Are large gas motor Rv's underpower?

d1725829 wrote:
My opinion, does it really matter how fast you get there as long as you get there ? I go for the thrill, I am in no hurry !

What my concern has been is that when we get stuck behind a very slow moving semi and we fall to their speed of 35 mph going up a steep hill, will I have enough 'umpth' to pass him without causing a major slow-down in the passing lane.

I'm not wanting to race anybody, I just want the ability to pass within reason instead of being stuck behind a very slow truck.

MM.

invmartyc wrote:
At the risk of getting someone angry for me asking the same question they don't want to see again, that no doubt some people feel has been discussed enough.

Are gas Class A's underpowered to tow a toad in any area with mountains or large hills.

I have read people with gassers say that they are powerful enough for them to tow a toad anywhere. And I have read that no gas motor Class A has the power and that you need a diesel motor.

What is the truth, not opinion on this?

Without specifying the general size of the MH and weight of the toad, this question is kind of meaningless.

My opinion, does it really matter how fast you get there as long as you get there ? I go for the thrill, I am in no hurry !

Dave, I probably have seen torque vs. HP debate but it just didn't register until I was looking at a heavier motorhome with not much more hp than what we have now. We have a 425 hp Cummins with 1,200 lb. ft. of torque and going to 500 hp with 1,750 lb. ft. of torque. I'm learning, when things are put in more mathematical terms, it clicks.

MM.

Mr. Mark, I am surprised you have not seen anyone talk about torque vs HP. We talk about it all the time. The problem with a torque spec is that it is incomplete, as it does not account for the torque multiplication of the drivetrain. If the engine is limited to low RPM, the drivetrain can not multiply the torque to the wheels because the gear ratio is too low. Basically, the diesel has little headroom to downshift. At freeway speeds, you can probably only go down one gear and gain a small amount of pull. I can drop 2 gears and smoke by most anything, including a lot of cars.

Torque gives the perception of power because all of the power the engine has is available without downshifting. When you do downshift, you don't gain much. For a high HP/lower torque gas engine, all of its available power is achieved by RPM, so when the engine spins faster, the gearing multiplies the torque to the drive wheels. Its simple math, but to prove it out, one needs to go through all the gear ratios and calculate wheel torque.

In the practical sense, very simply, the HP spec tells you how hard it will pull when the throttle is wide open. The torque spec gives you some indication as to how often you will downshift. People generally like a lot of torque, because they don't like the idea of downshifting. Ultimately while that is important, it doesn't tell you anything about how much it can pull up a hill.

BTY:There was an earlier comparison of a Duramax 300 hp/600ft-lbs torque. It was compared to a 454, which had 230 HP and 390ft-lbs of torque. 300HP vs 230HP which one is faster? well dah. Compare that to an 8.1 and the 8.1 will slowly pull away. (340HP /450ft-lbs torque) Still lower torque, but more powerful.

JetAonly wrote:
horsepower, the common unit of power; i.e., the rateat which work is done. In the British Imperial System, one horsepower equals 33,000 foot-pounds of work per minute—that is, the power necessary to lift a total mass of 33,000 pounds one foot in one minute.

work, in physics, measure of energy transfer that occurs when an object is moved over a distance by an external force at least part of which is applied in the direction of the displacement. If the force is constant, work may be computed by multiplying the length of the path by the component of the force acting along the path.

torque, also called Moment Of A Force, in physics, the tendency of a force to rotate the body to which it is applied. The torque, specified with regard to the axis of rotation, is equal to the magnitude of the component of the force vector lying in the plane perpendicular to the axis, multiplied by the shortest distance between the axis and the direction of the force component.

HP = TORQUE x RPM ÷ 5252

So you can see that without torque you have no HP. HP is calculated, not measured. Moreover , if you don't have RPM you don' have HP.

Here is a good review of this.

Further think about electric motors in cars. At the moment of power application the speed is zero, there is no HP because there is no rotation of the motor. What moves the car at zero HP? Why torque does. The reality is they are very closely related and it is the application of the engine that determines at what RPM you want your torque and where you want your HP. So you want your 60000 lb MH to use 22.5 inch tire which determines your RPM. ~500 RPM equal a mile on your 22.5 inch tire. At 60mph your tire has 500 RPM.

Bottom line, climb hill at peak HP, to get best speed up the hill. Cruise at peak Torque for best fuel economy.

Thank you, that is an explanation that I understand.

MM.

horsepower, the common unit of power; i.e., the rateat which work is done. In the British Imperial System, one horsepower equals 33,000 foot-pounds of work per minute—that is, the power necessary to lift a total mass of 33,000 pounds one foot in one minute.

work, in physics, measure of energy transfer that occurs when an object is moved over a distance by an external force at least part of which is applied in the direction of the displacement. If the force is constant, work may be computed by multiplying the length of the path by the component of the force acting along the path.

torque, also called Moment Of A Force, in physics, the tendency of a force to rotate the body to which it is applied. The torque, specified with regard to the axis of rotation, is equal to the magnitude of the component of the force vector lying in the plane perpendicular to the axis, multiplied by the shortest distance between the axis and the direction of the force component.

HP = TORQUE x RPM ÷ 5252

So you can see that without torque you have no HP. HP is calculated, not measured. Moreover , if you don't have RPM you don' have HP.

Here is a good review of this.

Further think about electric motors in cars. At the moment of power application the speed is zero, there is no HP because there is no rotation of the motor. What moves the car at zero HP? Why torque does. The reality is they are very closely related and it is the application of the engine that determines at what RPM you want your torque and where you want your HP. So you want your 60000 lb MH to use 22.5 inch tire which determines your RPM. ~500 RPM equal a mile on your 22.5 inch tire. At 60mph your tire has 500 RPM.

Bottom line, climb hill at peak HP, to get best speed up the hill. Cruise at peak Torque for best fuel economy.

rgatijnet1 wrote:

Mr.Mark wrote:

the silverback wrote:
I designed engines for 35 years. It is about horsepower and weight of the Motorhome. NOT TORQUE!! Except the more torque means more horsepower. So if a gas MH has a good horsepower to weight ratio it will do just fine.

Mr. Silverback, I'm certainly not questioning your advice because you probably have forgotten more than I'll ever know about engines.

But, you are the first person I've seen that says 'not torque' is not a major component to power an engine. All the diesel converstaion I've read about engines is torque, torque, torque.

For instance, on the new motorhome that we have ordered, the engine has been replaced from a Detroit with 515 hp and 1,650 lb. ft. of torque to a Volvo 500 hp with 1,750 lb. ft. of torque. The Prevost guys say that I should be able to tell a difference on the increase ability to climb steep hills. That's what they are telling me if I understood them correctly.

MM.

Torque at the rear wheels, which is all that really matters, is a function of gearing, which is why a hand held 1" air ratchet can produce 2000+ foot pounds of torque in a truck shop. Plenty of torque with the air ratchet but it won't get you up a hill very fast. :B
Then you talked to a salesman and there is the old adage about if his mouth was moving he was lying.

Honestly, I don't know what that means about the air ratchet compared to a diesel engine. The info I got is from the Prevost sight where all the gear-heads hang out, they are not salesman but very intelligent folks (apparently, they have me fooled) LOL!

MM.

Well if all you are talking about is power and not handling or breaking then here is our experience.

We had a 32 foot 2002 Southwind with 8.1 on a Workhorse chassis and pulled a jeep grand Cherokee. For the last 11 years we have had a 40 foot Revolution with 8.3 Cummins pulling a smart car. Without a doubt the Southwind would leave us in the dust on any hill anywhere anytime. Really I could care less. I still get to the top, but it's a lot quieter going up and with better handling and breaking overall. But the again, it's a bigger coach. If we hadn't gone full time we would still probably have the Soughwind. It served us well. Just a little small for our needs.

JMHO.

wa8yxm wrote:
Most gassers come with 5,000 pound rated hitches and are not able to tow quite that much... Read on however

Most Diesel pushers the hitch is rated for 10,000 and yes, they can do it.

That said... I have only had a problem one time with my 8.1L Vortec V-8 Gasser where I could have used about 1 more horse power... This was on a mountain road I should never have even attempted (More of a mountain goat trail than a road, in places the road was NOT as wide as the motor home) I had one peak where I had to drop the 4,000 pound plus cargo towed and drive it over under it's own power.. I will never take this RV (Or any other) down that road again.. Way too narrow.. Yes, Narrow.. I may some day take a Jeep though, was a lovely drive.

Are you Robin Williams?