350 HP ICE vs. 350 HP Diesel?

[RMIGHTY1]

OK, newbie here and aspiring RV'er. What comments can be made about the advantages/disadvantages of a 350 HP internal combustion engine vs. a 350 HP diesel engine in a class A camper (assuming all other variables are equal)?

[OhhWell]

wny_pat wrote:

tinkerer wrote:
the hemi has a lot of power but is constantly hunting for right gear because it's torque curve is small. I really don't care what other say about gassers as I go by the seat of my pants and the diesel wins hands down, JMHO.;)

And there in lays the answer! The torque curve of a diesel covers a much wider RPM range that the gas engine does. The diesel requires less shifting when going up a hill because of the wide torque curve.

The answer to which question?

Also, the diesel's curve covers a much SMALLER rpm range than the gas engine. It is a lot easier to have flatter curves when you have a smaller RPM range. Also, the forced induction has a lot to do with flattening a power curve as well. It is great for the Good 'ole seat of the pants Dyno etc but the stopwatch often tells a different story.

[wny_pat1]

tinkerer wrote:
the hemi has a lot of power but is constantly hunting for right gear because it's torque curve is small. I really don't care what other say about gassers as I go by the seat of my pants and the diesel wins hands down, JMHO.;)

And there in lays the answer! The torque curve of a diesel covers a much wider RPM range that the gas engine does. The diesel requires less shifting when going up a hill because of the wide torque curve.

[tinkerer]

Another thing that I hear from gas owner is that the coach has a tendency to hunt for the right gear. They have to shifting a lot more than a diesel because of the narrow torque curve. I have a diesel pusher and a Ram pickup with a 5.7 Hemi and the hemi has a lot of power but is constantly hunting for right gear because it's torque curve is small. I really don't care what other say about gassers as I go by the seat of my pants and the diesel wins hands down, JMHO.;)

[Matt_Colie]

Summary, CI (diesels) engines cost more than SI (gasoline) engines to build (buy) and maintain. Though performance is better at higher altitudes, there is little else to be gained. Past that, the differentiation can only be established by a complete economic analysis of each specific situation.

I don't think very many people here will like what I have to say, but the problem is that what I am about to say is fact. If you don't agree, you are free to do the research and post it here..

The biggest difference in CI vs SI over the road engines is very simple. Diesels(CI) do well with turbo-charging and spark ignition (SI) engines do not. I could get down one of the papers I wrote on this, but I don't have the time.

The simple answer is that air is what makes horsepower. Horsepower is Horsepower and I don't care where the torque peak is, gears can fix that. The reason air is important is that it is needed to burn the fuel (any fuel) to make horsepower. Turbo-chargers are an inefficient way to cram more air into an engine. The advantage that they have is that they are not dependent on the engine crankshaft speed to do this. They also exhaust into the local atmosphere as so they can increase the manifold pressure as altitude increases. This is a real advantage at altitudes above 8K MSL a turbo-diesel may (depending on calibration) have a significant advantage.

Another serious comparison has to be cost. CI engines cost more for lots of reasons. Maintenance costs of many CI engines is higher that most most SI engines. (This is a gross generalization, but typically accurate.) Over the life of a typical RV the fuel savings will never overcome the combination of initial cost and the differential cost of maintenance. This can only be determined by a complete cost analysis.

So, do the complete analysis and make your own decision what will serve you best.

Me, What do I know about this??
I ran contract engine testing laboratories for about 30 years.
I am a ship's engineer by trade and have owned 6 diesel vehicles.

Matt

[OhhWell]

rgatijnet1 wrote:

tempforce wrote:
one thing they forgot to mention above and in the test...
how long the engines will last and how much fuel each used to perform the test...

the diesel will usually last between 750,000 to 1 million miles before overhaul... the gas engine should last around 150,000 - 200,000 miles...
also the diesel engine will usually get 1/3 to double the mileage of the gas engine, with the same gearing/load ect.

Yep, I'm sure that Motor Trend could have modified the test so that the diesel performed better. Maybe change the gearing, load the gas coach more, etc. Since you brought up mileage, according to the test, the diesel got 18MPG max and the gas engine got 17mpg max. Not a huge difference, especially since diesel fuel costs more The OP asked for a comparison with all variables being equal. The Motor Trend test was pretty equal as to comparing the two different types of engines.

Yeah but some people don't like having their core beliefs challenged. HP is HP and you are stuck with what your engine puts out. Torque can be increased all day through gearing. It's just the way it is. The engine with more power (HP) is going to pull more weight faster (note, I didn't say vehicle, just engine).

I could take the engine out of our coach, install pedals under the steering wheel and enough gearing to deliver 2,000 lbs/ft of torque to the transmission. I could then pedal our 20,000 lbs coach right on up a 10% grade BUT I sure as heck wouldn't be going very fast.

[Hjudge49]

Let's get back to the OP's question...350hp Gas v 350hp diesel. The Hemi vs Cummins test pits 410 vs 385HP and the gearing and trannys are different. Hook both up to a trailer with a load of 15,000 pounds and let's see who can climb an 8% grade. That's where the torque commes into play. The biggest difference is that the Hemi makes it's power at about 4500-6000 RPMs while the Cummins makes it's peak torque around 1800-2000 RPMS. Diesels are designed to run literally all day at close to peak RPMs, gas motors not so much. That's why they wear out quicker. I repowered a fishing boats I once had with brand new Volvo 220hp diesels. The technician told me to run them wide open until they reached max rpm, then back off 200 RPMs as the proper cruising speed. I dare say nobody except a NASCAR driver is going to run a gas engine like that and have it survive very long. I put 5000 hours on those engines before selling the boat, and they were still running like new.

[OhhWell]

Oh lord.... Here we go.

OP, all other things would not be equal as there would be a very different home on top of each engine.

If all other things WERE equal for some reason, the smart person would buy the 350hp gas engine. HAHAHAHAH

Nice popcorn thread starter by the way...

[rgatijnet1]

tempforce wrote:
one thing they forgot to mention above and in the test...
how long the engines will last and how much fuel each used to perform the test...

the diesel will usually last between 750,000 to 1 million miles before overhaul... the gas engine should last around 150,000 - 200,000 miles...
also the diesel engine will usually get 1/3 to double the mileage of the gas engine, with the same gearing/load ect.

Yep, I'm sure that Motor Trend could have modified the test so that the diesel performed better. Maybe change the gearing, load the gas coach more, etc. Since you brought up mileage, according to the test, the diesel got 18MPG max and the gas engine got 17mpg max. Not a huge difference, especially since diesel fuel costs more The OP asked for a comparison with all variables being equal. The Motor Trend test was pretty equal as to comparing the two different types of engines.

[tempforce]

one thing they forgot to mention above and in the test...
how long the engines will last and how much fuel each used to perform the test...

the diesel will usually last between 750,000 to 1 million miles before overhaul... the gas engine should last around 150,000 - 200,000 miles...
also the diesel engine will usually get 1/3 to double the mileage of the gas engine, with the same gearing/load ect.

[wny_pat1]

There is a sticky at the beginning of the Class A Motorhomees discussion area. Helps answer your question! Choosing Gasoline or Diesel Powered?

In fact, there are two: The power equation...Horsepower- Torque -Gear Ratio- Weight.

My personal preference is the diesel. The diesel engine is more efficient in producing power than the gasoline engine. This is due to the higher compression ratio that a diesel can and does run at. How many gas engines do you see in tractor trailers? One I know of is the 534 Ford. But I don't know one RV manufacturer who puts a 534 Ford in a motorhome. If my motorhome has to have gas, I want a 534 Ford in it.

[mena661]

Ivylog wrote:
I'll bet the Motor Tend test was done close to sea level. I'll let you figure out the rest of a story.

What's to figure out? Oh, so you're saying that they should've made their test even more unscientific by giving one of the engines an advantage?

DougE wrote:
Quarter mile performance is mainly a horsepower and gearing thing. I wish they had done a high gear only hill climb up a long 5% or better grade.

So full throttle with a trailer in tow isn't a load?

[Ivylog]

I'll bet the Motor Tend test was done close to sea level. I'll let you figure out the rest of a story.

[Supercharged]

Stealth1 wrote:
In a word: TORQUE!!

You forgot smell, shake, oil on carpet, and last but not least, smoke.

[the_silverback]

In a few words it is pounds per Horsepower, not Torque. See my earlier posts or goggle acceleration Horsepower.

[Mike_and_Trish]

rgatijnet1 wrote:
The same diesel engine could probably pull 10,000 pounds more than the gasser but the test just shows that all things being EQUAL, the gasser will out perform the diesel

I got a chuckle from this. "Performance" encompasses far more than speed or ET in the 1/4 mile.

My preferred "performance" test would be to load them up EQUALLY with another 10-20,000 lbs or so (getting closer to the type of performance you need in a motor home) and drive them up a long grade in the Rockies, like approaching Eisenhower Tunnel from the west. Let's see which one gets to the top first.