HP and TORQUE.. know the diff.. FOO
A horsepower figure is simply insufficient information with which to understand the nature of an engine. Take, for example, a HondaCivic Si with 160 hp and a Nissan 240 SX with 155 hp. Being only 5 hp apart, you would assume that they feel similar, but if you know anything about these two cars, you probably already realized that the driving experience is entirely different.
If you look at a dyno pull for a 240 SX versus a dyno pull for a Civic Si you see two dramatically different engines. Ignore the fact that the 155 hp Nissan makes slightly more power at the wheels than the 160 hp Honda, and concentrate instead on the shapes of the hp curves. The 240 SX’s power curve comes on strong early in the rev band, peaks at 5000 rpm, and by 6000 rpm has started to take a nose dive. The Civic Si, on the other hand, is relatively lethargic (wow big word) until 5500 rpm, when the VTEC system switches to the big cams, and power comes on with a surge. Even without knowing the numbers, the shape of the lines tells you a lot about the personality of the engines.
We can get a better understanding of an engine if we at least say where that 155 or 160 hp is made. Remember, a 160 hp engine only makes 160 hp at one particular point on the powerband. It is a peak number, and therefore tells very little, but knowing where that peak number occurs should at lease give you an idea of what the power delivery is like.
Let’s witch from manufacturer’s claims to real, measured drive-wheel hp numbers, and say that the Civic Si makes 123 hp at 7500 rpm, and the 240 SX makes 127 hp at only 5100 rpm. Now you at least have the idea that the 240 SX has lots of grunt down low, and the Civic Si does all its screaming at the top of the tach. If you can’t look at a dyno chart, though, you really want more information than power and rpm. This is where torque comes in.
The relationship between torque and hp is one that generates a lot of confusion. You will always be able to find some chump to insist that hp is what’s important, while some other chodeball will say that torque really does everything. Ofcourse, as usual, everybody is wrong. Saying one is more important than the other is like saying the temperature in Fahrenheit is more important than the temperature in Celsius. Functionally, to the end user like you and me, they are the same thing. In fact, like Fahrenheit and Celsius, you can convert one to the other with a simple formula. To get torque from hp, the formula is …
Torque=hpx5250/rpm
To get hp from torque, the formula changes to…
Hp=torquexrpm/5250
An easy mistake to make, and this is probably the mistake at the root of everyone of these chumps’ arguments, is attempting to use this formula on peak values. Use this formula for the Civic Si’s 123 hp at 7500 rpm and you will get 86.1 lb-ft of torque. While that is the correct torque output at 7500 rpm, the peak torque output that you are really interested in is actually down at 6000 rpm, where we measure 93 lb-ft at the wheels. The same is true for the 240 SX. The conversion formulas are only useful for a single rpm.
So if hp and torque are functionally the same, why bother talking about both? Because.. quoting peak hp and peak torque numbers gives you an even better snapshot of what the actual power curve might look like. Knowing that the 240 has 135 lb-ft of torque at 4100 rpm really drives home the fact that it has a lot more low-end grunt than the Si with only 93 lb-ft way up at 6000 rpm.
Just quoting peak power and torque output worked reasonably well when engines had more predictable powerbands. The basic shape of the 240’s powerband used to be what pretty much every powerband looked like. Some were shifted higher in the rev range, some were shifted lower, but there was always a natural arc to the shape. Then came VTEC, variable-length manifolds and computer-controlled turbocharchers and everything changed.
The Si has what is now a typical Honda torque curve-almost dead flat with two small, but distinct humps representing peak torque for each of the two cam profiles. Knowing about the peak at 6000 rpm tells you nothing about the slightly lower peak at 2300 rpm. Audi’s 1.8 liter turbo engine makes the traditional peak readings even more irrelevant. Look at the torque of 123 lb-ft (at the wheels) could easily be at 2600, 3300, 4000, 4400, or 4800 rpm, but picking any one of these numbers would give you a false picture of the engine’s output.
Audi tries to express this by claiming peak power of 150 hp (at the flywheel) at 5700 rpm, and peak torque of 1555 lb-ft of torque form 1750 to 4600 rm. It’s a clumsy way to express power output, but short of a chart, it’s the best you can do.



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