Torque or Horsepower, which is better?

                                             By Michael Erlandson

      The question asked in the title, or at least some variation of it, is one that I hear come up often in motorsports conversations. If you’re anything like me, you were introduced to the wonders of torque, horsepower and dyno sheet numbers long before establishing an intuitive understanding of what those concepts really meant. What are torque and horsepower and how do they connect to the performance level of a rotating engine? In this post, I want to take a dive into the core concepts of torque, horsepower and the spinning nature of power output.

     People often view torque and horsepower as distinct metrics and it is my belief this is
largely due to the fundamental design philosophy of the muscle car-era. The conversation is usually centered on the ‘down-low’ power of the high-performance dreams we chased with badges like Ram-air 4, BOSS and Six-Pack.

To get around this efficiency deficit, we were given the phrase, “there’s no replacement for
displacement”, and to some extent that’s true. Lacking efficiency? Just make it bigger! The
turbulent nature of this lower efficiency, however, means the power is generated lower in the RPM range due to more complete mixing of the charge. So it follows that if you double the diameter of your cylinder bore, you quadruple the piston’s surface area and the force acting on it follows the square of the bore diameter; more CID = more Power, right? Well yes, but that might not be the whole picture.

       The definition of torque, crucially, is a force acting about a center of rotation. As an
example, something like a dedicated 675cc racing motorcycle might exert 55(ish) pounds of force out the end of a hypothetical 1-foot-long lever. Now, grab any torque wrench and crush a new head gasket onto your favorite old engine from earlier and you will undoubtedly exceed this torque output by a fair margin. Does this mean my human arms have more power than a vehicle capable of carrying me 160mph? The unequivocal answer is “no, my arms do not” and here is why. If you want to “go fast”, you must do Work, though torque alone will not get you there. 

If you look at any ‘rate’, whether it be Miles per hour as in this example, Feet per Second or even Revolutions per Minute, they all share a crucial component, Time. And so, the question of Power becomes a question of “how much force can you deliver per unit of time?” If you deliver more torque per time step or the same torque at more frequent intervals, you will have, by definition, more Power.

Now, a lawn tractor, with a similar displacement 656cc twin Briggs, can exert a tremendous amount of force, far more than a racing motorcycle is capable of, but in a race it’s not even a question, no amount of clever ‘gearing’ will ever make a lawn mower faster than a Triumph Daytona around a track. The lawn tractor can deliver a huge amount of torque but only at its slow engine speed, maybe 2500–3200rpm tops, so that force is delivered less often. The high revving bike on the other hand, spins up to 13,500rpm. The torque for the motorcycle, although lower in magnitude, is delivered much, much more frequently due to its high RPM versus the lawn tractor, and as such its absolute ability to do Work is considerably higher.

      Here’s the catch, horsepower (total capacity for Work), torque (force) and RPM (the time it takes to deliver that force) all mechanically have an inverse relationship, meaning you cannot use gearing or any other means of mechanical advantage to increase horsepower like you can torque.

To gain Power you must increase torque, RPM or both, and the main takeaway is that HP is invariant to things like gearing where an increase in torque comes at the cost of RPM, balancing the equation. Horsepower is a metric that relates torque to RPM and how often you can deliver that force defines your Total Power.

Torque and RPM are crucial ingredients in horsepower more than they are separate metrics to quantify an engine by. When you discuss horsepower, you are by definition, discussing torque and its relationship to RPM.

     A 2-stroke makes more power than a comparable 4-stroke, right? So, why do 4-stroke engines dominate most motorsports nowadays? To address this, let’s shift gears for a second and chat about a third consideration in engine performance and why Peak Horsepower may not be as important as you’re led to believe. 2-Stroke engines are often cited as having a higher peak power than a 4-stroke engine (ceteris paribus). A 2-stroke engine shines in the fact that it has twice as many power pulses in the same number of revolutions as a 4-stroke engine. With 2-stroke engines, each power pulse actually has less usable energy, less force on the piston and thus, less torque available to it but there are exactly twice the number of power pulses to speak of in the same amount of time, – higher peak horsepower. The interesting thing about 2-strokes is they are a resonating air pump and are dominated by frequency-based flow characteristics. This has the effect of making their power output heavily dependent on a specific engine speed (RPM). They make relatively little power until you “hit the pipe”, the light switch flips and then its “hold on for your life!”

Graphing this power curve produces a peak that is usually very steep and in a narrow RPM range. This is what is meant by the “peaky power” description often associated with 2-stroke engines. If you contrast this with a 4-stroke, while its peak horsepower might be less, that power is typically distributed over a much wider RPM range.

The total area under the curve of a typical 2-stroke power band is less than the area of a similar power band for a 4-stroke engine. Even though the 2-stroke might have a higher peak power output, the average power over all RPMs is oftentimes higher for a comparable 4-stroke (with occasional exceptions). This is one of the driving reasons motorsports are not dominated by 2-stroke engines.

     Finally, we have arrived at the fundamental metric of “what really is better”. Torque alone cannot fundamentally give you any insight as to how “fast” your car is because it does not answer the question of how fast or how often that force is delivered. Horsepower is a more insightful metric because it incorporates time and allows you to make a statement about the rate of torque delivery. Horsepower, though, is usually stated as a number, a single data point (426hp for example) but that is necessarily at a single RPM, a condition most vehicles don’t exclusively live in. So, what about all the time you’re not at that RPM? That peak number quickly becomes less relevant every time you shift to remain centered around the optimal RPM. I would argue that the area under the curve is the most important of any single metric for an engine and is probably the least talked about. A larger area under the curve allows you to accelerate for longer before shifting. Power is delivered in a more linear fashion making every bit of it more effective. It lets you maintain a given force of acceleration for longer and results in a higher speed in less time, all good things if we want to go fast!

      At the end of this all, am I trying to tell you that the engines of the muscle car era are exclusively slow or that torque is not an important consideration, no. I grew up doing more than just holding a flashlight for my father. Getting my hands dirty and wrenching on hotrods of the past, gave me some of my best memories and most valuable experiences I have. So, while they will always hold my interest, what I am saying is that just how fast they really were may have been a bit of an illusion created by the feel of an old engine's intrinsic characteristics. It’s an engine in a power range that is falsely indicative of its true performance level. They definitely had that wide eyed thrill the moment you put your foot into it but engines from that day delivered their power low in the revs. It was immediate and intense but for a shorter duration than engines nowadays are capable of. Power plants from those days had few choices other than to have low end grunt. This is a blast from the stop light and lends to easy drivability that sold cars but it does not necessarily imply absolute speed. A seriously fast street engine from 1970 might run an 11 second ¼ mile but a modern engine wouldn’t even get talked about. At the end of the day, a mountain of torque and that low-end twist hits hard out of the box but often dies off quickly while the car with a larger power curve area will keep putting lengths out front all the way to the finish line.