Gas vs Diesel Hp vs Torque

[Farmwithjunk]

Ok if horsepower is the same thing no matter whether diesel or what then why is it that my 15 horsepower b6100 kubota diesel tractor with a 60 inch mower deck will cut through all kinds of high thick grass with no problems. My 20 horsepower riding mower with a 36 inch deck will bog down and die in grass half as tall or half as thick. both of them were 6 speed and I believe the top speeds in 6th gear was not that much different. My gass mower had only to run a mower deck that was almost half the size. Could it be that my diesel kubota has more torque than the gas riding lawn mower and since torgue is what actually turns things that might be more important than the horsepower rating.

For beginners, a lawn mower engine and a diesel in a tractor are "rated" in a very different manner. Most "lawn mower engines" (That's what we'll call 1 and 2 cylinder "cookie cutter" utility engines for now) are rated without any load, no accessories, oil NOT normally used in such an engine, running wide open (sometimes WAY over normal RPMs) and under a very controlled environment. Several well known manufacturers use a "math formula" to determine their rating in place of an actual dyno test. Their findings are subject to liberal interpretation. With engines installed in tractors or trucks, generally they're tested and rated in a simulated working configuration, (on a dyno) and NOT stripped down, no accessories, oil being pumped and cooled externally, and without a load. A 20 HP Briggs & Stratton won't put 20 HP to the ground in any event. My Deere lawn mower has a Kawasaki 17 hp single cylinder engine. Old 9N's had about the same rated hp. That 9N would drag 4 or 5 of my lawn mowers backwards in a back to back "pull-off", weight being equalized somehow.

Back in "the old days", tractors were generally compared amongst farmers and typical end users with ONE NUMBER.... PTO HP. While engine hp has some use, it doesn't represent what your tractor is capable of doing "on the job". I'm of the belief that engine HP numbers are popular ESPECIALLY on lower hp units just because it makes people think they have a bigger tractor than what they do have. In the end, you've got what you've got. CLAIMING higher HP numbers won't make it any more powerful. You don't take just an engine out to do a days work. What matters is PTO HP or DRAWBAR HP. And those numbers aren't always as accurate as the engine builders would lead us to believe.

I'm still not impressed with all the numbers and "facts" that are used for comparison sake on todays tractors. They're virtually meaninless since no 2 manufacturers use the same "standard" of comparison. Seems a lot of folks just eat up this info. Without a standardized testing, anyone can claim ANYTHING.

Gas vs. diesel.... Diesel is better? At what. Gas is terrible? again, at what? We have a couple pick ups at work that are for running errends. One is a diesel (Dodge) One is a gasser (Dodge) Both are 2003 models. Both have around 90,000 miles on them. They make several short runs a day, never allowing it to warm up before hitting the road, seldom running long enough to sustain operating temp for more than a few minutes. The diesel is ragged out, and in need of work. The gasser is still in great shape. There's one case where the gas engine is far superior.

Without a doubt, a diesel is a better choice for sustained pulling or hard work. Gassers are better for short hauls. (Both come in a chassis that won't go 1,000,000 miles without extraordinary maintenance and repairs anyway) Gas is WAY cheaper here as compared to diesel. There are circumstances where a gas engine is superior or at least equal to a diesel in over-all performance/economy/longevity.

The "numbers game" manufacturers have everyone so concerned about is "smoke and mirrors". The ONLY real accurate comparison is side-by-side testing while doing an actual job. Obviously, by the fact that tractor manufacturers don't offer us an independent testing on the smaller rigs they sell, (like the Nebraska Test of bigger AG tractors) we can only assume they don't want us to know the REAL facts.

And making claims that "engine A" is better than "engine B" because one is diesel and the other is gas, or making the same sort of claim because on engine builder claims his 6-cylinder has 50 more HP than their rivals engine is pure folly. It's not that cut and dried.

It's all about how they actually PERFORM, and NOT how they are hyped by the folks trying to sell you one.

And keep in mind that no 2 people use their truck/tractor/lawn mower/ect in the same manner. What's best for YOU, might not be anything at all like what I need.

 

[john_bud]

Goodness sake John, don't believe everything I say. Sure hope you didn't bang the finger and have it fall off due to brittle failure.

Up here it's so darned cold that when I went to start the fire the match flame froze!

A diesel with torque: Image attached.

Egon,
Well, I didn't want to say anything, but it did! Fortunately - being so cold- I was able to pick up the pieces and glue it back on good as new.

So if I understand what you said then the torgue increases in my toyota the slower the engine is running. That means then at 100 rpm I have more torque out of my kubota than I have at 1000 rpm. I dont understand how that could be. Seems to me that I would have more torque the faster my engine turns. Theoretically if I could get my engine to run at 1rpm it would have the most torque it will ever have ?

Don't be silly. Maximum torque occurs at the point of greatest volumentric effiency. As hp is a calculated value, and the calculation multiplies the torque by the RPMs, it just works out that the torque peak is 99.999% of the time at a lower rpm than the hp peak. Your toy ota probably has a peak torque at 3000 rpm and peak hp at 4800 or so. Maybe higher.

I have to assume that you are just making silly comments and not asking a serious question, as those rpms are outside of the operational windows of all internal combustion engines I am familar with.


jb

 

[Dougd]

I have an air-powered impact wrench that is rated at 900 ft/lbs of "nut-bustin'" torque. Weighs about 8 pounds and runs on 90 PSI of air. More torque than a built-up 426 HEMI can produce at 7,500 RPM.
Is the torque of an impact wrench measured differently than that of the HEMI?

 

[john_bud]

I have an air-powered impact wrench that is rated at 900 ft/lbs of "nut-bustin'" torque. Weighs about 8 pounds and runs on 90 PSI of air. More torque than a built-up 426 HEMI can produce at 7,500 RPM.
Is the torque of an impact wrench measured differently than that of the HEMI?

I'm not entirely sure, but I think the nut busting torque is the maximum momentary load it can put out. The shock impulse you feel with each whack!

You can try and make an adapter to the input shaft of your Hemi's transmission to connect your Nut Buster. Take out the Hemi and put in a 10 hp briggs powered air compressor. Yep, I'll be you can run a 7 second quarter with that rig!

Uh ,er .... or not.

jb (Lame attempt at humor....)

 

[gemini5362]

Egon,
Well, I didn't want to say anything, but it did! Fortunately - being so cold- I was able to pick up the pieces and glue it back on good as new.






Don't be silly. Maximum torque occurs at the point of greatest volumentric effiency. As hp is a calculated value, and the calculation multiplies the torque by the RPMs, it just works out that the torque peak is 99.999% of the time at a lower rpm than the hp peak. Your toy ota probably has a peak torque at 3000 rpm and peak hp at 4800 or so. Maybe higher.

I have to assume that you are just making silly comments and not asking a serious question, as those rpms are outside of the operational windows of all internal combustion engines I am familar with.


jb

Actually that was a silly comment although I apologized I typed toyota when I meant to type kubota. I was talking about my kubota tractor. The point I was trying to make was that you said torque goes down as RPMs go up. I believe that is going to be a case of maximum torque is available at an engine at different RPMs for different engines. In the case of diesels maximum torque is found at lower RPMs I do have one serious question. The original poster talked about the new GMC gas pickups that have 375 horse power and I believe it was around 300 foot pounds of torque. My duramax diesel has 360 horsepower (pretty close to the same) but it has 650 foot pounds of torque. Now if you got the same gearing would the gas engine be able to pull as much as the diesel. For instance if I am pulling a 33 foot fifth wheel would the gas be able to pull it up a mountain just as well as the diesel.

 

[daTeacha]

Engine designers can make the same block deliver a wide variety of different torque curves and horsepower curves. Within limits, they can deliver your torque in a sharply peaked curve or over a wide plateau with a somewhat different maximum value by changing pistons, cranks and cam profiles. The same with horsepower.

When you spec out that new truck, the dealer should be happy to show you the torque/hp curves for the various engines available in the truck you want. You just need to decide which applications are the most important to you -- hauling two loaded hay wagons out of field or cruising down the highway on the way to work. Pick the engine best suited for what you do most.

Another example -- Audi has a new engine in the S8 series, a $100K plus luxo sedan. The engine is a V-10, the same basic engine used in the Lamborghini Gallardo, a high end Italian exotic sports car. The S8 is intended for autobahn cruising. The engine in the Audi delivers a wide plateau of torque at a lower rpm and with less peak horsepower than it does in the Lambo. You could buy them both and swap engines, but the cars would no longer serve their intended purpose as well as they do now.

With all this debate, you might be interested to know that I once owned a sports car -- a Triumph TR-4A (pre IRS for you sports car gurus) -- that had the same engine as a neighbors tractor. I found that out when I went to see this guy about buying a 9' wide spring tooth harrow for my 8N Ford. He was a mechanical type and heard my car (straight pipes) as I drove in, thinking it sounded familiar to him. We popped the hood and he asked me where I could get parts for the TR. I told him and he went to the Triumph dealer and got cylinder liners (if I remember right) for his tractor and put it back to work.

For first prize (3 days of nice weather in a row), what tractor was it that provided the engine design which served so well in the British sports cars? The TRs, by the way, were known for having lots of low end torque compared to other cars of the same vintage.

 

[john_bud]

Actually that was a silly comment although I apologized I typed toyota when I meant to type kubota. I was talking about my kubota tractor. The point I was trying to make was that you said torque goes down as RPMs go up. I believe that is going to be a case of maximum torque is available at an engine at different RPMs for different engines. In the case of diesels maximum torque is found at lower RPMs I do have one serious question. The original poster talked about the new GMC gas pickups that have 375 horse power and I believe it was around 300 foot pounds of torque. My duramax diesel has 360 horsepower (pretty close to the same) but it has 650 foot pounds of torque. Now if you got the same gearing would the gas engine be able to pull as much as the diesel. For instance if I am pulling a 33 foot fifth wheel would the gas be able to pull it up a mountain just as well as the diesel.

Past the torque peak, torque will drop as rpm's (and hp) raise. That's true.


As all answers go about this topic, it's yes ---- and no. It's yes if you can get optimum gearing and keep the gas engine in the exactly best rpm. It's no, if talking a real situation and you want to stay in OD or D and still go over the hill. The diesel having so much more torque, will be able to stay in OD (Allison 5 sp or 6 sp) and the gas engine will have to drop a gear or two or three to get to it's power band as the load increases.

{on edit - If you put a continuously variable transmission CVT in both vehicles and set them to the exact HP peak for each engine, they would go up the hill nearly identically. However, the duramax would be at about 3000 rpm and the gas would be at 6000 rpm. Both would be floor boarded assuming a max effort at max hp. }

It's also subjective on how you define "just as well". Same speed at the top? Same time bottom to top? Keeping it in OD the whole way or at most dropping to drive and not 2nd? Fuel economy? I think it has been shown that the Chev 8.1L was able to pull the same loads up the same hills as a duramax/allison, but it had to drop more gears. Some would say that was just as well. Some would not. It's subjective.

I know I traded off a 5.3L chev 1/2 for a 6.6 duramax. The 5.3l got the trailer where it needed to go, but it was burning itself up doing it. Had to be floorboarded on every onramp, etc. My guess would be that a dedicated 5th wheel person wouldn't be as happy with the gas engine as the duramax. But a person with occasional towing would probably be thrilled. Again, totally subjective.

signing off...
jb

 

[indianaEPH]

Past the torque peak, torque will drop as rpm's (and hp) raise. That's true.

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As all answers go about this topic, it's yes ---- and no. It's yes if you can Again, totally subjective.

signing off...
jb

I've got to say that john-bud is all over this question. He is right on.

No one uses an engine without a transmission on a vehicle (not usually....don't bother if your going to say you had a direct drive vehicle at one point.....I believe you). The transmission match and capability, as well as the engines torque curve, have a lot to do with the perceived performance. Theoretically, a really good transmission (such as a CVT) could eliminate the effect the lower torque rise that a gas engine typically has over the diesel engine. However, this theoretical transmission does not exist. It's hard to make a high torque CVT with good efficency and durability and cost. Better to get a good diesel with Allison 6 speed (7?).

 

[JDFANATIC]

While I'm in no way thinking or intending to get into a technical discussion with some of you that obviously know your mechanics and in some cases physics, I can add some things from my anecdotal learnings.

I used to own a JD425. 20 hp Kawaski twin with a 54" deck. If you tried to engage the deck at idle, all you would get is a stalled motor. (this same analogy also works with the current top-of-the-line JD X7xx gas vs. diesel). I now have a JD2210. 23 hp Yanmar diesel and a 62" deck. You can engage the deck at idle with no protestations from the engine. From my experience, this same power advantage makes itself known across the power bandwith (idle to full-speed-ahead). So, when I head off into tall wet grass, I want that diesel powerplant. When I till that section of earth, I want that diesel powerplant. When I move that object up that hill and over that piece of real estate . . . and nothing but net . . . then I want that diesel powerplant. Maintainance is less. Fuel gph is vastly less. Expected life expectancy is hugely better. What, other than the extra chin yang to afford the diesel, is there not to like?

 

[SkyPup]

I have a high performance one cylinder water cooled two-stroke racing go-kart that puts out 40+HP @ 17,000rpm and does 115 MPH.

My question is, should I remove the three cylinder diesel engine from my Kubota L-39 and install the go-kart engine to pick up a few more horsepower and more top end speed?