How much Horse Power do YOU need?

[jharrell]

Originally Posted by Daveinet

None of it makes any sense as the engine in the KL is a 3.2l 275 HP. The motorhome is 8.1 320 HP+ whatever the headers and Ultrapower add.

Doesn't make sense, I have heard the 9 speed has issues but that seems outlandish. I mean if the engine were at full throttle with enrichment it could be wasting fuel trying to keep itself cool, but you are saying half throttle.

I don't think that engine makes maximum power until like 6500 rpm. The BSFC of the 3.2 is supposed to be better than say the 3.6, but its not designed to pull a heavy load at WOT and my have pretty bad BSFC there, either way what your saying makes no sense.

[4x4ord]

Originally Posted by burgmark

My 35 foot Thor Aria has the 6.7 Cummins putting out 360 HP and 800 foot pounds of torque. I probably average 8 mpg but also tow a 5,000 lb. Grand Cherokee. My weight is 29,420 loaded as I typically travel plus the toad, so around 35k total.

I believe I read somewhere that the most fuel efficient RV tested was powered by the 6.7 Cummins, so its an efficient power plant.

I'm by no means that knowledgeable and my career was as an accountant, not an engineer. But I wonder, is horsepower the one measure that is important, what about torque. My high level understanding is that horsepower is more of a measure of work and that torque is the measure of power. Then too it seems like the gearing would greatly impact both.

Here is kinda a long explanation of how horsepower, torque, gear ratios and torque or power curves all come together:

Horsepower is a unit of measure for power. It is the amount of power required to lift 550 lbs one foot in one second. It can also be described as
HP=Torque x rpm/5252

Lbft are the units used for the measure of torque. Torque is a rotational force acting on an axis.

Work can be measured in ftlbs and requires movement ... force through a distance.

When torque is applied to a shaft work is accomplished as soon as the shaft begins to rotate. That rotational movement can be timed and represented in rpm.

If an engine is making torque it is also accomplishing work at a certain speed and so it is making power.

So, if an engine is said to have a torque rating of 1700 lbft @ 1000 rpm it will be making a specific power at that rpm .... in fact 324 hp = 1700 lbft @ 1000 rpm.

If that same engine has a peak HP rating of 500 hp at 2000 rpm it means it is making 1313 lbft of torque at 2000 rpm = 500 hp

The rear wheel torque required to propel an RV up a grade is largely determined by the steepness of the grade and the weight of the RV. Speed plays only a minor role.

So, for instance a 40,000 lb MH going up a 8% grade at 30 mph will require about 3700 lbs of force at the point where the rubber meets the road. If the motorhome has 36" tires that force will require a torque on the rear axle of 3700 x 1.5' or 5550 lbft of torque.
For that same motorhome to climb the grade at 60 mph the torque on the rear axle only increases about 9% to about 6050 lbft.

Horsepower is a different story: The HP required to move that 40,000 MH up an 8% grade at 30 mph is about 296 HP. For that same motorhome to climb the grade at 60 mph the required HP is about 645 HP .... a 118% increase in power.

So, here is how this all plays out in the real world:

You are going down the highway with your 40,000 lb MH at 60 mph when you hit an 8% grade. You push the accelerator to the floor and the transmission drops to fifth gear and the engine revs to 1990 rpm where it produces its maximum power of 500 crankshaft HP and 1320 lbft of torque. After a 12% driveline loss there is 440 HP at the rear wheels. At 60 mph the rear axle is turning 560 rpm so the torque on the rear axle is 4126 lbft (440 x 5252/560). Another way of determining the same thing is to take the 1320 lbft of torque at the crankshaft and multiply it by the gear ratios of the driveline. (1320 (engine torque at 1990 rpm) x .88 (driveline loss) x .74 (5th gear ratio) x 4.8 (final drive ratio) = 4126 lbft) The required torque to maintain a speed of 60 mph on an 8% grade is 6050 so the coach slows down. As the coach slows down the torque requirement drops slightly and the engine rpm drops off until 1470 rpm where it reaches its peak torque of 1700 lbft. At this point the rear wheel torque is 1700 x .88 x .74 x 4.8 = 5314 lbft of torque. Still not enough to maintain speed so the transmission drops into 4th gear (direct) and the engine revs back to 1990 rpm where the engine torque drops to 1320 lbft. The torque at the rear wheels is 1320 x .88 x 1 (4th gear ratio) x 4.8 = 5576 lbft. At this point the coach will slow down just slightly till the engine torque rises to bring the rear wheel torque up to about 5700 lbft of torque at about 1880 rpm and 42 mph.

[burgmark]

Originally Posted by jharrell

force without distance does nothing, stand on a wrench attached to frozen nut, you are putting torque on the nut and nothing is getting done and you will never get tired because you are performing no work, well maybe tired of standing around.

Gears are a form of lever they can amplify torque, there is no way to amplify power from an engine without another engine working together. A lawnmower engine can put more torque to the wheels than a 8.9 liter cummins with the right gears, the rpm however will be much lower and the vehicle will move more slowly because it has less power.

That first paragraph is partially my point, horsepower without sufficient torque to the wheels will not move a load. Gearing to some degree can overcome that issue. Diesel engines make their horsepower and torque at lower RPMs than gas engines, but as illustrated in a post above, boost the RPMs of the diesel and it produces a lot more horsepower.


When it comes down to pulling a load up an 8 degree grade starting from 65 mph, where many motorhomes must slow down and use lower gears and assuming the correct gearing, is horsepower all that matters or does the amount of torque matter as well? And one reason I'm asking is that someone mentioned earlier in the thread that two MHs with the same weight and horsepower would climb a hill at the same rate. But if one makes its horsepower at 1,800 RPMs and another makes its horsepower at 6,000 RPMs is that really true because torque won't the torque potentially be different?

[4x4ord]

Originally Posted by burgmark

That first paragraph is partially my point, horsepower without sufficient torque to the wheels will not move a load. Gearing to some degree can overcome that issue. Diesel engines make their horsepower and torque at lower RPMs than gas engines, but as illustrated in a post above, boost the RPMs of the diesel and it produces a lot more horsepower.


When it comes down to pulling a load up an 8 degree grade starting from 65 mph, where many motorhomes must slow down and use lower gears and assuming the correct gearing, is horsepower all that matters or does the amount of torque matter as well? And one reason I'm asking is that someone mentioned earlier in the thread that two MHs with the same weight and horsepower would climb a hill at the same rate. But if one makes its horsepower at 1,800 RPMs and another makes its horsepower at 6,000 RPMs is that really true because torque won't the torque potentially be different?

The steepness of the grade and the weight of the RV are the factors that determine the rear wheel torque required to climb the grade.

Horsepower is a function of torque and rpm ..... double the rpm and 1/2 the torque and the power remains the same. So, if you have HP you can use gears to slow the rpm and increase the torque.

If the grade/weight of RV are dictating you need 6000 lbft of torque at the rear wheels you can get there with gears regardless of your HP. Higher HP is capable of providing that torque at a higher rpm. It doesn't matter what fuel is being burned. If the crankshaft torque is 1500 lbft at 1600 rpm (457 HP) you need to slow the axle rpm down to 400 rpm to get to 6000 lbft at the rear axle (ignoring driveline losses) and you still have your 457 HP at the rear axle. If the crankshaft torque is 500 lbft at 4800 rpm (457 HP) you still need to slow the rear axle down to 400 rpm to get 6000 lbft of torque at the rear axle (457 HP). If your engine can only provide 500 lbft at 2400 rpm (228 HP) you will have to slow the rear axle down to 200 rpm to get your required 6000 lbft of torque at the rear axle and climb the grade at 1/2 the speed. (you're needing 12 times the torque (6000 lbft is 12 x 500) so you need to cut your shaft speed by 1/12 (2400/12 = 200 rpm))

[4x4ord]

Once you understand that torque is a component of horsepower you need to understand that an engine's torque rating is at a certain RPM. That torque rating along with the power rating describe a power curve for a particular engine. An engine with a flatter power curve will be able to put its power to the rear wheels over a broader rpm range .... so understanding the torque curve and taking into account the gear ratios in the transmission are all part of the equation.

[twinboat]

Originally Posted by 4x4ord

Once you understand that torque is a component of horsepower you need to understand that an engine's torque rating is at a certain RPM. That torque rating along with the power rating describe a power curve for a particular engine. An engine with a flatter power curve will be able to put its power to the rear wheels over a broader rpm range .... so understanding the torque curve and taking into account the gear ratios in the transmission are all part of the equation.

All torque and HP curvers are recorded at WOT, wide open throttle.
You need more then RPM to reach the ratings. Your not making any HP or torque coasting downhill at any RPM.

[F4Gary]

Torque = acceleration
Horsepower = top speed

[Daveinet]

I think what gets confusion for some is that they fail to understand a torque spec is an incomplete spec. I can put 1000 ft-lbs of torque on a torque wrench, yet if the bolt is tight, the RPM is zero and no work is being done. But now if the bolt rotates at a particular RPM, then work is being accomplished. So for torque to have any value at all, there must be RPM. As soon as you add RPM to the equation, you have just fulfilled the definition of horsepower. Horsepower is the quantification of torque and RPM.



SO the torque spec is not telling you anything about doing work ie pulling your motorhome up a hill. All it is telling you is what RPM your engine will be turning when you go up that hill. What gets tricky and often misleading is that a high torque low RPM engine spends most of its time running close to max RPM. This means that all the HP that engine has is available without the engine increasing RPM. Just step on the throttle and it pulls. In contrast, a high RPM low torque engine is not cruising close to its max RPM. So for it to access its HP. it must spin to a higher RPM ie downshift. SO when driving a diesel, there is a miss-perception of power, because a diesel has access to its power without downshifting. A gas engine will downshift to access its power, so there is a perception that it is weaker, because it downshifts.

[Rob_M]

Originally Posted by Daveinet

…….
What gets tricky and often misleading is that a high torque low RPM engine spends most of its time running close to max RPM. This means that all the HP that engine has is available without the engine increasing RPM. Just step on the throttle and it pulls. In contrast, a high RPM low torque engine is not cruising close to its max RPM. So for it to access its HP. it must spin to a higher RPM ie downshift. SO when driving a diesel, there is a miss-perception of power, because a diesel has access to its power without downshifting. A gas engine will downshift to access its power, so there is a perception that it is weaker, because it downshifts.

This is the best comparison explanation I’ve read yet.

[tuffr2]

HP and torque do go hand in hand. I like the above explanation and point of view. It is interesting the wide range of HP / Torque engines in the world. All HP and little torque you have a race car like an indy car. It can go around the track at 200 mph - work being done fast. All torque and little HP you have a farm tractor. It can pull a log out of the woods. Work is being done but slow.

If driving on the highway you need enough HP to go fast but also enough torque to move your RV. It is interesting they can build such an engine. I had a gen 1 Ford 6.7 diesel. It was almost a perfect 400 HP to go fast and 800 ft lbs torque to pull an RV. Now new engines are still about the same at 475 HP and 1,000 ft lbs of torque.
So HP has to be around half the torque number to be somewhat balanced for RV'ers IMHO.

HP, to me is how fast you can do the work.
Torque is how much work you can do.

When I see 475 HP and 1,000 ft lbs of torque I think you can do a lot of work fast.

[4x4ord]

Originally Posted by Daveinet

SO the torque spec is not telling you anything about doing work ie pulling your motorhome up a hill. All it is telling you is what RPM your engine will be turning when you go up that hill. What gets tricky and often misleading is that a high torque low RPM engine spends most of its time running close to max RPM. This means that all the HP that engine has is available without the engine increasing RPM. Just step on the throttle and it pulls.

I appreciate what you’re saying as far as comparing a high torque diesel to a high rpm gasoline engine. Very true that high torque diesels don’t need to rev up so much to get to their power which offers a sense of power. But that is not why they give torque specs. Torque specs are very useful to help describe how the engine will behave towing a RV up a hill …. Looking at the 2010 vs 2011 Cummins 6.7. A quick glance at the torque curves instantly tells you how the new engine will have much better pulling characteristics.

[Daveinet]

Originally Posted by 4x4ord

Torque specs are very useful to help describe how the engine will behave towing a RV up a hill …. Looking at the 2010 vs 2011 Cummins 6.7. A quick glance at the torque curves instantly tells you how the new engine will have much better pulling characteristics.

I was not trying to suggest that the torque spec is not useful. Just characterizing what happens and how it happens. Yes, one will be "nicer to drive" due to how it accesses its power. Having access to all of an engine's power without downshifting is certainly more pleasant. But assuming optimum gearing, both will pull a grade at the same maximum speed.

HP is the measurement is how much work can be done. (both weight and speed up a grade)
Torque is the method of how the work will be done.

So yes, we do care about how the work is done. Even though both get the same job done at the same maximum speed, one is more pleasant.

[Rob_M]

In the hotrodding world we call it “drivability”. Typically the larger less radical engine is more street “drivable” while the smaller c.i. cammed up engine making the same hp requires more shifting (and maybe less streetable gearing and/or torque converter) to keep it in its usable rpm range.

[Derr]

All that matters is that a person is happy with the way the RV gets to the destination, is happy with the fuel consumption, and arrives without problems or breakdown.
Everything else is just talk and very interesting graphs/formulas.