Why is it okay to rev a diesel vs. gas?

[JChott]

A guy at work runs a Suburban with a gas 6. 0L and we were discussing why a 300hp diesel will crush his 300hp gasser towing here in the AZ mountains. (Proven by another co-worker who just bought a Duramax GMC, not to mention much better mileage) I mentioned that I would be willing to run a diesel at or near the hp peak but that I would never do that to a gas engine. He asked why and I didn't have a real good answer other than his hp peak is near 5000rpm and mine is at 2900rpm. Is is really okay to operate the Cummins at hp peak for long periods of time? I'm pretty sure running a gas engine at hp peak will result in cast iron pieces on the ground if you do it long enough. Any opinions or facts?



Thanks,

Jim

 

[Hohn]

It's mostly to do with the fact that diesels rev lower (less strain), they are built MUCH stronger (heavy crank, pistons, etc), and most are designed for heavy load/commercial use.



All the 3 pickup diesels are used in much heavier trucks. VERY few gas engines can say the same. When's the last time you saw an F-650 with a 6. 8L V-10?



Gas engines CAN be built extra strong so they can run at 4000 rpm all day, but in that kind of usage, diesels rule the market. When did you last see a gas-powered Class 8 truck?



jlh

 

[catoiler]

You gotta remeber that we have the big advantage in torque!Just because they both put out 300hp doesen't mean much. Torque makes you move and the HP keeps you moving,so the diesel will not need to use all the hp. to move the same load. How much throttle does it take in a gasser to up your speed 10mph at fwy speed vs a diesel.

 

[Big_Daddy_T]

Diesels can run at peak hp under full load for 10xs longer than a comparable gasser. A lot of it is the revs. A gasser would be expected to live for about 200 hours under those conditions whereas a diesel would easily live 2000 hours. Diesels are just heavier duty. They are designed to work hard.

 

[bmoeller]

You gotta remeber that we have the big advantage in torque!Just because they both put out 300hp doesen't mean much. Torque makes you move and the HP keeps you moving

A gasser putting out 300hp, is probably only making 350 ft/lbs of torque, if you're lucky. Assuming we're not talking about a 8. 0L Dodge and maybe the Vortec 8. 1L. If its in a car, it is usually has LESS torque than it has horsepower.



An diesel, the torque is generally double, or more than, the horsepower. A in-line engine also makes more torque than a V8.

 

[cwarner]

I've held my 670hp 540ci gasser at 5800rpms for around 20 miles before !

So far it's still together.

I've always heard Diesels were designed to operate at peak hp for extended periods of time. That is why a lot of heavy equipment use a de-accellarator .

 

[Forrest Nearing]

horsepower doesn't keep you moving... it moves you...



torque = work



horsepower is a function of what RPM you make your torque...



horsepower wins drag races, torque pulls trailers



Forrest

 

[DPelletier]

- 1) lower rpm's



- 2) built stronger (more so in our case than the others)



- 3) Cummin's performs extensive full rpm and full load testing for simulated thousands of hours and millions of miles. I'm not aware of any gas engine that is tested to this level.



The main reason is RPM's. Ours can run at peak HP at half the RPM as a comparible gas engine. Less RPM = less friction = less wear and heat.



Dave

 

[ndurbin]

Diesels can run alot longer than 2000 hours under full load. I have seen more tractors than I care to count with 10,000+ hours w/o a major rebuild and I've seen alotta OTR trucks with 1 million+ miles w/o a major rebuild too.



Also another thing to think about is the average oil capacity of a gasser. The 5. 9 has double the capacity of most gassers. And a semi/tractor has 9-10x the capacity of a gasser.



Nathan

 

[MCummings]

Duty rating.....



Gassers were only designed to operate at 100% power for only about 25% of it's life.



Diesels were designed to operate at 100% power for 75% of it's life.



Gassers run with vacuum, so cylinder pressures are low, and usually on a vacuum.



Diesels run with a full intake of air every intake stroke... and with extra cylinder pressure (from boost) when you are accelerating.



Gassers have to deal with high vacuum, and pressure when running, while diesel's operate with a positive cylinder pressure all the time.



Having to deal with high cylinder pressures anytime the engine is running, the design of the motor has to be extremely heavy duty.



Merrick



EDIT: Would this fall under the category of, "If you can't Dazzle them with brilliance, Baffle them with bull honky" ?

 

[Deezul1]

I know Diesel rule when it comes to peak power operation for long periods but I was amazed some years ago when a friend took me out in his ski boat with a 460 Ford engine and we ran 5000 RPM all day long and he said that where he runs it all the time. I would never have done that with my F250 Ford truck 460 for fear that it would turn into scrap metal in 1/2 hour.

 

[mattymac]

I've held my 670hp 540ci gasser at 5800rpms for around 20 miles before !

So far it's still together.

That motor isnt your everyday run of the mill motor and its obviosly built to do something like this. A nascar engine can run at 9500-9900 Rpms at full load for hours on end, because they are built to do such tasks. (Except for the Rouch motors :-laf ) Also my snowmobile motor can turn 13,000 rpms and make almost 300 hp does that mean it can tow as much as a 300 hp 5. 9 cummins?

 

[Joe G.]

I'm not so sure that it is the fuel that makes the difference. A Hall-Scott gasoline burning truck engine from the 40s and 50s would run with the diesels. It was as big and heavy as a diesel. The last time I was at the auto museum in Reno there was one of them just inside the front door. I was kind of surprised since I had not seen one in years. I think that fuel costs finally did them in.

 

[Crunch]

That motor isnt your everyday run of the mill motor and its obviosly built to do something like this. A nascar engine can run at 9500-9900 Rpms at full load for hours on end, because they are built to do such tasks.

Aren't NASCAR engines torn down and rebuilt after every race anyway? I think there are a lot of engines that can be pushed to a high HP or performance configuration, but who wants to tear it down or change the oil every few days? And speaking of NASCAR, does anyone believe we might one day see the return of real stock car racing? NASCAR's OK for what it is, but to me it ain't real racing when they install equipment that limits the top speed and forces the drivers into unnatural driving strategies. Man, I miss AJ, Richard, Bobby and Al (not Jr. though), and especially their cars...

 

[mattymac]

Yes I was just making a point about how if engines are built for what they are intended to do then they usually can do it. As for the NASCAR engines yes they get tore down or simply replaced about every race and they are built for what they are intended to do, but also engines are cheap in that sport. As for the real NASCAR drivers and their cars I miss them just as much as you do (even though Im only 20) all my family is crazy about this sport but just seems now a days that it just sucks. Dont miss the old #3 though.

 

[WDaniels]

1. The diesels in the big 3 automakers in the US, the Ford and the Duramax are cataorigzed as light duty diesels. The Cummins is considered a medium heavy to heavy duty diesel. This is because the Cummins was designed for other applications and adapted to the truck. The Ford and Chevy were designed strictly for the truck.

2. The best way to understand torque and H/P is, torque is how much work the engine will do, horsepower is how fast it will do it.

hope this helps.

wd

 

[y-knot]

It is not a problem at all to run a diesel at it's max rpms for long durations of time. Ask any equipment opperator how they run them.

I have run alot of equipment and several were powered by the Cummins 4 model, they run just fine at max rpm's and will do it day in and out. One of the hardest pieces there is on engines has to be the big trenchers. They opperate ate full throtle and max power all day, constantly live in a very high dust enviroment, and offten run into obsticles that stall the engine often while at max power. I have used mainly Vermeer products, they produce a very fine line of heavy trenchers, several were tall enought to have there own elevator to lift you to the opperators area. These trenchers break many things, but I have never seen one break an engine, or really have engine related problems. All their HD service trucks are powered by the Cummins 5. 9L engine, and the service thecs I talked to all said the Cummins is bullit proof, and one item they very seldom had to worry about. I know how much Dodge owners worry about their own Cummins, but by construction standards out diesel engines live a very sheltered and pampered life. It took me a while to get use to the constant sound of killing a diesel that has been at full throtle and max power, the engine has a odd sound when it dies like that and the turbo is still wound up. But they just start them again with full throtle, and keep going. If you are digging in city areas were they have burried street car tracks or train tracks, the going gets very interresting. You can digg right thru a solid 6 foot thick concrete wall with re-bar, but those solid steel train rails are killers.

 

[JChott]

Thanks for all the inputs. I'll happily put the pedal to the metal and blast up these hills.



The other thing we have been discussing was a physics book talking about horsepower and hill climbing. The book contends that it's pure horsepower that pulls you up hills. (I know this is an old argument that everyone's heard before, hp vs. torque. ) So to try to understand it better, we wrote an Excel spreadsheet trying to prove that out and I tend to agree. At initial startup, big torque seems like the only way to get a load moving, but after you're rolling uphill, it really seems to be just horsepower. Horsepower is just this: hp=torque*rpm/5252 for any engine. For the 600, 590ft-lb*2900rpm/5252=325hp. Big torque makes much more horsepower at lower rpms and seems to be why diesels pull hills so well - they make lots of hp at the low rpms. I plugged in some numbers for trucks I have owned and compared to how they used to pull hills with various loads, and it seems to match pretty well. The guy with the Suburban can only pull his boat up a 6% grade by us in second gear at 55-60mph with the engine screaming, and the spreadsheet matches that. It also shows that my 04 should be able to pull the same hill in 6th gear at 70mph at 2000rpm, because the Cummins is making the same hp at 2000rpm as his gasser does at 4000rpm. We plan to hook up his boat later this summer to test it out. If anyone wants to try the spreadsheet out, email me and I can send it to you.



Jim

 

[klenger]

JChott:



I think you have nailed the HP vs touque issue dead on. HP is what actually gets work done and doesn't matter if it comes from a diesel, gas engine, electric engine, or steam engine.

 

[NIsaacs]

Klenger!!, You forgot the Horse!