Shopping for Horsepower

What Horsepower Ratings Mean to Work on Your Farm

(Progressive Farmer photo by Jim Patrico)

If you are planning to buy a new tractor and ask about horsepower (hp), you might want to rely on your experiences buying the right cartridge for your printer, a battery that fits your phone or a replacement razor blade that works in your razor.


Like any of the above experiences, finding the exact tractor to fit your needs may take several visits to a dealership or dealerships. Tractors of all colors bring with them a multitude of numbers -- some affect your work, others affect your checkbook. Sooner or later, the discussions will roll around to horsepower.

After all the diesel (and sales) smoke settles around all of the horsepower ratings, which rating really means something to you on your farm? How can one tractor have so many different horsepower ratings? As a columnist for The Progressive Farmer, the question that new smaller tractor owners ask me the most is, "Why won't my new 24-hp tractor (pick a brand) pull my plow like my dad's old 1945 24-hp tractor?"

With that question on the mind of so many readers, let's take a look at horsepower ratings and what they really mean to work on your farm.

First, let's compare an old tractor to a new tractor of comparable drawbar horsepower ratings to verify their "dusty" specifications. A Farmall H was a popular two-row tractor with a tested drawbar horsepower of 24.17 ( and is known to pull a drag-breaking plow with two 14-inch bottoms through the field at around 2 mph at a depth of around 7 inches.


However, readers tell me when they hook a newer tractor that also has been tested at 24 drawbar hp ( to the plow in the ground, it won't even move the plow. So, how can the old and new tractors have similar drawbar horsepower but perform so differently in a field-test?

Maybe a little history of the many definitions of horsepower and how they are calculated will help you understand what horsepower means.

Through the history of the tractor (originally called steam plows or traction engines), there is a common thread running through it all. Tractors got lighter, and they operated more efficiently. For example, the huge monster steam tractors that weighed several tons would require as much as 3,000 gallons of water every day, not to mention the wood, straw or coal needed to boil this water. Nowadays, tractors can run several hours on a gallon of diesel, and they only weigh a few thousand pounds.

But the weight-to-horsepower ratio of tractors sold today is the opposite of the original tractors used for pulling huge plows. It's common today to find a tractor rated at even 100 engine hp that will not think about pulling what the 15-hp steam tractor would pull in a tug of war.

Mainly, this "dusty math with horsepower" is possible because of the many different horsepower ratings. Where did the word horsepower come from anyway?


In the late 18th century, an engineer by the name of James Watt came up with a measure of power for his steam engine. At this time, draft horses were the most common source of power, so to help farmers figure out what steam engine power they needed to do the same job as the number of horses they were using on the farm, he came up with the name "horsepower."

Watt's field research showed that an average horse could pull 330 pounds a distance of 100 feet in 1 minute. With these numbers, he multiplied 330 by 100 and divided that number by 1 and found that pulling 33,000 pounds in 1 minute equals 1 hp.

But with this calculation in mind, can we really believe that a 24-drawbar-hp tractor will pull the same plow at the same speed as a team of 24 horses? So, what is there to believe about published horsepower ratings of tractors?

After Watt's definition of horsepower, several different categories of horsepower came into being. The two main categories are theoretical horsepower (sometimes called indicated) and useful (what is available at the end of the crankshaft) horsepower.

Theoretical horsepower is really what the engine's horsepower looks like on paper. In other words, in theory, what kind of horsepower will this engine produce before any deduction for friction and any other drain on the engine. But all of us in the tractor seat are about useful, or net horsepower, in a tractor. That is the power we have at the end of the crankshaft after all the drains (especially friction) on the engine are subtracted.


And there are more. Besides indicated horsepower, there are several other horsepower ratings.

-- Friction Horsepower. This is the difference in theoretical horsepower and usable horsepower. It's calculated by adding up the friction among engine parts, such as the crankshaft and bearings, piston rings and cylinder walls, and even the power to drive the piston up on the compression stroke.

-- Flywheel Horsepower. Sometimes called brake horsepower, this is power that's left after the friction horsepower and other engine horsepower drains are subtracted from the theoretical horsepower. It's measured by an engine brake that applies a load onto the engine, and it is measured in pounds. If the theoretical engine horsepower of an engine is 100 hp, and the friction loss is 20 hp, then an engine with a theoretical horsepower of 100 is now 80 hp. That is before all the other drains on the engine (alternator, fan, flywheel, water pump, etc.) come into play, which is a minimum of 8 more hp. So, what you have is an engine with a theoretical (indicated) horsepower of 100 that it is now down to 72. And that's before additional drains that come from powering anything else, such as the power-take-off (pto), hydraulic pumps, front-wheel drive, AC compressor and other functions, are subtracted.

-- Power-Take-Off Horsepower. This type of horsepower is calculated by torque and speed, and is measured at the tractor's pto. The gear reduction in the tractor increases the torque value but reduces the speed of the shaft. When measuring produced pto horsepower, the pto speed is held at a constant so the horsepower reading will directly measure torque using a scale that translates the results to horsepower. This reading is the actual horsepower available to do work.

-- Rated Horsepower. This is the rating that an engine manufacturer gives an engine as a way of defining optimum operating horsepower and engine revolutions per minute (rpm). This is how the engine should be set up for maximum life and reliability. You might hear words like "power boost" and "power bulge" thrown at you with this rating.

-- Drawbar Horsepower. This is the measure of the tractor engine's pulling power that it can produce (not will produce) when mounted in the tractor.

There is a difference in what it can produce and will produce. "Will produce" is actually pulling that two-bottom plow. But "can produce" is pulling the two-bottom plow but only if the tractor is equipped to deliver the horsepower to the ground.

Don't forget, the definition of horsepower (no matter what kind of horsepower) is moving 330 pounds 100 feet in 1 minute. It's easy to believe that drawbar horsepower is calculated by what the tractor will pull when tied to a spring scale or a sled. However, that is not how drawbar horsepower is calculated. The formula for drawbar horsepower doesn't allow for this straight pull test. The closest thing to a true drawbar pull test is a huge scale or a tractor-pulling contest.

So when we look back at the original question as to why your dad's old 24-pto-hp Farmall H will embarrass a new tractor in a straight pulling contest and pull the same plow faster and deeper than all of the new 24-hp colors, it's time to look at the two tractors' specifications.


It's easy to see the new tractor is rated to pull the same as the H, but the engine has to be packaged around a tractor built like the H.

The way I try to understand drawbar horsepower is to compare it to the amps a battery can deliver. A battery with 600 cranking amps can start a diesel engine, but only if the battery cable is large enough to carry the required amp.

So, when you go shopping for a new tractor, you now have all of the information on horsepower you need to ask the right questions.

Since most requirements for small tractors depend on pto horsepower to do the work (mowing, tilling, post-hole digging, etc.), that number is very important. Don't buy engine horsepower thinking you are buying pto or drawbar horsepower. The tractor sales associate can talk about engine horsepower, but the pto horsepower does the real work.