40' mh - how large an engine is enough?

[btlabow]

i'M LOOKING INTO FULLTIMING IN A 40' MH. TOP ON MY LIST NOW IS THE WINNIE VECTRA 40. I NOTICED THEY COME WITH DIFFERENT ENGINES. THE ONE I LIKE IS THE ISL400. WILL A SMALLER ENGINE BE AS EFFICIENT AND HAVE THE PULLING POWER TO PULL A LOADED COACH AND A TOAD? I'M NOT LOOKING TO DRAG RAG RACE THAT'S WHAT THE CORVETTE IS FOR BUT I WANT ENOUGH POWER TO GO UP HILLS AND IF I WANT TO PASS.
THANKS IN ADVANCE

[RV Wizard]

The ISL will have enough torque to do the job. An ISC might do OK depending on weight. My ISC gets about 9 mpg. Sometimes I wish I had more torque for long steep grades. I am not sure what the ISLs are getting.

[RVThere]

Just drove my 400hp ISL cross country for 6288 miles pulling a Cherokee and got 6.2 mpg. That was about $3000 worth of fuel. If you had a ISC and got the 9 mpg mentioned above, you would have only spent $2000 on fuel.

The power is great but it comes at a price!

[Norm4015]

I have a 370 hp Cummins ISL with 1200 ft lbs torque. The motorhome is 41' long and weighs 31,000 pounds loaded and tows a 3600 pound CR-V. I have crossed the Rockies several times and crossed the Canadian Rockies two times. It has all the power I need and I'm impressed with the fuel mileage. The last 21,000 miles it has averaged 9.2 mpg figuring with a pencil and paper so that is actual fuel mileage. This one is a keeper.

[Gary RVRoamer]

100 lbs per hp is the traditional rile of thumb and it applies to motorhomes too. That yields adequate if not blazing, performance. The voluminous low RPM torque & hp of a diesel will get you up the hills nicely. That doesn't mean you won't slow down on a long 6% grade, but you will still get up it fine.

My 2004 ISL (370 hp, 1200 lb-ft torque) gets 8.1 mpg cruising the interstates at 62-65, but falls off to the low 7's on two lane roads. That's with a 33,000 lb coach and 4900 lb toad.

Post 1/1/2007 engines get poorer mileage due to substantially increased emissions controls.
Supposedly the post 1/1/2010 engines buy some of the mpg back with an improved system, but you pay extra cost upfront for the engine and also every mile for something called DEF, which costs about the same as a gallon of diesel and is consumed at the rate of 2% of the fuel used.

[RickO]

That's a very nice coach you're looking at. My Itasca Ellipse 40FD is similar. I have the 400hp ISL w/1200 ft # of torque and power is certainly not a problem. I get 7.1 mpg at 62mph towing a Ford Explorer.

Good luck.

[btlabow]

Originally Posted by RV Roamer [Gary]

100 lbs per hp is the traditional rile of thumb and it applies to motorhomes too. That yields adequate if not blazing, performance. The voluminous low RPM torque 7 hp of a diesel will get you up the hills nicely..

Gary,
You speak of 100lbs. per hp. 100 lbs. of what? I'm assuming GVW. The sentence about the torque and the 7hp threw me. It seems there can be too much hp which lowers the fuel efficiency of the MH. I also noticed from the replies that while the hp ratings differed the torque ratings seemed to all be around 1200 ft. lbs.. Is the efficiency and or fuel mileage a function of the horsepower or the torque?

[JimM68]

The 1200 ft-lbs of torque is being limited by the miracle of modern electronics.
That's how much the transmissions can take.

[HD4Mark]

Our Vectra has the ISC 350hp and it has plenty enough power for us. Can't comment on the mileage though. Never figured it out 'cause we are going anyway

[Gary RVRoamer]

You speak of 100lbs. per hp. 100 lbs. of what? I'm assuming GVW. The sentence about the torque and the 7hp threw me. It seems there can be too much hp which lowers the fuel efficiency of the MH.

Typo - that was supposed to be "torque & hp", not "7 hp". All the ISL engines have 8.9L displacement and a max1200 ft lbs of torque, but the hp varies and so does the rpm where that hp is available. The miracle of electronic engine controls. as someone already said. Great peak horsepower does not mean greater fuel consumption unless you actually use that peak hp regularly. If it takes, say, 140 hp to keep a coach rolling at 62 mph, then you use 140 hp worth of fuel, whether it is a max 400 hp or a max 425 hp version of the engine. A physically smaller engine, e.g. the 8.3L ISC, might be more fuel efficient at that same 140 hp, but all the ISLs will be about the same.

Hp = torque*RPMs /550. You need to know the torque actually generated at the RPMs you run at for this to be meaningful. The peak torque (1200 ft lbs) is at a fairly high rpm value that you seldom use, and the peak horsepower RPMs rarely coincides with the peak torque RPMs.

If fuel economy is your major concern, get a smaller displacement engine (the ISC or ISB) and buy one built before 1/1/2007. And keep your foot light on the go-pedal.

[mythplaced]

Originally Posted by RV Roamer [Gary]

......

Hp = torque*RPMs /550. You need to know the torque actually generated at the RPMs you run at for this to be meaningful. The peak torque (1200 ft lbs) is at a fairly high rpm value that you seldom use, and the peak horsepower RPMs rarely coincides with the peak torque RPMs.....

????



Per cummins, http://cumminsengines.com/assets/pdf/4971107.pdf

My ISL 400 puts out max hp at 2100 rpm & max torque at 1300 rpm...and I see 1300 rpm all the time..

Do you know something I don't know??

[Jerry J]

Fairly high rpm? 1300? At a seminar a Cummins rep said the sweet spot is 1400 to 1600 rpm. More rpm will get you up the hill faster but use more fuel. I seem to drive in this range while driving down the highway.
1200 lbs. at 1300 rpm, 425 hp at 2100 rpm.

>>>>>>>>>>>>>>>>
The peak torque (1200 ft lbs) is at a fairly high rpm value that you seldom use, and the peak horsepower RPMs rarely coincides with the peak torque RPMs.
<<<<<<<<<<<<<<<<

[Always-RVing]

The 8.3 ISC does fine for flat to moderate hilly, rolling terrain cruising (31,000 lbs-35,000 lbs). When climbing more serious mountains, I really need more power. The engine lags and overheats easily. There is of course a cost trade-off with cost. The next size up (400-425 ISL can run $20-30,000 more) in coach cost plus you'll pay more at the pump with less MPG. Both engines are great. The ISL is a higher end motor and used in larger, higher end motorhomes.

Just to mention. The ISL is often trimmed back on torque so not to over power the tranny. One more thing... ISL's require a side radiator. Side radiators use hydraulic to power the fan which can take 25-75 horsepower. It's the requied size of the radiator that forces a side mount instead of rear. There is not enough room across the rear for it.

The motors are widely used commercially and often get 600-800,000 miles. They are both very durable, powerful and long-lasting engines. Bullet Proof

[Jerry J]

My Vectra has a rear radiator (425hp,ISL).
One of the best motorhomes I have driven was a 08 or 07 Alpine 36' with a 425 and active air. That was an enjoyable driver. I don't think there ever is too much power for hills or passing. I think weight to power ratio is what really makes the different.