Car enthusiasts and everyday drivers alike often find themselves in discussions comparing vehicle performance. The terms “horsepower” and “torque” frequently emerge, sometimes leading to confusion and misconceptions. You’ve likely heard the saying, “Horsepower sells cars, but torque wins races.” But is this age-old adage truly accurate, or is it a simplification that overlooks the complete picture of engine power?
This article aims to clarify the relationship between torque and horsepower in a way that’s easy to understand, even without a deep engineering background. We’ll explore how these concepts work together to define a vehicle’s performance and debunk some common myths surrounding them.
Understanding the Basics: Beyond Simple Numbers
In the world of automobiles, comparing specifications is commonplace. From casual chats about which car is “better” to intense online debates, horsepower and torque figures are often at the center.
“Your car only has 90 horsepower?”
“Yes, but it has 220 Nm of torque, so it’s quicker than you think.”
“My SUV boasts 103 horsepower.”
“Oh, but just 134 Nm of torque? That seems low. Can it even handle a full load of passengers uphill?”
“My BMW has 250 horsepower and 350 Nm of torque.”
“Well, my Mercedes is a torque monster with 440 Nm, so the horsepower at 190 is less important.”
These kinds of exchanges highlight the common, yet often misunderstood, importance placed on torque and horsepower. You’ve likely encountered the horsepower equation:
Horsepower = (Torque x RPM) / 5252 (when torque is in lb-ft)
OR
Horsepower = (Torque x RPM) / 7127 (when torque is in Newton meters – Nm)
Equations can seem daunting, turning discussions into a numbers game. But let’s shift perspective and visualize these concepts using a more relatable analogy: land area.
Imagine comparing plots of land. The value and usability of land are determined by its area. A larger area generally means more space for building and greater value, location depending, of course.
For simplicity, picture all land plots as rectangles.
Area = Length x Width
Image showing different rectangles to illustrate area, length and width
Length and width can vary. Sometimes a plot is long and narrow, sometimes short and wide, and sometimes it’s a perfect square.
Now, consider someone arguing that only the “length” of the land matters, dismissing “area” as irrelevant. They might compare two plots: Plot 1 (60m length, 25m width) and Plot 2 (50m length, 27m width), claiming Plot 1 is superior solely due to its greater length.
Area of Plot 1 = 60m x 25m = 1500 square meters
Area of Plot 2 = 50m x 27m = 1350 square meters
Plot 1 indeed has a larger area, making it potentially more valuable and useful. However, the “length-only” advocate insists length is the key factor, dismissing area as a fabricated concept.
Consider another scenario: someone boasts about their land’s 70m length, owning a 70m x 50m plot (3500 sq meters). Its value stems from the substantial overall area, not just the length.
Conversely, a 50m long plot with only 2m width (100 sq meters) is far less valuable than a 15m x 10m plot (150 sq meters). Focusing solely on length (50m vs 15m) misses the crucial aspect of overall usable space – the area.
Imagine a plot 30m long and 200m wide. Its area dwarfs the others, explaining its higher value. Those fixated on length alone struggle to understand why a “shorter” plot (30m length) can be more valuable, failing to recognize the importance of width and, consequently, area.
Owning land that stretches 500m might sound impressive, but if it’s only 1m wide, its practical applications are severely limited. Similarly, land 500m wide but 1m long is equally impractical for most purposes.
Image of a large engine, representing power
Back to Cars: Torque and Horsepower Decoded
Just like land area, horsepower represents the overall capability of an engine, the total work it can perform. Torque and RPM (revolutions per minute) are analogous to the length and width of our land plot analogy.
Horsepower = (Torque x RPM) / 7127
This equation clearly shows horsepower is directly proportional to both torque and RPM. Increase either, and horsepower increases.
When designing an engine for more power – whether for faster acceleration, higher top speed, off-roading, hill climbing, hauling heavy loads, or towing – engineers can theoretically increase either torque or maximum RPM to achieve this.
While either approach can boost horsepower, there’s a practical consideration we’ll address shortly. Ultimately, horsepower is the true measure of work output, and vehicles can be geared appropriately to optimize for acceleration, speed, or towing capacity.
For instance, an engine producing 100 Nm of torque at its redline of 9000 RPM generates approximately 126 horsepower. Conversely, an engine with 250 Nm of torque but a lower redline of 4000 RPM produces about 140 horsepower. Despite a 2.5x torque difference, the horsepower figures are quite similar.
Now, the intriguing aspect: engines don’t deliver consistent torque across their entire RPM range. Torque fluctuates as RPM changes.
Graph showing torque and horsepower curves of a Mazda 2.6L engine across RPM range
As you can see in the graph, torque (the darker line) isn’t constant. It rises and falls. This 2.6L Mazda engine produces around 120 Nm of torque at 1000 RPM (idle), peaks at about 210 Nm around 5500 RPM, and then gradually decreases to 190 Nm at its 8500 RPM redline.
This leads to a crucial question: “Does maximum acceleration or towing power occur at the RPM where peak torque is achieved (around 5500 RPM in this case)?”
The answer is no. This is a common misconception. Even as torque decreases after its peak, the increase in RPM is significant enough that horsepower continues to climb, reaching its maximum around 8200 RPM before it too starts to decline. This decline happens because the torque drop becomes more pronounced, outweighing the RPM increase. Therefore, around 8200 RPM is where this engine produces maximum power, delivering the strongest pull for acceleration or towing. Revving beyond this point actually reduces power.
Graph comparing torque curves of various SUVs
Here’s a torque graph of popular SUVs. Notice how torque varies with RPM. Most turbocharged diesel SUVs start relatively low at idle, surge sharply to a peak due to turbocharging, and then decline rapidly. The naturally aspirated petrol Outlander 2.4 is an exception, exhibiting a more linear torque curve and a higher redline (6400 RPM).
You might also observe that the SUV with the lowest peak torque isn’t necessarily the one with the lowest horsepower.
This torque fluctuation with RPM results in a pronounced power band for turbocharged diesel SUVs. This explains why experienced drivers suggest upshifting diesels sooner and why “dieselheads” advise against redlining – it often leads to noise and smoke without a significant power gain. You may have experienced this “flat spot” at high RPMs when driving a diesel SUV aggressively.
Real-World Driving Dynamics: Torque in Action
Consider the Ford Endeavour 3.2L diesel. It delivers peak torque between 1800-2500 RPM. However, maximum horsepower is achieved around 3000 RPM. This means the Endeavour pulls strongest for acceleration and towing around 3000 RPM.
In contrast, the petrol Mitsubishi Outlander’s power delivery extends higher into the RPM range. This allows its CVT transmission to hold higher RPMs under full throttle, producing the characteristic “boooooom” sound but also providing decent power from its 2.4L naturally aspirated engine.
Interestingly, the Honda CRV and Skoda Kodiaq diesels, despite having higher peak torque figures than the Outlander, produce less overall horsepower.
Looking at the power and torque curves of a Mercedes E250 diesel and petrol, you’ll notice the diesel boasts significantly more torque. However, the petrol engine generates more horsepower due to its ability to rev higher – 220 hp at 6000 RPM (petrol) versus 200 hp at 3700 RPM (diesel).
Now, imagine a gentle start from a traffic light, typically around 1000 RPM. Which car has more power available at 1000 RPM?
Analyzing the graph, the diesel produces approximately 70 horsepower at 1000 RPM, thanks to its high torque (around 250 lb-ft). The petrol, however, only delivers about 25 horsepower with its lower torque (around 125 lb-ft) at the same RPM.
This difference in low-RPM power explains why torquier cars often feel quicker in everyday driving situations. They have more horsepower available at lower RPMs, making them responsive and effortless.
However, if you floor the accelerator, the diesel’s 200 horsepower will be outmatched by the petrol’s 220 horsepower in a sustained acceleration run. The petrol engine will also rev up faster due to lighter internal components.
Your 400 Nm diesel SUV’s seemingly effortless low-speed performance isn’t because horsepower is irrelevant; it’s because that high torque translates to higher horsepower at lower RPMs.
The Practical Limits: Why Trucks Aren’t High-Revving
Image of a Tata SE 1613 truck
Consider the Tata SE 1613 truck. It features a large 5.7L engine producing just 136 horsepower but a substantial 490 Nm of torque. Why such a large engine for seemingly low horsepower? Because it delivers that horsepower at a low 2400 RPM.
“But 136 horsepower is still weak! Why use this old truck example when newer Tata trucks offer 300 horsepower and 1100 Nm of torque from just 1100 RPM, like the Tata Signa?”
This older truck illustrates a key point: even with modest horsepower, it can carry 16 tons due to gearing. With sufficiently low gearing, even low-horsepower engines can move immense loads, albeit slowly. Speed isn’t the priority; load-carrying capability is.
This is relevant for off-road enthusiasts who believe massive horsepower is essential or those concerned about smaller crossovers struggling on hills. Vehicles with lower horsepower can still climb steep inclines; they just won’t do it at high speeds. Downshifting and revving the engine are key.
So, why not use smaller, high-revving, lower-torque engines making the same horsepower in trucks?
Theoretically possible with extremely low gearing. However, imagine starting a fully loaded truck at 6000 RPM. Aside from clutch wear, the inefficiency of running an engine at high RPMs continuously to generate the needed power would be substantial.
Practically, high RPM truck engines are problematic. High RPM engines require lighter internal components (pistons, crankshafts). Truck engines with heavy-duty components designed for immense loads cannot rev as high. The heavy pistons have too much inertia to change direction quickly enough for high RPM operation.
Electric Motors: Instant Torque Revolution
Graph showing torque curves of various Tesla models
Examine the torque curves of Tesla electric vehicles. Notice anything remarkable?
Every Tesla delivers maximum torque from zero RPM. Unlike petrol or diesel engines, torque doesn’t build with RPM; it’s instantly available and remains flat across a significant RPM range. This is why EVs accelerate so rapidly from a standstill, but their acceleration gradually diminishes at higher speeds as torque and then power eventually decrease.
This instant torque also revolutionizes off-roading. Electric off-roaders eliminate the need for low-range gear selection and high RPM revving. Precise low-speed control becomes possible, enabling effortless crawling over obstacles without power limitations.
While electric off-roading offers performance advantages, traditional internal combustion engine (ICE) off-roaders retain their appeal for those who enjoy the sounds, sensations, and thrill associated with them. This discussion isn’t about EV vs. ICE superiority, though.
This dyno result of an older Toyota Prius hybrid reveals another interesting aspect. While dyno charts for hybrids and EVs can be less accurate due to their unique power delivery systems, we can observe that torque is maximized from the start thanks to the electric motor. However, horsepower increases with RPM as the petrol engine revs up.
Toyota hybrids use an e-CVT, which is fundamentally different from conventional CVTs. It lacks belts, pulleys, and clutches, employing a planetary gear set instead. The engine in a Toyota hybrid doesn’t “downshift” to increase torque at the wheels. Instead, it spins faster to generate more electricity for the electric motor, which then provides the necessary torque for movement. At higher speeds, the engine speed gradually aligns with wheel speed. This complex system warrants a dedicated explanation, but this simplified description provides a basic understanding.
Hopefully, this exploration helps you evaluate vehicles more comprehensively than just comparing peak torque figures. To truly assess engine performance, test driving is invaluable. If that’s not immediately possible, horsepower/torque/RPM graphs offer insightful and meaningful data. Peak torque alone is an incomplete metric. Acceleration times, which factor in the gearbox, can also be helpful indicators.
In Conclusion
Ultimately, horsepower does win races and sell cars. It represents the total work an engine can do. However, understanding the torque curve and how torque delivery changes across the RPM range provides a much richer understanding of a vehicle’s performance characteristics and driving feel. Both torque and horsepower are essential parts of the engine power equation.
For further insights and discussions, explore BHPian comments.
