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Old 07-24-2015, 11:10 AM   #1
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Cummins ISBXT 360

I am seeing the latest model of this engine as a 2011 in a Discovery with a max torque of 750 foot lbs of torque. I am buying a new 2016 NeXas Bentley class A 36 footer still on the factory floor which claims to have a cummins ISBXT 360 2400RPM, 800ft lbs of torque. By the 50lbs of torque per lb of weight rule, it maxes out at 560lbs of torque which means I will have 240lbs of torque to play with....am I right or did I go wrong someplace in the engine or the formula.

WDK
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Old 07-24-2015, 06:14 PM   #2
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Never heard of that "rule". Have heard of 100 lb/hp rule of thumb, though. You can convert torque to hp with this formula:

HP = torque x rpms/5250
You need to know the rpms at which the torque number occurs (it is thepeak torque rpm)

Not quite sure what you mean to have 240 ft-lbs "to play with". Extra torque just means you don't need to max rpms to get adequate performance.
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Old 07-24-2015, 07:38 PM   #3
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It depends on what your coach weighs. It could feel like you have some extra to play with or like your ankle is tied down with a 50# ball & chain.
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Old 07-24-2015, 09:59 PM   #4
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I am just trying to quote properly what I read....I could not find the exact formula to cut and paste......but as best I can recall it....50lbs of torque per 1lb of weight of your coach is the ratio you should be shooting for in an engine.. in my case, my coach weighs 28,000lbs. As per the formula, 28,000 divided by 50lbs of torque equals 560lbs of torque telling me that what ever engine I do have or choose should not have less than 560lbs of torque to properly push 28,000lbs. The cummins 360 ISBXT has a max of 800lbs of torque so according to the formula, I have 240lbs of torque more than the 560 needed. I read that on this forum so if I am getting it wrong I am here to be schooled it just seemed to good to be true that my torque to weight ratio was that good as per the formula.

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Old 07-24-2015, 10:52 PM   #5
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Cummins ISBXT 360

I suspect that the 50lbs torque per lb weight is probably the least amount you'll need to efficiently tackle the daily grind. I also suspect hills, wind and extra weight will take care of what you call extra to play around with. Try climbing steep mountain pass and it won't seem like extra...you'll be wishing you had more. According to the your math my coach 33k needs 660 ft lbs of torque. No way would that work...I'm at more than double that (1450) and at times still wish I had more. IMHO It's just a numbers game or at best maybe a loose guideline.


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Old 07-24-2015, 11:21 PM   #6
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I'm a bit confused regarding the way the rule is stated...it seems that dividing the weight by 50 yields the desired ft-lbs of torque... but it doesn't seem like you want 50 ft-lbs per pound rather you want 1 ft-lb per 50 lbs of weight. Unless I am misunderstanding something...At 50 ft-lbs per lb you would want 50 x 28,000 or 1,400,000 ft-lbs of torque...That would tear the road up!
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Old 07-25-2015, 10:04 AM   #7
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Quote:
Originally Posted by ecurb74 View Post
I suspect that the 50lbs torque per lb weight is probably the least amount you'll need to efficiently tackle the daily grind. I also suspect hills, wind and extra weight will take care of what you call extra to play around with. Try climbing steep mountain pass and it won't seem like extra...you'll be wishing you had more. According to the your math my coach 33k needs 660 ft lbs of torque. No way would that work...I'm at more than double that (1450) and at times still wish I had more. IMHO It's just a numbers game or at best maybe a loose guideline.


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2000 Foretravel U320 4010
Cummins ISM 450hp
Allison HD4060R 6 Spd W/Retarder
Even if the formula states the minimum torque value one should look for in the size coach you want to buy, you have defined my point or at least what I was asking in that at 28,000 pounds and 800lbs of torque I do have at least more then the minimum as per what the weight of the coach is.

While torque and horsepower can be measured, the performance of any vehicle is relative to the expectations and desires of the person driving it....you may think that your struggling to get over these massive hills you speak of hills at 65 or 70 miles per hour while I may think 50 is just fine. No offense to old ladies or old men......but some are doing 35 in fast lane of a major freeway and its probably feeling like 75 to them....its all relative.

WDK
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Old 07-25-2015, 10:17 AM   #8
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My rig is about 28,000 with the 360 and 800 torque. Forget about the formulas, you need all 800 pounds.
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Old 07-25-2015, 01:58 PM   #9
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My rig is about 28,000 with the 360 and 800 torque. Forget about the formulas, you need all 800 pounds.
Thank you, while its not so bad to know that I will need all I got.....the thing I was really trying to find was weather or not is was grossly under powered for the weight, adequate, or more than enough.....the choice being in that respect for what I am buying in the house portion of the new NeXas Bentley versus the drive train and the price overall, I am more than happy.

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Old 07-26-2015, 10:25 AM   #10
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I don't think 1 ft/lb of torque per 50 lbs of coach weight is anywhere near enough for decent performance (and I'm not talking a hot rod either). My Cummins ISL has 1200 lb-ft of torques, so that rule of thumb would say I could pull 60,000 lbs adequately. My coach runs 37,000 lbs (including a toad) and the hill-climbing performance is no better than fair, dropping to around 40 mph on 5-7% grades. The 50:1 formula would suggest I need only 740 lb-ft of torque.

But torque is the wrong measure anyway. An ISB, ISC, or ISL can all be configured to produce 360 or so horsepower and pull the same load, but their torque ratings are widely different. The ISL is 1.5x the torque of the ISB! More torque simply allows the engine to operate at lower RPMs, which in turn make acceleration a bit quicker (doesn't have to rev up as much to get going).

I suppose in saying this that I have opened up the torque vs horsepower can-of-worms. Sorry 'bout that...
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Old 07-26-2015, 10:38 AM   #11
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Here's the way I see it. I have the ISB 360 XT that produces 360hp and 800 ft-lbs of torque. My coach loaded weighs 32k lbs and I pull a 5k lbs toad. While driving not in the mountains it performs well. I get about 8 mpg and cruise at 70. While in the mountains it performs satisfactorily. I do slow to 35-40 on the way up long steep grades but I don't want to go much faster than that up a mountain with switchbacks etc.


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Old 07-26-2015, 10:38 AM   #12
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Quote:
Originally Posted by WillandJane View Post
Thank you, while its not so bad to know that I will need all I got.....the thing I was really trying to find was weather or not is was grossly under powered for the weight, adequate, or more than enough.....the choice being in that respect for what I am buying in the house portion of the new NeXas Bentley versus the drive train and the price overall, I am more than happy.

WDK
The owners we know with the 360 are very satisfied with their power. The previous version in our rigs was a 340 with around 600 torque and Allison 2500 which is lacking.
Our 360 has the Allison 3000 which handles more torque.

I would add some of the more important numbers are the weight carrying capacities.
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