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Old 01-19-2012, 02:37 PM   #57
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Maybe this is what confuses people.....you are not accelerating going up a steep grade, you are maintaining speed by maintaining a constant engine rpm and changing speed by changing the gear you are in. If you have to slow way down then you had better downshift and keep those engine rpms up there and constant or within and narrow band or that MH is going to slow right down real fast.
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Old 01-19-2012, 03:08 PM   #58
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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?
esoteric >adjective intended for or understood by only a small number of people with a specialized knowledge

Become an esotericist. So here is how the HP torque relationship is arrived at. As mentioned in previous post HP is the rate of doing work, one HP = 33,000 ft-lb/min.

One foot pound of torque would be one pound of force applied to a 1 foot lever arm pivoting on the crankshaft main bearings.

One full rotation would be a distance of 2 x r x pi.

pi = 3.14159 and using analogy above r would be one foot.

2 x 3,14159 = 6.28318, this is the distance in feet the force is applied in a full revolution of the crank.

As the force, or torque, goes up, so does the HP.

HP = Torque x 2 x pi x rpm all divided by 33,000.

Note that 33,000 divided by 2 x pi = 5252. That is where the 5252 comes from, nothing to do with rpm.

In otherwords HP = Torque x rpm divided by 5252 as stated in a previous post.

After the peak torque speed is reached while the torque drops off the resultant of the torque number multiplied by the rpm in virtually all engines will still be increasing.

Think of riding your ten or fifteen speed bike. Come up to a hill in tenth gear and you can exert full body weight on the pedals but you will likely slow down. Shift down, spin the pedals faster but now you cannot now stand up on the pedals because of the speed yet you go up the hill faster. Going up the hill faster is generation of more HP the rate at which work is done.

Give me HP before torque everytime on a hill. Just optimize the rpm to get the HP.
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Old 01-19-2012, 03:12 PM   #59
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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.
I always STOP at the vistas in the Shasta area, I better let my 454 know it should not be able to get back up to speed.
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Old 01-19-2012, 04:09 PM   #60
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Goldwinger, after a vista stop I can get up to speed again but it is not blinding acceleration. I weigh 43,000 lbs and pulling a toad that weighs a little over 3000 lbs, how much do you weigh, 19,000 lbs if that? Coming off of those vista stops is normally enough less steep road to build up speed. What I am saying is bad is when you get behind a heavy truck on one of those real steep sections and the 4 wheelers cut you off so you have to match speed behind the truck on 6% grade then pull out and get around him. Don't tell me that doesn't present a problem for your big block Chevy. I do know a little about engine performance.
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Old 01-19-2012, 04:28 PM   #61
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OK, first, I'm enjoying this and, second, I think (most of us) we're really saying the same thing only differently? I was trying to present my case in the most practical sense by, literally, talking about it from the "seat of the pants" perspective. Stalin said that at some point quantity is quality, and I understand that at some point, enough HP will be more than enough to take you up that hill or down that strip (work), but, it's still the torque that's measuring how much work is actually being done? I know that my (Dinan modified) BMW 335I, with 3.0 liters and its two turbochargers will scream up any hill I want, but, it is screaming in order for me to really feel it in my pants, so to speak. My buddy's ZR-1 'vette - man you feel the (torque?) right from the get go. In the real world however, where starts and stops are commonplace, I prefer the 1250 pounds of torque to get me moving and keep me moving (uphill) when a load is applied and I don't get deafened. You're right, I may not accelerate at all, but, I won't slow down either and my ROM will remain more or less constant. As for your assertion that RPM has nothing to do with this discussion other than in an esoteric fashion , I disagree. So ....

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 1/2 horsepower (16,500 foot pounds per minute), and so on. Therefore, the following formula applies for calculating horsepower from a torque measurement:
Torque * RPM Horsepower = ------------ 5252This is not a debatable item. It's the way it's done. Period

That does sound kinda harsh doesn't it? Not meant to be.
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Old 01-19-2012, 06:44 PM   #62
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And the original question was: "Going to look at six DPs on the week-end and was wondering...which size of engine is best?"

I'm glad we were able to clear that question up with a clear, concise, definitive answer. I only nodded off twice reading the last two pages.

Now, back to the OP. Ask us about holding tank sizes, or tire failures.

Ken
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Old 01-19-2012, 06:59 PM   #63
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Don't see what holding tanks sizes or tire failures have to do with the original question. The more important part of the original question was "Is bigger better"? To answer that question one needs to get into rpm, torque and horsepower. Sorry the answers bored you but I bet somebody learned from it all. I even learned a thing or two or three.
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Old 01-19-2012, 07:22 PM   #64
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Well I enjoyed the techno discussion on torque vrs hp.... after all this a MOTORhome forum. With all the discussion about floorplan being "everything" I was beginning to think I had stumbled into the 5th Wheel forum.
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Old 01-19-2012, 07:40 PM   #65
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Such sa Horse and torque are the same.
Nope, they are not the same. HP is only a calculated figure from the torque and rpm, nothing more and nothing less. No chassis dyno measures HP directly, they measure the torque output and the rpm then the HP is calsilated from the formula I posted earlier

Due to the factors used in deriving the calculation it favors higher rpm engines. It was originally derived to show mine owners how many horses a steam engine could replace.
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Old 01-19-2012, 07:43 PM   #66
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Give me HP before torque everytime on a hill. Just optimize the rpm to get the HP.
Try putting a Formula one engine in a MH, you'll get lots of HP and rpm, but it sure won't move the MH very fast.
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Old 01-19-2012, 09:52 PM   #67
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And the original question was: "Going to look at six DPs on the week-end and was wondering...which size of engine is best?"

I'm glad we were able to clear that question up with a clear, concise, definitive answer. I only nodded off twice reading the last two pages.

Now, back to the OP. Ask us about holding tank sizes, or tire failures.

Ken

LOL ... I nodded off a couple of times entering it!
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Old 01-20-2012, 02:30 PM   #68
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Goldwinger, after a vista stop I can get up to speed again but it is not blinding acceleration. I weigh 43,000 lbs and pulling a toad that weighs a little over 3000 lbs, how much do you weigh, 19,000 lbs if that? Coming off of those vista stops is normally enough less steep road to build up speed. What I am saying is bad is when you get behind a heavy truck on one of those real steep sections and the 4 wheelers cut you off so you have to match speed behind the truck on 6% grade then pull out and get around him. Don't tell me that doesn't present a problem for your big block Chevy. I do know a little about engine performance.
H.P. wise we are in the same boat, 80+h.p. to a pound. Torque and transmission you have it. If I get cut off, it's first gear and chug, chug, chug. Never had it happen on that stretch of road, plenty of times on the Cajon Pass going to Vegas, just part of the ride. When I see the "slow trucks" sign that's my signal to stay to the right. It's not a problem for the BB Chevy, every engine has it's limits, even the ones you build and tune for the racing world.
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Old 01-20-2012, 05:44 PM   #69
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Don't see what holding tanks sizes or tire failures have to do with the original question. The more important part of the original question was "Is bigger better"? To answer that question one needs to get into rpm, torque and horsepower. Sorry the answers bored you but I bet somebody learned from it all. I even learned a thing or two or three.
Well, maybe I missed it. Which post answered the question? Did you get any agreement on whether HP or Torque was most important? I see some don't agree which takes you up the hill, torque or HP. A 5 page discussion isn't a clear concise answer. A new poster who is looking to buy their first motorhome probably didn't fully understand how a small block chevy's drag race career fit in with a diesel motorhome.

The other two topics relate to this one in that they get discussed at length with no definitive answer at the end either.
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Old 01-20-2012, 05:59 PM   #70
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Sorry you missed it. It is torque that pulls/accelerates the MH from a standstill and it is horsepower that lets you maintain a steady speed to climb a steep grade. Like I have said before if you think it is torque that will get you up a steep grade then just try holding your engine rpm at the max torque rpm and see if you can maintain a speed. No way. To maintain speed on a steep grade you have to hold the motor at the rpm specified for peak horsepower. If you don't believe that then I am more than glad to demonstrate that with a 846 cubic inch Detroit Series 60 with peak torque of 1650 ft lbs at 1100 rpm and 525 HP at 1800 rpm that I cannot maintain or climb a steep grade holding the motor at peak torque at 1100 rpm. It just will not do it and will overheat. It has to be at 1800 rpm to climb or maintain speed going up a steep 6%+ grade. If it gets to 7 or 8% then I have to downshift will maintaining that 1800 rpm. If my motorhome drops to 1100 rpm I am in deep trouble. If you still think it is torque then you either do not know where you peak HP occurs at or you have never climbed a steep grade.
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