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06-09-2012, 12:00 AM
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#1
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Senior Member
Join Date: Jan 2012
Location: Taft, TN 38488
Posts: 592
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Does a higher HP diesel engine have a better mpg ratio
Ag a 400 HP 40 footer will it get better mpg than say a 350 HP?
Thank Tom
"searching for the best full-time rig"
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06-09-2012, 12:13 AM
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#2
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Senior Member
Country Coach Owners Club Solo Rvers Club iRV2 No Limits Club
Join Date: May 2011
Location: Vancouver, WA
Posts: 37,725
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Depends mostly on the driver. Testing by truck companies show as much as 30% difference between the best and worst drives even after changing rigs.
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2009 45' Magna 630 w/Cummins ISX 650 HP/1950 Lbs Ft, HWH Active Air
Charter Good Sam Lifetime Member, FMCA,
RV'ing since 1957, NRA Benefactor Life, towing '21 Jeep JLU Rubicon Ecodiesel
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06-09-2012, 04:45 AM
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#3
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Senior Member
Vintage RV Owners Club
Join Date: Aug 2009
Location: Central Florida
Posts: 671
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It takes the same amount of HP to move the coach down the road no matter how big the engine, having said that a higher HP engine doesn't have to work as hard to get to speed and maintain that speed then a lower HP engine. As D said a lot depends on the driver.
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92' Holiday Rambler 1000
The wife, me and two furry kids
If you have it, a truck brought it.
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06-09-2012, 05:00 AM
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#4
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Senior Member
Join Date: Dec 2011
Location: Chaffee, MO.
Posts: 504
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More ponies = more hay
Happy Trails
Jim
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MGySgt USMC(R) 1961-1991
2004 Winnebago Brave 34D
2019 Jeep Wrangler unlimited Amateur Call N0LZS
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06-09-2012, 05:16 AM
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#5
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Senior Member
Join Date: Aug 2011
Posts: 1,546
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While not totally the same the principles are.
We had both a super charged Grand Prix v6 and a gas friendly Pontiac Vibe 4 banger. Going to Long Island I actually use to get 3-4 MPG better in the Grand Prix (if I drove them both the same).
Keeping the transmission from hunting for gears and the rpms down helps. Sometimes that extra HP/torque gives you just enough to keep everything efficient.
Summary:
If you drove them exactly the same probably so, of course like the Grand Prix on local driving, the first time you put the pedal to the metal you just blew your MPG savings.
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2017 F350 Lariat Diesel Dually, White, Hitch Kit.
2013 Dutchman Voltage 3200 Epic II 5th wheel.
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06-09-2012, 06:27 AM
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#6
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Senior Member
Join Date: Nov 2009
Location: Bainbridge Island, Wa
Posts: 557
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I dont believe that a high hp engine will out perform a lower hp engine in fuel economy if all things are equal except the hp. If that were true, every one would be buying the same engine for a 34 footer as with a 43 footer. Right?
A diesel needs only two things to run, air and FUEL.
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06-09-2012, 08:47 AM
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#7
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Senior Member
Join Date: Oct 2002
Location: Madison, MS
Posts: 10,527
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Quote:
Originally Posted by SeattlePirat
...........A diesel needs only two things to run, air and FUEL.
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Make that "some" diesels.
Most modern diesels won't run on JUST air and fuel, they also need a computer and lets don't even start talking about all the downstream stuff.
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06-09-2012, 09:58 AM
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#8
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Senior Member
National RV Owners Club
Join Date: Jun 2010
Posts: 4,581
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Years ago my buddy and I both had the same year Ford pick ups - his had the 460 engine and mine had the 390 engine. We had the same size cab over campers and we traveled together to go camping in the mountains. His 460 always got better mpg than the smaller 390 on those trips. Maybe it was just the efficiency of the those particular engines but my thought was the larger engine did not have to work as hard as the smaller one. I had the pedal to the metal when he only used 3/4 pedal going up some of those mountains. I know this thread is about diesels so am only putting this in as a theory that the larger HP engine may not have to work as hard and therefore get better mileage depending on vehicle weights, terrain, and other conditions.
Bob
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Bob & Sandi, dogs Tasha a Frenchie and Tiki a Skipperkey
SW OREGON 2005 34 foot DolphinLX
If towing: a bright red 2016 Mini Cooper on a tow dolly.
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06-09-2012, 12:15 PM
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#9
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Member
Winnebago Owners Club
Join Date: May 2012
Posts: 67
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Does a higher HP diesel engine
I had a 1999 HR with the 275 cummins and kept track via spread sheet of every drop of fuel over 127,000 miles towing and overall average was 10.347 mpg. When considering a change of motorhomes I contacted Cummins that the new unit with a 400 cummins would be about 4,000 lbs more overall. They suggested it would run around 9.0 mpg. Well after 19,000 miles we are averaging 7.58 mpg.
I then contacted Cummins shops as well as Cummins and then gave the response that as they reduce emissions since 1999 that there will also be a reduction in Mpg. I fail to understand burn more fuel and reduce emissions but I do not have a degree in science only common sense.
I finally came to the conclusion that as fuel price increase's and HP increases the overall mileage is reduced. But I will not give up the lifestyle till we are out of $$$
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06-09-2012, 01:07 PM
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#10
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Senior Member
Join Date: Aug 2010
Location: Over the next hill, around the next curve...
Posts: 5,663
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As others have pointed out, so many factors in this...
And, one key thing to remember is that the 'same' RV with say mid block vs big block engine - will not be the same 'exact' RV. The Big Blocks, lets just talk Cummins in this example. Compare a ISL (as I have) with an ISM. The bigger blocks weight more, and require more cooling so more weight, requires bigger trans with the Allison 4000 so more weight, and usually heavier duty running gears for the differential and axle shaft so more weight.
So this means the same RV with mid size vs Big Block - are not the same RV.
All that being said, it comes down to power to weight ratios, and you can find that the mid block actually has more power to weight ratio then the Big Block, in the identical rigs. (Country Coach Intrigues, can be found with the ISL400 or the Big Block Cats, as one example.)
So, if driven the same way by the driver, it is very possible for the bigger engine rig to get better MPG then the mid size engine. It is a case by case analysis.
In general, IMO, if available - go with the biggest engine you can get in the rig. As it is more then just MPG, it is also a safety feature to have reserve power capacity for acceleration needs. And remember most Big Blocks, will have more compression to aid in braking, many times with a three stage Jake vs a mid size engine's two stage Jake Compression, and or exhaust brake in some units.
So, suppose the answer is - it depends !
Best to all, be safe, have fun,
Smitty
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07 Country Coach Magna Rembrandt 45' ISX600
Roo II was our 04 Country Coach Allure 40'
OnDRoad for The JRNY! Enjoy life...
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06-09-2012, 01:10 PM
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#11
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Senior Member
Join Date: Dec 2009
Posts: 1,920
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__________________
Paul
2006 Patriot Thunder C13 Allison 4000
2010 Ford Flex Ecoboost AWD
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06-09-2012, 01:22 PM
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#12
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Senior Member
Monaco Owners Club Freightliner Owners Club Holiday Rambler Owners Club
Join Date: Jul 2009
Location: Spring Hill, Florida
Posts: 1,380
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6 speed tranny helps and a light foot.
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98 Endeavor DP, ISB275
RX300, Falcon II hitch, BB Vantage Select
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06-09-2012, 05:51 PM
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#13
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Senior Member
Fleetwood Owners Club American Coach Owners Club
Join Date: Mar 2011
Posts: 6,768
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Quote:
Originally Posted by SeattlePirat
...A diesel needs only two things to run, air and FUEL....
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You forgot one...MONEY!
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2014 American Eagle 45T
DD 13, 500 HP
Pulling a Honda CRV
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06-10-2012, 04:17 PM
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#14
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Senior Member
Join Date: Dec 2009
Posts: 1,920
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Quote:
Originally Posted by 1ciderdog
...his had the 460 engine and mine had the 390 engine.
Bob
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Worth noting that the 390 was an FE engine, the design of this family began in 1957, and it had a peculiar design were the push rods essentially pass through the intake manifold. The 460 is of the 385 family of engines designed in the late 60's with better breathing characteristics. Of the FE family only the 427 Single Overhead Cam (SOHC) didn't have that problem because the Ford engineers eliminated the push rods with an overhead cam design.
Really hard to make a generalization about fuel economy and power when comparing to such different architectures.
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Paul
2006 Patriot Thunder C13 Allison 4000
2010 Ford Flex Ecoboost AWD
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