350 HP versus 400 HP

[O1dSoldier]

Hi Folks,

We're narrowing down our search but are finding that many of the coaches we like have 350 HP motors versus 400 HP. Obviously 400 HP is better for getting up hill, has more torque, etc. but what I am wondering if there is really that much difference, I mean if after driving 400 HP rigs would I be disappointed with the 350 HP?

Thanks in advance for your input.

 

[Mark_K5LXP]

Sounds to me about the difference between toad and no toad. Mine is spec'd 330 at full tilt and not anywhere near the weight of some of the larger class A's and it's just "OK" without a toad. Some grades I've wished for more but not worth enough to me to buy another one. I would place ride, handling and floorplan above HP but that's me.

Mark B.
Albuquerque, NM

 

[Kirk]

I mean if after driving 400 HP rigs would I be disappointed with the 350 HP?

If you were to go from driving a 40+ foot motorhome with a 400 to one that is 30' and has a 350 you would probably like the 350 better on the road. You have to compare the weight as well as the hp and the temperment of the driver also plays a big part. A patient driver who drives conservatively probably wouldn't notice much difference between the two even in the same size and weight of motorhome but someone who is an agressive driver who likes jackrabbit starts and being first out of the traffic lights would notice.

 

[O1dSoldier]

Sounds to me about the difference between toad and no toad. Mine is spec'd 330 at full tilt and not anywhere near the weight of some of the larger class A's and it's just "OK" without a toad. Some grades I've wished for more but not worth enough to me to buy another one. I would place ride, handling and floorplan above HP but that's me.

Mark B.
Albuquerque, NM

I'm very much a beginner when it comes to RV, but I question if having a 350HP diesel even in a 40-footer means no TOAD. That's one of the big reasons to go diesel over gas - the ability to tow greater than 5000 pounds.

 

[Mark_K5LXP]

I've never driven a diesel RV but by all reports the torque band is much greater than gas so you're not flogging it all the time. Plus it's all relative, I can't tell much difference in mine fully loaded with full tanks vs bare and empty tanks (it's a slug either way). At the end of the day you load up/hook up all your crap and down the road you go, whether you're doing a 45 second or 60 second quarter miles becomes somewhat secondary. As far as grades go, if they're that steep they're often accompanied by curves that preclude high speeds, or there are others just as slow and you're in good company.

Mark B.
Albuquerque, NM

 

[touchracing]

What are you more likely to run out of? Patience or fuel money?

 

[Larry N.]

I'm very much a beginner when it comes to RV, but I question if having a 350HP diesel even in a 40-footer means no TOAD.

That 350 and 400 referenced ARE diesel. And it's not NO toad, just a comparison of responsiveness with the 350 and no toad being roughly equivalent to a 400 WITH toad. Thje 350 (or my 360 HP) WILL pull a toad just fine, but with slightly reduced perfofrmance compared (all else being equal, which it rarely is) to the 400 HP unit. Of course weight of toad, loading of the RV, etc. affect it, too.

 

[Lou Schneider]

I've heard the rule of thumb is 1 HP for every 100 lbs of the Gross Combined Weight (GCW, the weight of the motorhome + toad). So 350 HP will give acceptable performance up to 35,000 lbs GCW, 400 HP up to 40,000 lbs GCW. Really, the difference between them is only 12.5%, not that much.

 

[O1dSoldier]

What are you more likely to run out of? Patience or fuel money?

Normally I'd go with patience but now that fuel is $79.99 a gallon, I might have to go with fuel money. ;-)

 

[UTTransplant]

We have a 40’ motorhome with a 10,000 pound towing limit. We also have a 360 HP diesel engine. The actual weight of our toad is 5,100-5,400 pounds, depending on what’s in the Grand Cherokee, while the MH is pretty much maxed out at 31,500-32,000 pounds. With that set up I pass loaded semi trucks on the mountains, and cars pass me. That is just fine for me. The amount of power you need is directly correlated to the size of the rig. My 360 HP would be well under what a 45’ tag axel rig needs. It is probably oversized for a 30’.

 

[Skookum]

We have a 40' motorhome with a 20k towing limit, 360hp Cummins L9 with 1150 lb-ft. Our toad weighs about 4600lbs (2-door JK Wrangler with steel bumpers, winch, etc). Gross combined for the motorhome is rated 60,000 lbs and the rig wet and loaded by itself is north of 35,000. I didn't get a weight this last weekend but my napkin math says just below 40k combined with the Jeep behind it.

We drove from Seattle to Newport, OR, down I-5 and then crossed over to the coast on Highway 20 at Corvallis. 20 goes over a minor mountain pass but there are a few sections with some good grades, maybe 5%. The L9 was happy to pull the steepest of those at 45 or so, which, honestly, a little more lethargic than I was expecting, but it's barely broken in. 1800-1900 RPM seems to be the sweet spot and once it gets into a gear where it can run that consistently, I feel like it could pull much more at that speed. Total trip was about 625 miles, averaging around 7mpg.

Now, I've towed a lot with a smaller diesel. Had an '06 Jayco Seneca 34ss, which was about 36', had a 300hp 550 lb-ft Duramax V8 and 5-speed automatic. We pulled a 7k trailer with that from Seattle to Minneapolis a couple of times and it did very, very well considering. 35-45mph on the steepest of passes including the Rockies, but that was everything it had at 2800rpm. It was fully loaded to its 26k gross combined limit and maybe then some. Fuel mileage was much better, 8-9 while towing and as good as 11mpg when not towing when keeping speeds around 65.

Look into a TSD Open Roads card if you get diesel. We saved $70 across two fill-ups this last weekend alone.

 

[Groo]

actually, if you look at torque curves, the higher HP engines usually just pull higher. There will be thresholds that have higher toque than others at different areas, but occasionally the lower HP motor will have more useful torque.
are you talking a new L9?

https://mart.cummins.com/imagelibrary/data/assetfiles/0064235.pdf

https://mart.cummins.com/imagelibrary/data/assetfiles/0043067.pdf

notice in the second link that the 330hp version (along with the 300hp and 285hp) has more low end torque than the higher hp versions.
I am guessing it has a smaller turbo.
the difference between the others is just going to purely be a software thing, possibly different injectors in the mix too. The transmission is going to be a limiting factor as well. They can limit peak torque in lower gears to let the transmission survive, type of thing.

 

[Groo]

We have a 40' motorhome with a 20k towing limit, 360hp Cummins L9 with 1150 lb-ft. Our toad weighs about 4600lbs (2-door JK Wrangler with steel bumpers, winch, etc). Gross combined for the motorhome is rated 60,000 lbs and the rig wet and loaded by itself is north of 35,000. I didn't get a weight this last weekend but my napkin math says just below 40k combined with the Jeep behind it.

We drove from Seattle to Newport, OR, down I-5 and then crossed over to the coast on Highway 20 at Corvallis. 20 goes over a minor mountain pass but there are a few sections with some good grades, maybe 5%. The L9 was happy to pull the steepest of those at 45 or so, which, honestly, a little more lethargic than I was expecting, but it's barely broken in. 1800-1900 RPM seems to be the sweet spot and once it gets into a gear where it can run that consistently, I feel like it could pull much more at that speed. Total trip was about 625 miles, averaging around 7mpg.

Now, I've towed a lot with a smaller diesel. Had an '06 Jayco Seneca 34ss, which was about 36', had a 300hp 550 lb-ft Duramax V8 and 5-speed automatic. We pulled a 7k trailer with that from Seattle to Minneapolis a couple of times and it did very, very well considering. 35-45mph on the steepest of passes including the Rockies, but that was everything it had at 2800rpm. It was fully loaded to its 26k gross combined limit and maybe then some. Fuel mileage was much better, 8-9 while towing and as good as 11mpg when not towing when keeping speeds around 65.

Look into a TSD Open Roads card if you get diesel. We saved $70 across two fill-ups this last weekend alone.

60,000 lbs GCVWR is transmission limited. You have an Allison 3000 series. I have seen multiple heavy duty chassis where the "cheap" transmission limits the vehicle weight below the axle ratings. I think your torque output will also be throttled back in your lower gears as well.

 

[Skookum]

are you talking a new L9?

Yes

60,000 lbs GCVWR is transmission limited. You have an Allison 3000 series. I have seen multiple heavy duty chassis where the "cheap" transmission limits the vehicle weight below the axle ratings. I think your torque output will also be throttled back in your lower gears as well.

I’d believe it. And yes, Allison 3200 TRV. It’s a weird sensation, it doesn’t feel like the truck is struggling much. It just gets slower and seems to maintain its speed even if the grade increases slightly. Maybe that’s torque. But it does feel like it’s holding back and decides to let a few more horses out of the barn when it really needs it.

 

[Scottro]

As Mark said from above, you will not notice any significant difference. Our coach is a diesel 40’er,with 340 hp and it weighs right around 30k. I do not remember the torque rating off the top of my head. We often travel with my mother and father-in-law and they have a 45’er with 450hp. Their rig weighs right around the 45k range. Not sure of the specs on his torque either. We both tow vehicles behind and from a complete stop I’ve noticed that he cannot keep up at low end with me on flatland. When it comes to pulling grades I have not noticed a significant difference either. Several times I have been in front of them and they definitely do not go flying by us or necessarily even pass us. So ultimately this supports the weight to horsepower ratio mentioned from above.
I drove their coach on our summer trip this year only for a short period, 10 to 15 miles on flatland and the handling of theirs seemed to be a bit better,but again the take off from a start,I felt ours was better.
Scott,Orlando

 

[Groo]

Seat of the pants feel is all about low end torque. The heavy trucks I deal with have a mix of 9l engines and 12l engines. The hp outputs are often the same or similar, but you can really feel the difference between the two in city traffic. It is a drastic difference. Out on the Highways, imagine that the differences close up some, but still, the basic engine family matters more than the hp rating. Someone asked me if the upcharge for the 375hp L9 was worth it over the 350hp L9. I suggested dropping to the 330 hp, higher torque version instead. There would be zero difference between the 350 and the 400hp versions for a decent chunk of the PowerBand. They will have identical low end and midrange. It's not till you cross 1500rpm or so, that there will be a difference, and as mentioned earlier, even then, it might only be there in your top few gears. I would think that the dealer should be able to get ahold of an expert to confirm or refute what I am saying. I am not a power train expert by any stretch of the imagination. Now like I mentioned, an RV is going to use that higher end power more than the stuff I'm used to, but you will certainly not have a 25% powerboost across the board with the higher hp version of the same engine. That is the main point I and driving at.

 

[Gary RV_Wizard]

The diesel torque basically just makes plenty of horsepower available at low Rpms, thus improving the acceleration & responsiveness. Torque & Hp are intrinsically linked; you can't have more of one without more of the other. The ratings that get bandied about are a snapshot of the peak, x Hp and y Lb-ft @ z Rpms. That's not the whole story of engine performance and the reason that drive train .

When you have differing Hp versions of the same engine, there will be fairly minor differences in the torque/Hp curve, but when comparing different engines, e.g. ISB vs ISC vs ISL vs ISX, the differences may be greater, The Cummins ISL in particular has been offered in a wide range of Hp configurations for motorhomes, at least 350-450 hp, all of them with a peak torque of 1100 lb-ft as far as I know. The Cummins B/ISB also covers a wide Hp range, with earlier years as low as 230 hp and the recent ones as high as 360.

As Lou said, the key to perfromance is weight per Hp. RV manufacturers use 100 lbs/hp as a rule of thumb, but they tend to use GVWR rather than GCWR for the weight, at least at the entry-price end of the scale. Luxury models typically are more generous with the Hp. In any case, 100 lbs/Hp is deemed "adequate" but not great performance. Your car or pickup is probably in the range of 10-15 lbs/hp, so a motorhome at 100:1 or even 90:1 is going to be a dog in comparison.

Use weight per Hp if you want to compare probably performance. Either GVWR or GCWR, as long as you use the same rating for both. Frankly, though, it's one of the less important measures unless you know you get frustrated by weak acceleration and slow hill climbs.

 

[O1dSoldier]

We have a 40' motorhome with a 20k towing limit, 360hp Cummins L9 with 1150 lb-ft. Our toad weighs about 4600lbs (2-door JK Wrangler with steel bumpers, winch, etc). Gross combined for the motorhome is rated 60,000 lbs and the rig wet and loaded by itself is north of 35,000. I didn't get a weight this last weekend but my napkin math says just below 40k combined with the Jeep behind it.

We drove from Seattle to Newport, OR, down I-5 and then crossed over to the coast on Highway 20 at Corvallis. 20 goes over a minor mountain pass but there are a few sections with some good grades, maybe 5%. The L9 was happy to pull the steepest of those at 45 or so, which, honestly, a little more lethargic than I was expecting, but it's barely broken in. 1800-1900 RPM seems to be the sweet spot and once it gets into a gear where it can run that consistently, I feel like it could pull much more at that speed. Total trip was about 625 miles, averaging around 7mpg.

Now, I've towed a lot with a smaller diesel. Had an '06 Jayco Seneca 34ss, which was about 36', had a 300hp 550 lb-ft Duramax V8 and 5-speed automatic. We pulled a 7k trailer with that from Seattle to Minneapolis a couple of times and it did very, very well considering. 35-45mph on the steepest of passes including the Rockies, but that was everything it had at 2800rpm. It was fully loaded to its 26k gross combined limit and maybe then some. Fuel mileage was much better, 8-9 while towing and as good as 11mpg when not towing when keeping speeds around 65.

Look into a TSD Open Roads card if you get diesel. We saved $70 across two fill-ups this last weekend alone.

The motorhome we are thinking of making an offer on is 40', it has a 400 HP cummings so should be more than enough but there are a few we like, also in the 40' range that have 350 HP. I don't know if I've looked at any with a 360 but I might have.

One of the next questions I was going to post actually was about fuel card options and any RV or travel groups to think about joining, AAA, etc.

 

[Laura & Charles]

I suggest you start a new thread to ask about fuel cards and another new one about travel groups/roadside assistance.
(Or just do a search here on TSD and roadside assistance… both topics have pretty thoroughly discussed here; and recently, too.)

 

[O1dSoldier]

Thanks everyone for your insights.

As we shop around and look at different vehicles, we are buying used, I know I want diesel. I will be carrying a motorcycle via lift. My research tells me that a gasser can't support the weight hanging off the back plus I do want the more comfortable ride and less noise that comes with diesel. Also, while I doubt that I'll ever be pulling an enclosed trailer I want the ability to carry the motorcycle and pull a vehicle at the same time. Again, my research tells me that I can't do that with a gasser. Please correct me if my information is wrong.

In general, getting up a hill at 40 MPH is better than doing it at 20 MPH. I believe everyone feels the same way I don't care who you are or how much of a hurry you claim to not be in. Further I don't want to be the guy who holds everyone up when merging because my 350 can only get up 20 MPH on the ramp versus 30 MPH on the ramp with the 400. So based on numbers alone the 400HP seems to be the way to go but if in reality only there are only minor differences between the performance of 400 and 350HP it then becomes a matter of getting the most bang for my buck, i.e., for x amount of dollars I can get this nice rig with 400HP or for the same amount of money I can get this even nicer rig with 350HP.