engine........Spartan for one, Freightliner seems to still offer the ISC on their Chassis.........

The chassis/coach builders like the ISB on the coaches that overlap with gas chassis and up to a GVWR of 30,000 pounds or so. The move up to an ISC also requires the Allison 3000 so the up grade is pretty expensive. I think Cummins pushes the ISL at that level for max h/p and torque for the money. They have already sprung for the bigger transmission and the marketing folks love writing up the ISL at 400-450 h/p.


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Old 12-17-2014, 04:01 PM   #7
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The chassis/coach builders like the ISB on the coaches that overlap with gas chassis and up to a GVWR of 30,000 pounds or so. The move up to an ISC also requires the Allison 3000 so the up grade is pretty expensive. I think Cummins pushes the ISL at that level for max h/p and torque for the money. They have already sprung for the bigger transmission and the marketing folks love writing up the ISL at 400-450 h/p.


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I agree Steve, Does the ISB with up to 360 HP also need the 3000 series tranny though?
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Old 12-17-2014, 04:03 PM   #8
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Ours has the ISC 8.3L mated to the Allison.
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Old 12-17-2014, 04:14 PM   #9
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Just simple arithmetic: the ISB now has sufficient horsepower (340) to meet the needs of most coaches up to around 40 feet. The ISC used to fill the slot between 300 and 400 hp, but the ISB is doing that now a a lower cost. If 380-425 is needed, the ISL fits the slot nicely.

How much is needed? Just about all RV makers use the 100 lbs/hp rule of thumb.
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Old 12-17-2014, 05:01 PM   #10
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From what I have gathered, since using DEF allowed higher HP and torque,
the ISB is now often used in 36' and under and then the ISL is used above that.
The ISC is 8.3L and the ISL is 8.9L, both straight six cylinders, so not
a lot of difference in output. With the extra HP available with DEF,
the ISC was no longer needed, as it was quite close to the ISC.
And yes, the ISC is still made, it is a popular choice in marine applications
that do not have tight emissions standards, as it easily puts out 550-600HP.
It seems some chassis guys are still using it, and it is an excellent engine.
I'm not sure I would want an ISB in a 40' coach. My ISC 360hp/1050
gets down to 37-39 mph on some steep grades, and I get 8.2 mpg overall.
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Old 12-17-2014, 05:17 PM   #11
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Guys - can a 2014 ISB 340 horse and 2500 tranny be upgraded to have more HP and Torque. The 36M has 360 hp and a 3000 tranny?
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Old 12-17-2014, 05:54 PM   #12
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I agree Steve, Does the ISB with up to 360 HP also need the 3000 series tranny though?

The current max limits for all the Allison's below the 3000 are 340 hp and 575 lb/ft torque. The 3000 now has a limit of 450/1250 and the big 4000 is good up to 650/1950. With these limits the ISB has maxed the 1000-2500 transmissions. The ISL is at the limit of the 3000 and the ISX15 is at the max for the 4000.


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Old 12-17-2014, 06:12 PM   #13
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I was told that the ISB 6.7 340 has 660 torque? Was I mislead?
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Old 12-17-2014, 06:18 PM   #14
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I was told that the ISB 6.7 340 has 660 torque? Was I mislead?

Depends. Cummins rates the ISB 340 at 660. I'm guessing that the engine ECM communicates with the Allison TCM to limit torque in certain gears or other specific circumstances to protect the transmission. The electronics really allow some fine tuning of components.


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