It occurred to me to chip in on this discussion of torque ...



Torque and Horsepower

It occurred to me to chip in on the discussion of torque and horsepower because most folks, even if they learn how to calculate one from the other, still don't have a common sense feel for the difference between torque and horsepower, and how they can be used to understand automobile performance.

Let me get away from the math and make a simple analogy to human performance. An Olympic weight lifter demonstrates great torque. He lifts the weight once, lets it crash to earth, and retreats behind the stage to recover. High human torque occurs in big men with big muscles.

In contrast, the cyclist Lance Armstrong epitomizes great horsepower. During any one push of the pedal he is not exerting the torque of the weight lifter, but he does it over and over again at high rpm, which powers him up the Alps. What Armstrong has that the weight-lifter doesn't is the delivery of great volumes of oxygen and fuel to the muscles. High human horsepower occurs in reasonably torquey people who also have great heart and lungs.

The high-torque weight lifter, despite his massive thighs, probably isn't going to climb the Alps on a bike. The high-horsepower cyclist, despite all his athleticism, is barely going to budge the weight set. It is easy to see that there really is a difference between torque and horsepower.

How does this relate to engines? Like the weightlifter pushing up a bar, torque is a function of the push generated by piston strokes. Torque begins dropping off at some point as engine rpm increases because air flow and combustion efficiency begins to fail. In effect, the engine gets winded. It is gasping, like the weightlifter toiling up the Alps. It cannot get enough oxygen and fuel into the combustion chamber, and it cannot flush out all the exhaust residue. Cars that can maintain their torque at high rpm because of good air-fuel efficiency will generate greater horsepower than comparably-torqued engines that lack efficient airflow. This was the basis for the success back in the 1960s of the now-outdated hemi head, and it is the reason behind the greater power of today's fuel-injected, multi-valve pentroof heads. (There are other things that affect hp for an engine of a given torque besides just air flow, all related in some way to maintaining torque at high rpm, but let's stick to the simple point here.)

Everyone wants a car that has the force of the weight-lifter, but which can apply that force at the grueling repetitive pace of the cyclist. Here's a comparison of three 2005 sport wagons that differ in engine size and valve number so we can compare what happens to torque and horsepower. I'm not trying to promote or diss any of these cars, I just picked them because they are about the same general size and design on the outside with really different engines sizes, oxygen delivery, torque and hp.

size torque hp valves

Chevrolet Equinox 6-cyl 3.4 liter 210@3800 185@5200 12 Subaru Outback Wagon 4-cyl 2.5 liter 166@4000 168@5600 16

Audi Allroad Quattro 6-cyl 2.7 liter 258@1850 250@5800 30

The 3.4 liter Chevy engine cranks out a lot of torque. (When most people refer to a "powerful" engine in the common-sense English use of the word, they mean torque.) The Chevy has 26.5% more torque than the little 4-cylinder Subie. But the Chevy generates only 10% more horsepower than the Subie, and it peaks at a lower rpm. In other words, the 2-valve-per-cylinder Chevy, like the weight-lifter, gasps for breath at higher rpm and in the end has little power advantage over the smaller 4-valve Subie. Because torque and hp values intersect at 5252 rpm, you can see that the Subie must have steadily maintained its less impressive torque up to high rpm values. Now look at the 5-valve-per-cylinder Audi – it has excellent low-end torque and then holds it as engine speed rises. By 5252 rpm the torque must have dropped off a bit, but it was still good enough to generate 250 hp at 5800 rpm. That car can really breathe.

The Chevy is the Olympic weight lifter, the Subie is Lance Armstrong. The Audi combines qualities of both with a forceful, aerobically-efficient engine (as a human athletic analogy, think NBA power forward). Subaru sells a turbocharged version of the same Outback H4 engine – in effect, force-feeding it air and fuel at high rpm – and that model generates 250 hp@6000 rpm, matching the Audi V6. It would be like Lance Armstrong wearing an oxygen mask.

Most people's intuition about a "powerful" car is actually torque. Horsepower increases if you can utilize your torque efficiently at high engine speeds.

Now back to the math. Horsepower = (torque*rpm) / 5252. In other words, horsepower is merely a function of torque times rpm (divided by a constant that matches the arbitarry units used to measure torque to those of horsepower). There is no pure weight-lifter's torque in car engines because even at the lowest speeds an engine is operating at hundreds or thousands of rotations per minute. Nobody wants to crank the crankshaft once. The ability to function well at higher rpm requires a car with great air-fuel delivery that allows it to just keep feasting on oxygen and fuel as rpm rises. It also requires great mechanical efficiency and low friction (issues we haven't brought into this discussion). But the point is this: by breaking down the things that contribute to high rpm performance, the mathematical formula can be re-cast conceptually like this:

horsepower = torque * airflow * combustion efficiency * mechanical efficiency.

In other words, horsepower is a more complex beast that torque. A car with relatively high torque but proportionally low horsepower is probably just a big lunker that is not engineered very well. A car with high horsepower compared to its torque production is one that has a lot of engineering prowess built in.

Think about your typical torque and horsepower curves. For traditional engines, the big ones operating like a weight-lifter get their torque peak at lower rpm. In contrast, those engines operating like a cyclist get their horsepower peak at higher rpm. Much has been made of the fact that these two curves always cross at 5252 rpm, but that is just an arbitrary accident of history resulting from the fact that somebody decided there would be 60 seconds in a minute. It is the relationship between falling torque and rising horsepower rising that is important. In almost all situations, flat torque curves are better than steep torque curves, because that means the horsepower continues to be delivered even as oxygen and fuel become scarcer commodities.

In reality, most high horsepower vehicles start out with a lot of torque to begin with. Without high torque, it would be tough to have high horsepower. You cannot typically feed a wimp more oxygen and make it a lot stronger. In theory, you could make a low-torque, high horsepower car. Take a Honda Insight and its torque rating of 66@4800 and rev that baby up to 15,000 rpm like a Formula-1 racer and our math equation tells us we get a respectable 188 hp, barely edging the Chevy Equinox. But can you imagine the engineering headaches trying to get that engine to hold together and deliver enough air and gas at the kind of rpm?

In other words, the best way to get high horsepower is start with high torque, and then engineer it well for efficient air delivery, combustion, and minimal friction. Another option to generate notable horsepower is simply to engineer a normal sized engine to maximize efficiency of oxygen and fuel delivery at high rpm. The key point is that the power of an individual piston push is not as impressive as the ability to maintain that push in a high performance, high rpm environment.

Tab Rasmussen, Edwardsville, IL

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