Does a higher HP diesel engine have a better mpg ratio

[WeatherTodd]

Originally Posted by SeattlePirat

...
A diesel needs only two things to run, air and FUEL.

why do factories turbocharge/intercool them???

[dennis45]

Originally Posted by distaff

See attached. How you drive is key to mileage.

http://www.catrvclub.org/PDF_Docs/Un...nding_Perf.pdf

Distaff,
Thank you for posting this. Excelent document!
I particularly like the section on Exhaust Brakes. So many new DP RVer's don't have a good grasp on the concept of Exhaust brakes and how they function to control speed.

[blackf3504dr]

Originally Posted by distaff

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.

I think you meant to say the push rods run through the intake runners....

[blackf3504dr]

Originally Posted by WeatherTodd

why do factories turbocharge/intercool them???

Turbo charging packs more air into the combustion chamber which produces more power and the cooler the air the more power, hence the intercooler. I know that's a pretty simple explanation but basically that's it.

[bbeane]

As modern diesels go they are going to get within 1-2 mpg of each other. I'll take the big motor any time. As far as the rv goes I long ago quit fretting about weather I get 7.9 or 8.4 mpg. The best thing you can do is look at the book on your engine, it will tell you where the peak torque and hp RPM is, you will get the best performance, and the engine will run cooler if you stay in that range. With that said you may have to manually control the transmission to keep it in that range depending on road conditions. Way back in the old days (70s and 80s) before high torque rise and gear fast run slow technology we just tried to keep em turning at governed speed, it was a no no to lug a diesel, of course we only got 4-5 mpg. My how have times have changed.

[SeattlePirat]

Originally Posted by edgray

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.

True, but the computer monitors and adjusts the fuel input to meet EPA standards and even out fuel consumption.

No air or fuel no workee.

[SeattlePirat]

Originally Posted by WeatherTodd

why do factories turbocharge/intercool them???

turbo's pack more air, so in essence you lower the amount of fuel you need to keep the mixture levels correct.

[bjsheldon]

There is so much incorrect or partially correct info in the above posts.
1. As correctly said above, a given coach of a given weight uses the same HP to push it down the road at a given speed, requardless of the size, HP, turbo or etc. of the engine.
2. Varrious engines have different efficiencies at the same HP output, thus as in 1. above, one engine may give better fuel economy that another because of this efficiency.
3. The time and effort to get to that given speed is indeed driver and HP dependent. If the driver puts the pedal to the floor, more HP will get to that given speed sooner and thus is able to back off the pedal sooner. However, a driver with a smaller HP engine who doesn't put the pedal to the floor may actually get to that given speed using less fuel; it will just take longer.
4. Turbo charging pushes more air into the cylinders, thus with more air and the proper amount of fuel will result in more HP. This allows for a smaller engine, ie less weight, to produce the required HP to maintain that given speed and usually better efficiency .
5. An intercooler cools the intake air making it more dense. (As air heats up its density goes down). Denser air has more oxygen, thus for a given amout of air put into the cylinder, the more oxygen the more HP.
The long and short of things, MPG is much more effected by the driver than the size of the engine. The other consideration is duarability. If one engine has to work at 50% of its rated power to maintain that given speed, while anothe engine may only have to work at 25% of its rated power. This allows engine 2 to have reduced loads on its components.

[laj]

Originally Posted by SeattlePirat

turbo's pack more air, so in essence you lower the amount of fuel you need to keep the mixture levels correct.

A no, that would give you a lean condition.
Basic physics

[laj]

Originally Posted by bjsheldon

There is so much incorrect or partially correct info in the above posts.
1. As correctly said above, a given coach of a given weight uses the same HP to push it down the road at a given speed, requardless of the size, HP, turbo or etc. of the engine.
2. Varrious engines have different efficiencies at the same HP output, thus as in 1. above, one engine may give better fuel economy that another because of this efficiency.
3. The time and effort to get to that given speed is indeed driver and HP dependent. If the driver puts the pedal to the floor, more HP will get to that given speed sooner and thus is able to back off the pedal sooner. However, a driver with a smaller HP engine who doesn't put the pedal to the floor may actually get to that given speed using less fuel; it will just take longer.
4. Turbo charging pushes more air into the cylinders, thus with more air and the proper amount of fuel will result in more HP. This allows for a smaller engine, ie less weight, to produce the required HP to maintain that given speed and usually better efficiency .
5. An intercooler cools the intake air making it more dense. (As air heats up its density goes down). Denser air has more oxygen, thus for a given amout of air put into the cylinder, the more oxygen the more HP.
The long and short of things, MPG is much more effected by the driver than the size of the engine. The other consideration is duarability. If one engine has to work at 50% of its rated power to maintain that given speed, while anothe engine may only have to work at 25% of its rated power. This allows engine 2 to have reduced loads on its components.

Agree mostly with 5. The new engines with a pf
Using def needs to run closer to rated power in order to create x amount of heat to properly clean the egr, pf
Loafing around with them will get you about a 1500 dollar bill to take it to a shop to have it cleaned so the system will work properly.

[jeffp]

Rear end gear ratio has a lot to do with mileage.A 355 uses less revs at hwy speed a410 is a great pulling gear.Big rigs spec their trucks for the type of loads and work they do.The size of engine coupled with the size of gears,also the size of the unit and weight Along with your driving will determine the mileage you will get for that unit.This can be a very good study to pick out your rv and when you ask and compare mileage.Having fun rving and listening to all the comparrisons.jeffp

[dennis45]

Originally Posted by dennis45

Distaff,
Thank you for posting this. Excelent document!
I particularly like the section on Exhaust Brakes. So many new DP RVer's don't have a good grasp on the concept of Exhaust brakes and how they function to control speed.

The only "accurate" information in this whole thread is the post by distaff. Written by an engine manufacture who knows what they are talking about.


http://www.catrvclub.org/PDF_Docs/Un...nding_Perf.pdf

[Mr_D]

Originally Posted by dennis45

The only "accurate" information in this whole thread is the post by distaff. Written by an engine manufacture who knows what they are talking about.


http://www.catrvclub.org/PDF_Docs/Un...nding_Perf.pdf

Nope, mine was also accurate.

[Cat320]

Originally Posted by WeatherTodd

why do factories turbocharge/intercool them???

That's what makes these things so powerful when climbing the Rockies. The turbo keeps packing the air in the engine so it doesn't know if it's in thin air of Colorado or Florida. Where a gasser, non turbo, loses 3% of it's power for each 1,000' of altitude, the TD does not.

Regarding the leaning/riching of the fuel mixture. The higher you go, the leaner the mixture should be.