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Old 01-19-2012, 12:40 AM   #43
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I've never had a problem driving cars, trucks or motorhomes. I guess it's all in how you approach it. Although I was raised on the West coast and have driven in mountains all my life.
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Old 01-19-2012, 05:18 AM   #44
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No such thing as to much horse power when you have to move weight. But you need to consider where you will be useing your MH most, if most of your time is in the east or midwest it's not such a big deal, out in the west long pulls, wind, high elevations, big horsepower is nice to have. In reallity you spend very little time pulling big hills so you go over the top at 40 mph instead of 50 and it takes an extra 5 min. Generally bigger engines come at a price, both up front, and at the pump.
With all that said take a look around there is a blue million gas motorhomes, and trailers on the road all over the country and they get where they are going just fine. In the end it boils down to finding a coach that has what you want and fits your wallet, and if it has a bigger engine thats great.
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Old 01-19-2012, 05:32 AM   #45
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Well said Bruce.
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Old 01-19-2012, 07:40 AM   #46
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Barbara,

Sorry we confused you. First, the longer motor homes are not harder to drive down the road. Actually, the longer they are, the more stable they are on the road, BUT they are more difficult to maneuver and park. Some manufacturers models are more "road wild" than others because of steering design, weight distribution, and front tire size; so make sure you drive your prospective coach out on the highway before you buy it.

Cummins is a diesel engine manufacturer that labels their engines with a three letter identifier. ISB is the smallest engine at 5.9 Liters (for us old school folks a liter is 61 cubic inches). Next in order is the ISC at 8.3L, the ISL is 8.9L, the ISM is 10.8L, and the largest on road Cummins engine is the ISX at 14.8L. It is slightly more convoluted in that each engine can have several horsepower ratings. For instance, an ISL can range from 370hp to 450hp.
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Old 01-19-2012, 08:26 AM   #47
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I'm surprised I haven't seen in 4 pages of this thread (unless I missed it) the other significant advantage to the larger engines (or at least the Cummins ISL) -- the availability of a true engine brake. HP/torque gets you up the hill, but an engine brake (or to a lesser extent, an exhaust brake) helps get you back down, safely. For those unfamiliar, an engine or exhaust brake helps slow you down using engine/exhaust backpressure (I'm sure someone will correct my explanation), reducing wear and overheating of your service brakes.

After 2 years driving my ISL 425 with 2-stage Jake brake, I wouldn't have a coach without it. Because these features are more commonly available in the larger engines, I'd consider it a major factor in engine choice.
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Old 01-19-2012, 09:09 AM   #48
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Quote:
Originally Posted by Mr_D View Post
In ALL applications there is a direct correlation of HP/Torque since the HP is only a calculated figure derived from torque and rpm.
You can easily convert torque to horsepower by multiplying torque by rpm/5,252.
The calculation favors high rpm engines though. But it's still the way it's done.
Very good. Many do not understand the relationship between torque and horsepower. Ultimately it is HP, not torque, that gets you up the hill.
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Old 01-19-2012, 10:09 AM   #49
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Quote:
Originally Posted by Mr_D View Post
In ALL applications there is a direct correlation of HP/Torque since the HP is only a calculated figure derived from torque and rpm.
You can easily convert torque to horsepower by multiplying torque by rpm/5,252.
The calculation favors high rpm engines though. But it's still the way it's done.
Please explain this correlation please...:

My 21 year old 10+ liter Caterpillar engine...:
(Cat model# 3176 inline 6 cylinder)
325 BHP @ 2,100 rpms
1,225 ft lbs. of torque @ 1,300 rpms
51% torque rise

325 horse power and 1,225 of torque?

TORQUE = rate of accelleration
H.P. = keeps you @ speed
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Old 01-19-2012, 10:11 AM   #50
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And I have said that in several previous posts. If you think it is torque that gets you up a steep hill (6%+) then try doing it at you peak torque RPM. My peak torque RPM is 1100 and I have 1650 ft lbs of it there. There ain't know way I can climb even a small grade at 1100 rpm. I need to be at 1800 RPM where my 525 HP peak is. I can climb anything at 1800 rpm. My turbo is fully spun up at 1800 rpm and I am making full boost.
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Old 01-19-2012, 11:03 AM   #51
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Having Owned 8 motorhomes. The most Important Thing is what/which floor plan you like.. When you post A HP. question you will get a lot of personal opionons; Such sa Horse and torque are the same. If that wasn't so stupid it would be funny.. The gas engine may be 430 HP and have 320 foot lbs of torque, VRS. the diesel 300 HP puts out 900 Foot lbs, (numbers Appx.). And torque gets you up the hill.. For get the Engine. Buy a main line coach you like.. One that has repair facilities In every Town.. ( no need rof the wise guy to respond; So you don't have to run all over heck to get it repaired;;
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Old 01-19-2012, 11:14 AM   #52
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I have a 2007 Crosscountry with 300 HP Cummins. What I like about the small cummins is MPG, I get 10 plus towing a jeep grand cherokee. With a 90 gal tank I can go 700 miles before stopping for a fill up. I have only been to the Smokie mountains and had not problems maintaining 55 MPH on the interstate.
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Old 01-19-2012, 11:57 AM   #53
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Darwin;
had not problems maintaining 55 MPH on the interstate.



How perfect is this.
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Old 01-19-2012, 12:51 PM   #54
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For those who have the spare time, and the inclination, I have posted one of my favorite articles on this subject (apologies to forum execs for using so much of your resources here - LOL). What I know personally is that we bought a 2005 Winnebago Adventurer, 39' long on a W24 chassis, with a 8.1 liter vortec engine. Other than learning that Winnie should never have been allowed to put so much coach on so little chassis with so much rear overhang, I learned allot about my personal tolerance level. We took the "coach", pulling our Grand Cherokee, on its first major outing. We chose I-81 from I-90 to I-64 into Norolk. By the time we had completed the third "lift" on I-81, each lift is about 6% I would guess, I was ready to scream about as loudly as the 8.1 that had not stopped screaming all the way up those "lifts"!! DW and I couldn't talk to each other on the way up! I'm old enough to remember when a big v-8 screaming at about 4800 rpm was either going down a dragstrip or a deserted road in an industrial area at about 2.00 a.m. - either of which is fun. This was not fun. We kept that rv 8 months, took the bonk on the head by trading it, and wound up with our first DP.
I hope this helps some folks.


WorkThe transference of energy that is produced by the motion of the point of
application of a force and is measured by multiplying the force and the
displacement of its point of application in the line of actionForceStrength or energy exerted or brought to bear; cause of motion or changeFoot PoundsThe unit of work equal to the work done by a force of one pound acting
through a distance of one foot in the direction of the force.TorqueA measure of twisting force and is measured in foot-pounds.HorsepowerA unit of power equal in the U.S. to 746 watts and nearly equivalent to the
English gravitational unit of the same name that equals 550 foot-pounds of
work per second.

Now that we have a common understanding of terms, let's talk about engine torque and horsepower. To begin, work is what happens when a force is used to move an object some distance. The energy expended to do the work is not the same as the work itself. For example, you could try to push your house off its foundation with your bare hands but obviously no work would be done even though energy has been expended. You would end up being very tired from the experience, but you would have accomplished nothing. If you selected something a bit smaller than a house, say a piano, you could expend a tremendous amount of energy and move it only a few feet. Some, but not much, work would have been done, and it could be measured in foot-pounds.

Next we need to look at how the idea of horsepower came about. In the 18th century, James Watt made some observations. He concluded that the average horse of the time could lift a 550-pound weight one foot in one second, thereby performing work at the rate of 550 foot pounds per second, or 33,000 foot pounds per minute, for an eight-hour shift, more or less. He then published those observations, and stated that 33,000 foot pounds per minute of work was equivalent to one horsepower. Everybody else said, "Okay!" This has been the accepted standard ever since.

When talking about motorized vehicles it is important to also understand how to measure the force from rotating objects such as crankshafts. The term we use for such a "twisting" force is foot-pounds of torque. A foot- pound of torque is the twisting force necessary to support a one-pound weight on a weightless horizontal bar, one foot from the fulcrum.

It is also important to understand that no one on the planet ever actually measures horsepower from a running engine on a standard dynamometer. What we actually measure is torque, expressed in foot-pounds and then we ?calculate? horsepower by converting the twisting force of torque into work units of horsepower. The formula for that process is:

Visualize that one-pound weight we mentioned, one foot from the fulcrum on
its weightless bar. If we rotate that weight for one full revolution against
a one pound resistance, we have moved it a total of 6.2832 feet (Pi * a two
foot circle), and, incidentally, we have done 6.2832 foot-pounds of work.

OK. Remember Watt? He said that 33,000 foot-pounds of work per minute was
equivalent to one horsepower. If we divide the 6.2832 foot pounds of work we've done per revolution of that weight into 33,000 foot pounds, we come up with the fact that one foot pound of torque at 5252 rpm is equal to 33,000 foot pounds per minute of work, and is the equivalent of one horsepower. If we only move that weight at the rate of 2626 rpm, it's the equivalent of ? horsepower (16,500 foot pounds per minute), and so on.
Therefore, the following formula applies for calculating horsepower from a torque measurement:

This is not a debatable item. It's the way it's done. Period.

To get vehicles moving, we use some kind of engine to convert chemical energy (fuel) into force. In an internal combustion engine, the fuel in the combustion chamber is ignited by a spark (gasoline engines) or compression (diesel engines), creating high-pressure gasses that push down on the pistons. As the pistons go down the connecting rods push the crankshaft throws or arms. Bearings control the crankshaft, allowing it to only rotate. Since each crank arm is offset from the centerline of the crankshaft (the rotational axis), we have torque. Voila.

In the MotorHome world this translates to the ability to get a load moving. Diesels produce massive amounts of torque at low RPMs. This of course makes them popular in large trucks, busses, and MotorHomes. Gasoline engines tend to produce maximum torque at higher rpm's. You will see, feel, and hear the difference between the two types of engines in terms of power production. While climbing a grade of say 6% the "staying" power of a diesel MotorHome can be illustrated by the sound of the engine and the RPMs registering on the tachometer. The engine begins to growl and the RPMs will probably stay in the 1500 to 2000 range with appropriate downshifting, very much the same range as normal highway speeds on level ground. Type of transmission is a key player here as is managing the gears. Diesel engines supply a lot of torque without high RPMs. It is like lifting a heavy object with a long lever and a steady force. A gasoline engine operating in the same environment will require more throttle and more RPMs to achieve the same amount of torque. It is like trying to jack up a car with a small jack. You need to pump on the handle a lot of times to raise the car. A gasoline engine then will not maintain the "staying" power because it cannot produce and maintain a torque range to move the MotorHome uphill at a constant speed. More and more fuel is required. Part of the problem is that gasoline powered MotorHomes typically aren't geared" appropriately to move the weight. Engine design, fuel systems, etc. all change the shape of the torque curve. In most cases, torque range rises to some peak value and then decreases.

What all this means is that as you make decisions about purchasing a MotorHome, you need to consider exactly what you expect it to do in terms of speed, hill/mountain climbing, and towing. As stated, diesel MotorHomes generally have the appropriate horsepower and torque ratings along with the properly sized transmission to take you down the road safely and in good fashion. The key word here is generally. Just because the coach has a diesel engine doesn't mean it's a "cruiser". Some manufacturers have managed to sell inadequately powered diesel coaches, and the coach owners in a lot of cases don't attend to the weight ratings of their coach. A good rule of thumb when planning a coach load is you should have a minimum of 10 horsepower for every 1000 pounds of coach weight you want to move. Remember your tow vehicle should be in the total weight equation, too.

Keep in mind. We are not assaulting gasoline powered coaches. For the appropriately sized coach and the total weight you want to move, gas coaches are just fine. In a lot of cases however, we want to move considerably more weight than the gas coach is capable of sustaining.
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Old 01-19-2012, 01:29 PM   #55
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Thanks Tony, but are we saying it is torque or it is horsepower that pulls my 43,000 lb MH up from Atlanta to Maine up I-81 three times a year? I climb I-81 all the time and have zero problems and normally do not have to get to my 1800 rpm HP peak except for about three "lifts" as you call them. Hope one day you can get to drive I-5 in Northern California from Redding to Grants Pass, Oregon going by Mt Shasta. It a a 6%+ grade up and down for over 10 miles. You had better not loose speed with your MH and let that rpm drop or you are going to come to a standstill. The 18 wheelers run it at about 20-30 mph at their peak HP rpm if they are lucky.
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Old 01-19-2012, 02:04 PM   #56
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First of all, from a driver's perspective, torque, to use the vernacular, RULES :-). Any given car, in any given gear, will accelerate at a rate that *exactly* matches its torque curve (allowing for increased air and rolling resistance as speeds climb). Another way of saying this is that a car will accelerate hardest at its torque peak in any given gear, and will not accelerate as hard below that peak, or above it. Torque is the only thing that a driver feels, and horsepower is just sort of an esoteric measurement in that context. 300 foot pounds of torque will accelerate you just as hard at 2000 rpm as it would if you were making that torque at 4000 rpm in the same gear, yet, per the formula, the horsepower would be *double* at 4000 rpm. Therefore, horsepower isn't particularly meaningful from a driver's perspective, and the two numbers only get friendly at 5252 rpm, where horsepower and torque always come out the same. In contrast to a torque curve (and the matching pushback into your seat), horsepower rises rapidly with rpm, especially when torque values are also climbing. Horsepower will continue to climb, however, until well past the torque peak, and will continue to rise as engine speed climbs, until the torque curve really begins to plummet, faster than engine rpm is rising. However, as I said, horsepower has nothing to do with what a driver *feels*.

I've driven most of the interstates out west and a variety through the Rockies and the Smokies. Our coaches have been from our Border to the Pacific three times now, always pulling a Grand Chrerokee (Diesel of course). I like the grumble of the big old Diesel 43' behind me pulling at about 2000 rpm as opposed to the gas engine between our legs screamin' at 4500 rpm or even an ISB workin' its heart out back there about 36-40' is all. I get the hp / torque relationship and I know, that if you want to get up that hill then hit it fast and the HP will take you up. I also know that if you don't get to hit it so fast that the torque will keep you goin' anyway. We had a Super Pro / Super Gas car that ran a 350 small block running about 9.15 sec. / 110 mph - we were the only small block. All the others were big blocks with lots more torque - you could always hear our engine in the crowd, but we still got it done. Nuff said on this subject you think?
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