20 replies to this topic

[amh210]

As a newbie still in the research phase of RV ownership I've become curious about the cost/benefit analysis of increased diesel horsepower. I've seen motorhomes on the market with HP from 275 to 550. One way to look at it is that "more is better." I suppose the more you have the more you can haul and the faster you can go uphill. On the other hand, I suspect that more HP results in lower MPG and how important is it to climb a mountain at the speed limit anyway?

I would presume that a manufacturer wouldn't market an under-powered motorhome (untested presumption).

Sure, a heavier motorhome may require more engine power, but I haven't noticed that there is all that much difference in weight (prior to loading) of equivalently sized motorhomes.

Are there other factors at play here? All other things being equal (which they never are) is bigger indeed better?

I value all of the experience and good advice shared freely here on the SKP RV Forums. I look forward to joining you on the road someday!

Andy

[Bruce T]

Rather than answer your specific questions I would like to make just a couple of comments re horse power.

When did RVing become a race to the top of the hill?
Why was 170 hp enough a few years back but now it isn't?
Have the hills got any bigger?
What percentage of your time in the RV is spent drive v camping?

So you see my point?

Yeah more hp is better. Whatever 'better' means. But I think that it's way down the list of 'must haves' when it comes to the RV wish list. I would rather a usable floor plan than 550hp.

But then I maybe on a different wave length to many others.

[Bill B]

amh - To an extent yes - more HP more fuel

At idle (600 rpm) our D12D uses 0.6 gal per hour. At 1500 rpm (no load just pushing on the go pedal - noload in neutral) it consumes 2.5 gph.

The 7.3L in the Ford I figure about 0.2 gph and 1 gph at 2000 rpm. Both no load.So engine volume is going to play a part.

So, there you have a baseline of just running the engine based on engine losses. Now, probably the biggest factor is going to be the nut behind the wheel. If you like to run fast, it is going to suck fuel. If you drive it 'easy', smooth starts, look way ahead, pickup speed on long straight downers and bleed it off on the next uphill, That will have a lot more impact. Usually on big hills, I will drop 2 gears (12 speed) to 10th which is a direct (no overdrive) gear in my transmission. Stops some of the parasitic losses. On the last trip, we actually passed a car. One in 250 miles. We get 8.3 mpg at 60 mph and if I run 70, it will drop to 6.5 mpg.

Am I first to the top of the hill, No Way. Could I be, maybe. Do I care, nope. A diesel, for the most part other than base losses, will use the amount of fuel that the go faster pedal tells it to use. Pulling a heavy hill I can drop to 1 mile per gal or 1 gpm.

Personally, a larger engine, within reason and let it loaf. In doing dyno runs, I was told that a Dodge 3/4 ton at 60 mph needs 35 hp to maintain. Flat level ground, sea level 68*F (ideal conditions) With the barn door that we are driving, guessing closer to 80 hp. Everything above that is for hills, getting up to speed, etc.

Best of luck with you choice.

[Barbaraok]

Rather than answer your specific questions I would like to make just a couple of comments re horse power.

When did RVing become a race to the top of the hill?

When someone had enough torque (not just horsepower) to make it up the hill before his buddies. Remember there are a lot of people who buy RVs, but don't really understand the RV lifestyle that some of us enjoy - - they are still trying to be first in line for everything, including getting to the top of the hill.

Why was 170 hp enough a few years back but now it isn't?

Because we travel with more things in our coaches and the coaches are bigger. We now have larger basements for carrying more gear, washer/dryers (and dishwashers) on board, residential refrigerators, more slides (each slide adds a significant amount of weight) and all of the furniture on the slide, longer length, etc.

Have the hills got any bigger?

People are now driving roads with steeper grades because they can - - couldn't do it with a smaller engine.

What percentage of your time in the RV is spent drive v camping?

You have a lot of people who only have x amount of time, so they are driving long hours to get to their destinations as fast as they can before they have to turn around and go back to work. Thankfully that is not us anymore.

So you see my point?

Yes, but that is like saying, why replace the horse with the horseless carriage, they both get you to the same place. Things are never static, thank goodness. Would be horrible if new changes never came along.

Yeah more hp is better. Whatever 'better' means. But I think that it's way down the list of 'must haves' when it comes to the RV wish list. I would rather a usable floor plan than 550hp.

Ahh, but that ideal floor plan is heavy, so it needs more power to move down the road.

But then I maybe on a different wave length to many others.

You sound like a person who would like to redo a 20-30 year old classic.

Barb

[docj]

As a former physics teacher, I'd like to explain that the parameter that manufacturers should be talking about is torque rather than horsepower. The reason they don't is that they can make relatively inexpensive mods to engines to increase horsepower but they can't do much about the torque. Furthermore, most consumers don't understand torque because it isn't featured in car ads.

With diesel engines torque is important because it is what gets you moving. Diesel torque is roughly related to engine size. Current Cummins engines come in roughly 3 torque ranges ~1,000, ~1,200, ~1,500 foot-pounds which correspond to the ISC, ISL, and ISM engines. For CAT products the engines are the C-7, C-9, and C-12 with roughly the same torque ranges as the Cummins engines.

Diesel engines are different from our gas car engines because as you slow down a diesel the torque increases; think of it getting more "oomph" as it goes more slowly. Although you can stall a diesel you really have to work at it. With a gasoline engine the torque peak occurs at a fairly rapid rpm; as you reduce engine speed from there torque falls. Anyone who's ever stalled a stick shift car knows what happen when you try to get a gas engine to work hard at speeds below its torque peak.

Horsepower is related to torque (for homework the student can develop the mathematical relationships ) but suffice it to say that the HP of a diesel will determine the speed at which you can climb a steep hill. My somewhat over-powered 32,000 lb coach can climb a 6% grade at a speed >60 mph. With a 300 HP engine it would be limited to 45-50 mph. If you would like to better understand how HP affects performance you can download a CAT technical paper here: Weiss Travels

It should be noted that fuel economy is not as directly related to engine size as one might think. With the data from our SilverLeaf computer I can demonstrate that our MH delivers about 8 mpg at ~63 mph over a broad range of road conditions. Headwinds reduced it to 7 mpg one day in TX and a recent trip on 2-lane roads at 55 mph produced over 9 mpg even though we encountered some fairly steep grades. The CAT technical paper I referenced provides a good analysis of the factors that affect diesel fuel economy.

Personally, I do believe that many manufacturers do produce and sell underpowered MH's because they can keep the price down that way. For some people the performance doesn't matter that much, for others it does. Personally, I like to be able to keep up with the flow of traffic regardless of road conditions. When I purchased cars for fun they were usually powerful ones. It's a personal decision; I very much enjoy the purr of my CAT as she climbs hills.

Edited by docj, 24 August 2011 - 04:45 PM.

[Bruce T]

Barb I would not only like to own an old classic but also live in it!
And I don't agree that the ideal floor plan needs to be heavier. Depends on what's in it.

Having said that I think most of us RVers are like most folks in all walks of life. We all want something bigger and better when in reality we could easily get along ok with something far less complicated.

We could get by with a black and white TV but why when we can have a colored one?

It's a symptom of modern life. No one's at fault. It is what it is. I guess our parents would argue the same point!

Luckily we are all different and all have different desires. For me less horsepower is not a worry. (We have 350hp now and it suits us just fine in a 40ft motorhome). I'm also one of those folks who would buy a 'simple' high quality RV. But do they exist?

[Jim2]

Andy, more HP (& more torque) is better up to the point where you have adequate HP (& torque) to reasonably accellerate your fully loaded rig and maintain a reasonable speed going up hills. After that point, more is just more; and yes larger engines usually burn more fuel (depending on how you drive it) and raise the maint costs (more oil, more coolant, larger filters, etc).
If you look closer, you'll see that in some cases the exact same engine, a CAT C7 for example, can be set for 300, 330, or 350 HP, purely by changing settings in the electronic engine control unit (and sizing the cooling sys accordingly). Those options contribute to why you see so many HP variations.

"...how important is it to climb a mountain at the speed limit anyway?"
Especially on 2 lane hills with limited turn outs, it's important that you are able to maintain a reasonable speed. otherwise you can expect many of those impatient folks stuck behind you to run you and others off the road trying to get around you.

"I would presume that a manufacturer wouldn't market an under-powered motorhome (untested presumption)."
I wouldn't put much faith in that assumption. Mfgs will market whatever they can sell at a profit, with little regard to what is adequate or reasonable. A max speed of 10 MPH uphill, may be adequate to some, but most would consider that limitation unreasonable.

"Sure, a heavier motorhome may require more engine power, but I haven't noticed that there is all that much difference in weight (prior to loading) of equivalently sized motorhomes."
Its been a few years since I shopped motorhomes, but there were some significant differences in dry weight and more importantly GVWR & CGVWR. The engine should be sized for the full CGVWR, since most folks will use up all the cargo capacity and pull a vehicle behind the MH.

Whether its cars, trucks, motorhomes, motorcycles or lawn mowers; the question of "how much HP is enough" has a different answer depending on our personal preferences, priorities, intended uses & driving styles.

[Stanley P. Miller]

One thing you will notice with more power is that your stress level will go down. Easier merging, quicker passing and less time going slow with a string of cars trying to get past you.

We had 275 horsepower and 800 pounds of torque to pull our 30,000 pound MDT/fiver combo and it was decent at merging, passing wasn't fun and it would not hold highway speeds on any significant hill. It would happily go up a 6% grade in 4th running near the redline at 55 MPH, if you got slowed down you'd be in 3rd at 40 MPH.

I'd say that we were marginal on power, enough to be safe but not enough to make some of the tougher stuff stress free. I'd guess that 325 would have been a good bit nicer but that was somewhere around $6,000 and a load of grief to get permission to build a truck that way. The 300 was about $3,000 and almost no hassle.

When it comes resale time your initial savings on power may evaporate. I'd have gotten more than the $3,000 cost back on what I sold for and had the extra 25 horsepower to use.

Talking to other owners with 250, 275 and 300 engines mileage was pretty close.

[Bill Baxter]

Bigger engine(more horsepower) does not mean less MPG.
Example: 350HP and 1100 ft/lb torque average MPG 8.1. Same coach, same driver and same routing with the engine computer reset to 400HP and 1350 ft/lb torque and yielded 9.2 MPG. Had 9.6 MPG until the hills on I84 in CT and in heavy traffic. Yes you legally can drive slow up the hills but I refuse to cause an accident because I'm trying to save fuel.
Hills were not a problem. I could run with traffic, when necessary, but usually chose not to hustle the hill. You don't want a grossly over sized engine. But, you don't want the weakling that struggles to climb a long 6% grade at more than 20MPH.

Bill

[amh210]

Fascinating!

My sincere thanks to the respondents.

My first synthesis of the information is based on Jim2's comment that the engine should be appropriately sized for the CGVWR. That makes sense of course but what is "appropriate" ? Are their guidelines that suggest "x" HP is appropriate for "x" CGVWR?

Or, using Docj's approach, if one converts HP to Torque, what is the appropriate Torque for a given CGVWR?

Most importantly, should HP/Torque play a significant role in choosing a motorhome? Is it reasonable to presume that a motorhome built within the last 5 years is going to be adequately (whatever adequate is) powered? Should I ignore the HP listing as I read motorhome advertisements? All the other factors are important, no doubt, but this "power" issue is one I have trouble wrapping my head around. Like everything else about my future RV, I want comfort. Creature comforts I can postulate on my own. But how much power/torque does one need to travel safely and comfortably?

Again, my sincere thanks!

Andy

[Barbaraok]

We have a Cummins ISC, 350 HP, coach is ~31,000#, climb the Rockies and Cascades 2-3times each way each year, never have a problem. We don't do 70 mph going up, but then we don't do 70 on flat land driving either. Barb

[Stanley P. Miller]

I sure wouldn't assume anything out there had adequate power. It might be the manufacturer offering a low-ball engine for the price sensitive buyer, a buyer that drives from the storage lot to the RV park twice a year and wanted a cheap engine or a host of other reasons you could end up with a rig that won't meet your needs.

Your needs really come into play in deciding what you want in the way of power. Driving from the central US to Florida you'd be happy with less power as it is flat and there are lots of four-lane roads. Crossing the Rockies several times a year you'd likely want more power, off the Interstates on mountain back roads you'll want more power and maybe even a Jake brake over an exhaust brake.

As someone mentioned a while back under 200 horsepower is probably plenty to safely get you anywhere you want to go but the stress is going to be high. Don't underestimate that stress and the impact it will have on your lifestyle. We have known a lot of folks that under-powered their rig and then gave up on getting to a lot of beautiful places due to the stress of getting there. Others went from doing a lot of travel to sitting in parks to avoid having to drive. Really sad when you invite someone to a prime backwoods camping spot and they don't feel they can make it in their rig even though there are no hills over 6% on the route.

You can't have too much power when you want to pass/merge/climb or when you want to trade your rig so don't skimp thinking it is going to be good enough or a big money saver. If I was motorhome shopping for something about 40 feet I'd probably set my minimum at 350 but if more was available at a reasonable price I'd go larger. If I had a bunch of stuff to haul or a big toad I'd lean to a bit more than 350 too.

[Bruce T]

The rule of thumb that has often been quoted is 10hp per 1000 pounds.

Don't forget other items such as side mounted radiators and a/c drives that all affect how much hp you have or will need. Also things like flat towing v a box trailer. There are many factors to take into account rather than just weight and hp.

[docj]

My first synthesis of the information is based on Jim2's comment that the engine should be appropriately sized for the CGVWR. That makes sense of course but what is "appropriate" ? Are their guidelines that suggest "x" HP is appropriate for "x" CGVWR?

Or, using Docj's approach, if one converts HP to Torque, what is the appropriate Torque for a given CGVWR?

It's not that I have "converted" the issue to torque, but rather that both torque and HP are important. Because manufacturers can get smaller engines to produce the same HP as larger ones, there are a lot of apples and oranges comparisons being made in their advertising. You can make a Cummins ISC produce 350 HP but it will ALWAYS produce a lot less torque than a Cummins ISL engine.

Torque is basically the force or energy required to move something. Torque is the measurement of force, and force is measured in reference to a twisting or rotating shaft. In English terms torque is measured in pounds-feet, but is more commonly referred to as foot-pounds. So in keeping it simple let’s just say that torque can be thought of as the amount of turning force it takes to move one pound of weight the distance of one foot.

Horsepower on the other hand is torque X RPM’s. Torque is how much work is being done, and horsepower is how fast you get the actual work done. What’s interesting is, an engine rated at 350 horsepower only produces that horsepower at a rated peak power RPM.

In a diesel engine the horsepower peaks at a relatively low RPM, and there is more torque at a lower RPM compared to a gasoline engine. This results in a diesel engine having much more power at a lower RPM, around the RPM range you will be towing at. This higher torque and higher horsepower at a lower RPM equates to better towing.

If you read the CAT paper I referred to in my post you can decide for yourself what the appropriate HP is for your particular CGVWR. The paper was actually a CAT presentation to an RV group a number of years ago at a time when they sold engines for RVs. The paper models MH's of 3 weights with different engines and calculates the performance, fuel economy, and HP usage for all three. In my case, I have the largest engine they modeled in the lightest weight coach, but I can still use the graphs in the paper to calculate my MH's performance.

With respect to what manufacturers produce and sell, I think it is fair to say that every MH sold will be able to drive on any road in the US in a safe manner. This is largely a function of the fact that diesel engines produce their largest torque at very low rpm. Therefore, they can "trudge" up any mountain albeit slowly.

You have to be the one to decide if the performance of any particular coach is acceptable to you. Personally, I don't think I would have been happy with my coach if it had the C-7 engine (Cummmins ISC class) which was the other option for it. On the other hand I probably would have been Ok with a C-9 (Cummins ISL class). My C-12 is admittedly overpowered, but it is a "low stress" engine; I don't have to worry about merging in traffic; I can accelerate nearly as fast as some small cars.

[amh210]

I've now read the CAT monograph Docj suggested. Can it be summarized as follows?

• 350 HP is likely to be adequate for most newish RVs and most travel (including mountainous terrain).
• 400 HP is likely to be comfortable for most RVs and most travel
• 450 HP is likely to provide a generous and "HP stress-free" travel experience
• 500 HP and up is "lux" when it comes to power

From my exploration of RV advertising, engine HP is NOT the most significant factor in RV purchase price. Quality/lux level of installed equipment, appliances, and finishes seems to impact price more than engine size.

Based on what I've learned here, it would be wiser to take torque into consideration, but since RVs aren't advertised for torque, that should be a detail that is considered after engine size is reviewed.

Again, thank you to all for your information and assistance! Thanks also for providing responses to my musings and out-loud thinking.

I hope the thread will continue with more sharing of points of view.

Andy

[Sheldon]

Buy as much power as you can afford considering initial purchase and operating costs. Few people wish they had less power. That is the rule of thumb for boats, particularly fishing. Seems like it might apply similarly to motor homes.

Edited by Sheldon, 25 August 2011 - 10:12 AM.

[marvmarcy]

Just some thoughts on the subject.

One horsepower is 746 watts (US) or 550 foot-pounds per second (UK). The latter gravitational term is easier to relate to engine power. Torque is rotational (twisting) force. One foot-pound is the force of one pound applied at one foot from the rotational axis. An engine developing 1,000 foot-pounds of torque applies a force equivalent to 1,000 lbs one foot from the rotational axis (or 500 lbs @ 2 ft, or 2,000 lbs @ 6 inches, etc.).

I've had four dpmhs between 36' & 43'. The smaller two had DD6V92 engines, one with 300hp and 820 ft-lbs, the other with 350hp and 1020 ft-lbs. The last two had ISMs with 500hp and 1450 ft-lbs. Because of the weight differences, they all had similar performance but the engines hauling the heavier loads got noticably lower fuel economy.

I had an F550 with 7.3 PSD, then my current Volvo with ISX (450hp & 1650 ft-lbs) both pulling my 20,000 lb fifth wheel. I get better fuel economy and much better performance with the ISX than with any previous diesel. I attribute this to higher torque and moderate load.

A big engine loafing is better than a small engine straining.

[Stanley P. Miller]

Old saying "Torque comes out of an engine Horsepower comes out of a slide-rule calculator computer" but like many things advertisers and salesmen pick something to their advantage, not yours.

With these diesel engines you will see many different horsepower/torque offerings but many of them will be the same basic engine. The power differences in some cases are only a function of the engine computer and you can roll into a service bay, hand over your credit card and with a brief computer link roll out with a lot more power. Sometimes you'll need to change a few things like injectors or turbo to make a big jump.

[grumpydoc]

Someone more knowledgeable than I once said "Displacement is King, Torque is Queen, and Horsepower is the Court Jester". Sort of sums it all up quite nicely. Just my personal preference but I would want the engine with the most displacement and the most torque after that I don't care what the calculated horsepower is. I also would not let fuel economy keep me from buying the best engine for the job which is safely hauling the load to the destination. A few hundred dollars difference in fuel expense over the year would be a small price to pay to avoid the stress and anxiety of driving an underpowered rig. For me the final straw with our small 7L engine was going 35 mph up big grades on the interstate in 3rd gear and then getting behind a big rig going 1 mph slower and not being able to pass him. Too much stress worrying about getting creamed in the rear by someone going 70 and not paying attention. Our 12 L MBE will go up the same hills at 50-55 and only gets 1-2 mpg less economy. Probably costs me an extra $500 a year to get to TX for the winter and then back home to VA, well worth it to me to have the big engine. Only you can make those decisions. but if you make the wrong one and buy the little engine it will cost a lot to turn around in a few years and correct the mistake, believe me, I and others speak form personal experience, Best wishes, Jay

[docj]

Here's how this translates into specific engines:

• 350 HP is likely to be adequate for most newish RVs and most travel (including mountainous terrain)--usually this is a Cummins ISC or Cat C-7.
• 400 HP is likely to be comfortable for most RVs and most travel--usually this is a Cummins ISL or Cat C-9 but beware of ISC's tuned for extra HP since the torque is a lot less
• 450 HP is likely to provide a generous and "HP stress-free" travel experience--Cummins ISM or CAT C-12
• 500 HP and up is "lux" when it comes to power--Cummins ISX or CAT C-15

It's also worth noting that ISC and ISL engines (or CAT equivalents) will have Allison 3000 series transmissions. The larger engines will get the heavy duty Allison 4000 series.

From my exploration of RV advertising, engine HP is NOT the most significant factor in RV purchase price. Quality/lux level of installed equipment, appliances, and finishes seems to impact price more than engine size--Many buyers go for the "glitz" without too much understanding of the drivetrain options.