RPM, CID, HP and fuel consumption

I am trying to understand how those four factors relate to each other. Specifically I am looking at two tractors and trying to figure out why one has more HP than the other. I have an MF35 gas which according to tractordata should have 32 on the drawbar and 37 on the PTO. It has 134 CID. I am planning to soon acquire a Farmall H which is only rated at drawbar 24 and belt 26 yet has a 152 CI engine. Is the difference in the RPM the engine runs, in the design of the engine or gearing or is it something else I haven't thought of? Which one would get better fuel economy doing the same job(say, pulling a wagonload of firewood)? I am sure there is an obvious answer to this question, but I am not clever enough to reason it out. Thank you in advance to anyone who responds.
Zach

The same reason my 5.9 cummins turns 305 hp and my 8.1 Deere turns 285. It is in the design of the engine, yes sped has something to do with it, Fuel economy usually has to do how efficent the engine uses the fuel. I guy once told me that he and his friend each had a 20 ft boat, both identical in every way, except his had a chevy 454 and his friends had a 350, both had the same size tank but the guy with the 350 had to fuel up more often because he was using all his available hp. Lets say it takes 45 hp to pull a plow. Now if you have a 90 hp tractor it will use 50%of its hp to do the job. but a 50 hp tractor will use almost all of its hp to do the same job. Do you understand ? I hope some others will chime in and give a more scientific view.
What is the RPMs of both tractors and bore and stroke?

Don't forget Torque...Most "heavy work" engines such as tractors have a longer stroke,run at half the speed of an equal displacement automotive engine and are designed to operate,basically within a fairly narrow"powerband"(RPMs),or working RPM where they generate the most efficient"work" and fuel efficency.
Having trouble explaining what I'm try to say,but kinda...A longstroke engine delivers more torque to the crank,like a long wrench delivers more leverage to a stuck fastener.The lower RPM translates to less cylinder charges of fuel burned,lowering fuel use...then after the "flywheel" TORQUE,up to the breaking strength of driveline components is manipulated within the transmission(lower gear-more torque),higher(road gears)less torque-more speed,AND Torque also figures in at differential gearing,wheel diameter,and somewhat even at drive tire size/weight.....clears as mud?Right?

Like Mike says with the plow and boat scenarios, in those cases the larger engine was more efficient. Years ago, I drove for a trucking company that found that out. An M11 vs N14 Cummins would be running maxed out more of the time than the larger N14.
It's a little different with the Farmall H vs the MF35. The Cummins engines are pretty much the same except for displacement. The H has a lower compression ratio which makes it's engine less thermally efficient. The higher the compression ratio, the better the engine is able to clear out exhaust from the previous combustion event. Exhaust doesn't burn, so getting as much out as possible improves efficiency. This is one of the reasons engines with a turbo or blower make more power. There is always some valve overlap and the turbo or blower is able to help remove some of the exhaust.
Another reason is the MF is newer and has benefited from improved technology. Higher compression, improved intake/exhaust manifold design, improved combustion chamber design, and better carburetion to name a few.

Adding to everything else the other guys said, The MF35 was made betwen '60 abd '65. Farmall H between '39 and '53. Tractor tech had come a long way in that time frame, and being tractors, the focus was on more power-less fuel. Two ways they advanced was in compression ratio and cam shaft design. The Farmall H's comp ratio is about 5.9:1...the MF35 was much higher. The higher the compression of the air/fuel mixture, the more power produced.

The H, with its 38 inch rear tires, will do a better job of putting the horsepower to the ground, so it might be a better tractor for pulling that trailer load of fire wood, but I doubt you will ever notice much difference.

I was curious about how my tractors compared to each other concerning power output. I multiplied the CID and RPM (to give total displacement for a given amount of time) and divided by the HP. All three tractors gave almost the same number. These were two old gasoline models and one newer, but old diesel. The diesel does not have a turbo. If you choose to try this, the lower the answer is, the more the engine is putting out per amount of "breath" it takes.

#1 Engine size matched to work load
#2 Compression ratio
#3 Engine worked at correct RPM for peak torque, work done at peak of torque curve = best eff.
#4 HD or LD advertised horsepower rating

Fuel ecomony with any tractor directly relates to engine size versus the job being done.

For example, and old Ford flathead, gas engine 8N can run a 4 or 5 foot brush hog more
efficiently than a new 150 horsepower turbo-diesel Deere tractor. That because most
tractors that are very efficient when worked hard, are inefficient when used at low power.

One example - running a 5 foot brush hog that might need 15-25 horsepower, The higher
the number, the more fuel efficient. An old Deere B or Ford 8N is almost twice as efficient
as a new 160 horse Deere in this situation:

Deere B - 11.7 eff. test rating
Ford 8N - 11.1 eff. test rating
Deere 7630 turbo-diesel - 6.5 eff. test rating.


An 8N Ford can make 25 horsepower with a rating of 11.l horsepower hours per gallon.

2007 John Deere 7630 turbo-diesel. When worked at 161.8 horsepower it is rated at
18.16. When worked at 92 horse it drops to 14.3. When worked at 62 horse it drops to
12.4. When only worked at 31 horsepower it drops way down to 7.6.

Another factor is how horsepower is stated. Horsepower alone means very little. Horsepower that is sustainable over time means a lot more. With heavy-duty diesels, usually, or at least formerly, horsepower was expressed as a figure an engine could produce all day long, and not in short spurts called "max horsepower."

HD engine makes in the past often expressed horsepower both ways.

The more conservative the power rating, the longer the engine was projected to last doing that kind of work. Very often identical engines would have totally different ratings between HD equipment and LD equipment. Even different yet if installed in electric generator plants that might run for months at one speed.

Horsepower is the product of torque and rpm. So if two engines produce the same horsepower at different rpms, then the one that is turning at the slower rpm is producing more torque at rated horsepower. Folks talk about engines with a "lot of torque", but what they really mean is the torque curve: An engine with a flat torque curve will seem to have better pulling capability than one with a peaky curve, even if they're rated at the same horsepower. The reason is the "torquey" engine will continue to pull as the load increases, but at a slower speed, while the "peaky" engine will lose power and have to shift to a lower gear.

Fuel consumption is usually defined as "specific fuel consumption", which is typically described in units of "pounds per horsepower-hour". Two similar engines will have the same specific fuel consumption, even though their displacement is different. If the efficiency of an engine is increased, typically by increasing its compression ratio, its specific fuel consumption will decrease. So if you have two otherwise identical engines, the one with the higher compression ratio will have lower fuel consumption.

Another factor in fuel consumption is engine load. Gasoline engines have greatest efficiency at wide-open throttle, because no power is lost pumping air past the throttle plate. If an engine is lightly loaded, it will typically use more fuel than a smaller engine doing the same task.

In the case of the two tractors you're comparing, I'll bet the H has a much lower compression ratio than the 35. Of course, someone may have installed high-compression pistons in the H, which would make it more powerful and more efficient.

Thank you all very much. That is a lot of information and I have read it all twice, but it'll probably take me a while to get my head around it all. Maybe I should do a non-scientific experiment and see which one seems to use less gas for particular jobs.
Zach

If you read the full versions of Nebraska Tests, and not the abbreviated tests that are often published instead, it gives you all that info.

You can look at any tractor tested and it will show you power and fuel efficiency at every load.

If you buy the big Nebraska Tests book, it will NOT show you the full versions.

This shows where the Farmall H is more efficient then the MF35 at certain loads. All from the full Nebraska Tests.

Farmall H can make 26 horse with an efficiency number of 11.59. MF35 does it at 10.8.


Test # 333 Farmall H gasoline 1939

26.2 horsepower,1652 RPM, 2.26 gallons per hour, Eff. 11.59. 100% load

23.8 horsepower, 1652 RPM, 2.08 gallons per hour, Eff. 11.4

17.9 horsepower, 1652 RPM, 1.76 gallons per hour, Eff. 10.2

14.2 horsepower, 1652 RPM, 1.56 gallons per hour, Eff. 9.06.

6.1 horsepower, 1652 RPM, 1.15 gallons per hour, Eff. 5.32.

1.3 horsepower, 1652 RPM, .93 gallons per hour, Eff. 4.42

MF35 (Ferguson 40) test 596 in 1956 same 134 Continental Red Seal engine

32.8 horsepwer, 2000 RPM, 3.09 gallons per hour, Eff. 10.6. 100% load

29.3 horsepwer, 2000 RPM, 2.66 gallons per hour, Eff. 11

17.5 horsepower, 2000 RPM, 2.04 gallons per hour, Eff. 8.5

15 horsepower, 2000 RPM, 1.94 gallons per hour, Eff. 7.7

7.6 horsepower, 2000 RPM, 1.5 gallons per hour, Eff. 5.0

1.4 horsepower, 2000 RPM, 1.18 gallons per hour, Eff. 1.2

HP is a function of how much air-fuel mixture the engine will pump through it per hour.
Take the 350 Chev for example with small high velocity ports, 76cc combustion chambers and a short duration cam. It will make lots of low rpm torque but mid range and peak HP will be limited.
Take the same 350 shortblock with a set of Edelbrock RPM heads with 64 cc chambers and a high lift/long duration cam. Peak torque may be even equal to or even greater than the previous low speed engine's version. How ever the peak torque will occur at a higher rpm. Of course max HP will be greater and occur at higher rpms.
Fuel consumption per HP per hr of the engines could be identical. Every body thinks a 400HP engine uses a lot more fuel than a 200HP engine.Well yes it does but everybody forgets the 400HP engine is performing twice the work per hour.

Rough comparison of hp to rpm- ignoring engine size and compression- note the 1652 RPM of H compared to 2000 RPM of MF35: H is 80% speed of MF35 so 80% of HP would be proportinate. 26 hp compared to 32/33 hp seems about right rough percentage. 26/33 X 3=78/99= close enough to 80% for our discussion. RN

Thank you jdemaris, that makes a lot of sense. I didn't think of looking at the F40, and the only MF35 listed that I could find was diesel in 1960. I don't know how many HP I am calling for when pulling wagons, but it looks like the Farmall might be the better way to go, and it will be handier for firewood anyway since the Massey has a loader on it that makes it harder to steer and requires more room to turn.
Zach

It's kind of amazing that the Farmall is almost four times more fuel effiecent at a very low load, as compared to the MF35 or Ferg 40.
And, a single-cylinder flathead "lawnmower" engine beats them both. But, the Farmall is close.

It goes to show what load-matching is all about.

A Briggs & Statton engine was tested at Nebraska at 2.19 horsepower and had an efficiency rating of 6.7 Eff. (horsepower hours per gallon).

The Farmall made 1.3 horse at 4.4 Eff.
The MF35 made 1.4 at 1.2 Eff.

What gets folks confused is only looking at "horsepower" and not torque- horsepower is just a calculated number, based on torque (the actual force that moves the load) and RPM. "Horsepower", being a calculated number, can be manipulated- these days with the new "horsepower" quotas and regs, many engines can be easily made to fit under the 50hp threshold- just turn down the governor to a lower RPM. Same engine, torques just as much, but HP is less. We sold all of our I-R air compressors a couple years ago, and bought new under 50hp compliant rigs- they even say "49hp" on the back in big letters. Look at a D8- enormous torque, but very low RPM- so "horsepower" is only about 300- like that little small block Chebbie.
Take the old 350 Cummins, and the derated 315- pretty much the same engine, but turned down to 1800 RPM- so "horsepower" calculates to less
John Deere even has a section on their website that helps you derate their engines just by adjusting the RPM

Hey! Hook that lawnmower to a two bottom plow and see how they compare.

On the same tractor chassis,the lawnmower engine would be the most efficient engine of the three.

Yeah, but I'm talking about fuel efficiency, not total power. These sorts of tests show why a log splitter with a 5 horse engine is a heck of lot more efficient than a farm tractor hooked to a splitter doing the same job.

The little low-tech, low compression, flathead Briggs is more efficient because it is just the right size for the job it was doing as tested.

It also why some farmers like to have small and large tractors around, and size-match for the work.

That is a very interesting fact. It seems like it is only on the extreme bottom of the power range that such a large difference between the tractors occurs. I am trying to imagine an application for either tractor that would only call for 1.3 or 1.4. Would that be what a hydraulic pump might require? This is getting complicated, and there are a lot more variables here than I had thought.
Zach