Horsepower and Torque
Let’s say we’re talking about the new Shelby GT350R. When most people refer to how much power the R has from the factory, you’re going to hear 526 horsepower. This isn’t wrong, but there’s another number that some seem to forget. And that would be 429 pound/feet of torque. That 429 lb/ft of torque statistic is important because it describes another side of the power than the 526 we’re all used to hearing.
In the most basic terms, an old saying describes the difference between the two numbers:
Horsepower is how fast you hit a wall, whereas torque is how far you will push the wall.
A great example of this would be a motorcycle and a semi truck. A motorcycle can hit a wall at 100 mph and the wall probably won’t move that much due to the fact horsepower is what got him to that speed. Take a semi truck going just half that speed, and the wall will move quite a bit further than the bike would have pushed it. That’s the difference between horsepower and torque on the most basic level.
Digging Deeper Into The Differences
"horsepower and torque are always equal at 5,252 RPM"
Believe it or not, on a daily basis, most drivers will only see the torque out of their car’s engine. This is due to the fact that horsepower doesn’t really come into play until after about 5,200 RPM. If anything, prospective car buyers who regularly drive their car on the streets rather than the track should be more concerned about torque rather than horsepower.
Without getting too deep into the physics behind it, Horsepower = Torque x (RPM / 5252). Where 5,252 is the constant between horsepower and torque. We could go into a ton of mathematics and physics to explain why that’s the case, but the most simple way of explaining it is that horsepower and torque are always equal at 5,252 RPM. That’s why on dyno graphs you can see that the horsepower and torque curve always converge at that RPM. Pretty cool, right?
What Is Torque?
Edmunds.com did a really good job of explaining the action of torque and relating it to something that most of us can understand.
The measurement of torque is stated as pound-feet and represents how much twisting force is at work. If you can imagine a plumber's pipe wrench attached to a rusty drainpipe, torque is the force required to twist that pipe. If the wrench is two feet long, and the plumber pushes with 50 pounds of pressure, he is applying 100 pound-feet of torque (50 pounds x 2 feet) to turn the pipe.
Coyote Swapped Fox Body Mustang
Another way to think about this is the crankshaft on your Mustang. The force required by the engine to spin the crankshaft is torque. From there, the power is transferred through the transmission, driveshaft, rear axles to your wheels. Another thing to keep in mind is drivetrain loss of about 15%. That’s why you see different power numbers from your engine output (typically what is advertised from the automaker) and what actually makes it to your rear wheels, or rear wheel horsepower (RWHP).
But what does this mean for the driver? Well, another way to explain the feeling of torque would be upon acceleration. When you launch your Mustang from a standstill, you get that seat of the pants feel almost immediately as you hit the gas pedal. This would be torque. That’s why you feel the most punch out of your Mustang at peak torque rather than peak horsepower.
What Is Horsepower?
Unlike torque which is strictly a measurement of force, horsepower is a measurement of work over a given amount of time. The math behind horsepower is a bit more confusing than torque, but the main takeaway is that horsepower is work over a period of time.
If you recall the equation earlier, Horsepower = Torque x (RPM / 5252), the general takeaway is that horsepower is dictated by torque and RPM. If torque is higher, so will horsepower. And if RPM is higher, horsepower will increase as well. This is why you see peak horsepower numbers at the top of your powerband. The more RPM that your engine is spinning at, the more horsepower that will be produced. Be sure to note that the reason horsepower drops off towards the top of your RPM range is due in part to the torque dipping further and further down and the mechanical limits of your engine.
Horsepower vs Torque On A Dyno Graph
A good visual example of horsepower and torque at work would be on a dyno graph. If you’re not familiar with a dyno or dynamometer, it’s essentially a treadmill for cars that measures the power output at the wheels.
Peak torque on most engines is usually mid-range in the powerband where peak horsepower is at the top of the powerband. High horsepower cars are naturally high top speed cars, where high torque vehicles (i.e. diesel-powered engines) are great for hauling and towing.
The graph below is from a bone stock 2011 Mustang GT. As you can see from the dyno graph, peak horsepower is at 366 hp @ 6,350 RPM and peak torque is 346 lb/ft @ 4,300 RPM. So at 4,300 RPM, you’ll be getting the maximum thrust or seat-of-the-pants feeling out of the Mustang due to the highest torque number at that point. And you’ll be seeing maximum horsepower at 6,350 RPM where your car will be accelerating at its highest rate of speed.
Torque And Forced Induction
Continuing on the dyno graph subject -- One effect of forced induction (supercharger or turbo applications) setups is the effect they have on the torque curve. The above graph was a naturally aspirated example of what a torque curve should look like. Relatively smooth and flat throughout most of the powerband. This can change from car to car depending on cam timing and other factors, but for the most part, this is the case.
However, what happens when you throw a turbo or supercharger into the equation? The shape of the torque curve changes. Why does that matter? Because that’s the difference you feel. For a flat torque curve on naturally aspirated cars, it feels like a smooth, steady acceleration throughout the rev range.
On turbo cars, that could change. Take our 2015 Mustang EcoBoost with a COBB Accessport Stage 1 Tune for example. This 2.3L turbocharged four-cylinder may be a different motor from above, but the same rule applies when it comes to the shape of the torque curve.
So, ignore the horsepower numbers for a second and focus on the shape of the torque curve. See how the torque kind of hits you in the face mid-range, then drops off quickly as the RPMs rise? Unlike a naturally aspirated engine where the torque curve will stay relatively flat, after peak torque on a turbo car, the curve falls off quickly.
A turbo car will accelerate differently than a naturally aspirated car. Where the acceleration is steady for an N/A car, a turbo car will kind of “hit you in the face” once the boost hits. It’s a pretty cool feeling.
2015 Mustang EcoBoost - Stock vs COBB Tune Dyno Graph
Furthermore, adding a twin-screw or roots-type supercharger in place of a turbo will give you a different kind of torque curve. These types of superchargers are known for how smooth and instant the acceleration could be. This is due to the design and various other factors.
On Bill’s 2015 GT with a Roush Phase 1 Supercharger, he made a respectable 555 horsepower to the wheels. But more importantly, check out the torque curve. As he said in the video, the power (torque) is linear and smooth all the way to the top which is great for driveability. This is in part why Ford favored these type of superchargers on the 2007-2014 GT500s.
Your Next Mustang Modification
And if you’re considering a forced induction setup for your Mustang, you know how the car will behave under acceleration on a turbo vs supercharged car in comparison to your currently naturally aspirated setup. If you’re looking for more information on forced induction and which route to go with your build, be sure to check out our tech article that goes further in-depth on Superchargers vs Turbos.