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Old 06-09-2012, 12:00 AM   #1
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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?
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Old 06-09-2012, 12:13 AM   #2
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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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Old 06-09-2012, 04:45 AM   #3
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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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Old 06-09-2012, 05:00 AM   #4
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More ponies = more hay

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Old 06-09-2012, 05:16 AM   #5
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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.
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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Old 06-09-2012, 06:27 AM   #6
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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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Old 06-09-2012, 08:47 AM   #7
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Originally Posted by SeattlePirat View Post
...........A diesel needs only two things to run, air and FUEL.
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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Old 06-09-2012, 09:58 AM   #8
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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.
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Old 06-09-2012, 12:15 PM   #9
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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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Old 06-09-2012, 01:07 PM   #10
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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!

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Old 06-09-2012, 01:10 PM   #11
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See attached. How you drive is key to mileage.

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Old 06-09-2012, 01:22 PM   #12
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6 speed tranny helps and a light foot.
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Old 06-09-2012, 05:51 PM   #13
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Originally Posted by SeattlePirat View Post
...A diesel needs only two things to run, air and FUEL....
You forgot one...MONEY!
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Old 06-10-2012, 04:17 PM   #14
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Originally Posted by 1ciderdog View Post
...his had the 460 engine and mine had the 390 engine.
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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