Power under the curve. 

Torque doesn’t matter. Wait, torque is everything. What is Torque again? I will admit to you that despite being a passionate car person and avid engineering fan that the concepts of torque and horsepower and their interactions with one another have never been very clear to me. Yes, I know the definitions, and I know how they feel, but what is torque, exactly, and what does it do? Isn’t HP all that really matters? Now I’ve done some research, and made some sweet graphs and I feel prepared to explain what I think I know about the subject.

The first thing I discovered is that HP is all that matters. Before you get all ballistic, allow me to explain that torque totally matters, just not on its own merit. To accelerate a car you need a force applied over time: F=Ma (Force = Mass x Acceleration). Torque is a force, but it doesn’t have a time component. Think about it like this, you can apply 150 lbs-ft of torque to a lug nut, but it’s not actually moving [in practical terms].

Advertisement

My “lightbulb moment” was, um, a light bulb. A light bulb consumes power, measured in watts, named after James Watt who also gave us the Horse Power unit…or so says reliable sources. An electrical watt is defined as volts x amps; That is, the voltage multiplied by the current. A 60 watt light bulb on a 110 volt system therefor draws .55 amps (110x.55=60) or 60 watt light bulb on a 220 volt system draws .275 amps since it’s a linear relationship; the greater the volts, the “slower” the current for the same “power”.

HP is defined in a similar way; HP = (Torque x RPM)/5252. Torque you know and RPM you know…the 5252 is simply a unit conversion number and we don’t need to worry much about it. Using electricity as an analogy we’ll call horsepower watts (Kw is actually a common use for engine power in many regions), torque is voltage RPM will be the current or amps. So, an engine producing 60 hp would be 100 ft-lbs at 3151 rpms. I could double the rate to 6302 rpm and half the torque for the same HP, or I could double the torque and half the rpms, same difference.

The [electrical] watt is king, its what produces light, or makes things turn on or go. You can have volts without amps, or amps without volts, but without both you have no watts and no power.

Advertisement

Same story with torque, horsepower and rpm; You could have a 1000 ft-lbs engine, that tops out at only 500 rpm and end up with only 95 hp [Hp=(tq x rpm)/5252 = 95]. Torque is force, but not work until its spinning (rpm) and unless we are producing that work over time we don’t get power, and we don’t get acceleration and acceleration is what matters in practice for a car. And when I say “acceleration” I mean, ANY acceleration so a physics definition not a track day one.

So if horsepower is all that matters…what’s the deal with diesels? Lets break it down:

1. We know HP is what accelerates a car

2. We know that torque x rpm [all divided by 5252] is what creates HP.

So the Faster you spin an engine, the more HP you make. Makes sense right? Now lets learn how to read a dyno chart and put it to good use

(thanks Automobile)

1. HP is an engine speed variable, we just learned this, but because engine speed has a far FAR greater spread than the input torque of an engine (engines may have a 7000 rpm spread where the torque spread is only a 150-300 ft-lb or so) it means that large HP numbers are going to be a natural consequence of high rpm where even a modest amount of torque being applied at a very fast rate will equal a lot of power. This is why Formula 1, street bikes…anything with a high revving motor makes so much peak power.

2. Where and how you make torque matters. Diesels make torque…lots of torque…and the make it way low down in the rev range. This low speed torque is that “shove” you feel driving a big lazy v8 or a diesel engine. But, its not the torque that gives you that feeling, as we described above, it’s the power.

To show you what I mean I picked a modern small diesel from VW; the CJAA 2.0 Liter TDI. The engine’s variable geometry turbo spools up to peak torque (236 ft-lbs) at a low 1700 RPM and hangs onto it 2600 RPM, a consequence of diesels being able to ingest HUGE amounts of air pressure and not explode or ignite the fuel until they are good and ready. 236 lbs-ft at 1700 rpm equates to 76 hp, at 2000 rpm you have 90 hp and at 2600 rpm you have 116 hp. That doesn’t sound like a ton, but that’s 85% peak power (136 HP) at 2000 rpm. You can see it in the graph that the HP line takes a strong upward bend where the torque is.

(VW CJAA TDI power and torque curve)

Compare that to a similar displacement naturally aspirated gasoline engine. In this case, the Subaru FA20 in the BRZ. The FA20 trumps the CJAA handily in the HP department at 200 peak HP and is obviously a better “sports car” engine. However at 1700 the FA20 is only making 105 Lbs-ft which equates to only 34 hp, at 2000 its 115ft-lbs/43 hp, at 2600 rpm its 137/68. In fact the FA20 isn’t making more HP than the CJAA until 3900 RPM, right about where you would shift for daily driving. This is why the BRZ feels lacking in power, when in fact it has plenty.

(Subaru FA20 power and Torque curve)

Take a look this graph of the two compared side by side; You can see the diesel bends the curve upwards for HP low down in the rev range. This “area under the curve” is all the additional HP you are making, and for nice long stretches way down low at everyday speeds.

(The orange area is all the area where the TDI has more power than the “more powerful” Toyota/Subaru)

Advertisement

Specifically look at the 900-4500 area between the blue tdi and the red FA20 and notice how much more horse power the TDI is making. Now 200 hp will out accelerate 136 hp any day, everyday, but while the BRZ is laboring slowly up to that peak power, the torque machine has been cranking out solid hp for a good long while, out accelerating the BRZ. Turbo lag is an exaggerated form of this kind of action: when the turbo is off boost, there is no torque thus no power thus bogging like a dog, when it kicks in the engine makes torque/power/speed.

Another way to express this concept is to talk about average horsepower over a given rev range, say between 1100-4000, which is typically the daily driving range. In this zone the The FA20 averages 67 hp, the CJAA averages 107 hp. What this means is that if you were to never rev your BRZ/FRS past 4000 rpms…aside from being a tragedy…the lowly diesel would trounce your sports car engine in average HP nearly 2:1! This is why torque feels fast, and this is what the phrase “power under the curve” means; More time at a higher average HP means more time accelerating at a greater rate.

Power under the curve could be compared to eating well all meals in the week but never really feasting, as opposed to peak power which is feast or famine.

Advertisement

The trouble is that because, as mentioned previously, RPM is a much wider band variable than torque output the amount of torque you can add down low to affect power is relatively limited. i.e. In a practical sense, you will gain more power adding engine speed (which is relatively easy) than you will with adding torque (which is relatively hard or costly). This is one of the big reasons diesels make lousy sports car engines (generally speaking.)

This is an exaggerated example of course since the FA20 is a high revving sports car engine and the TDI is a low revving diesel so another brief comparison is in order I think this time between 3 naturally aspirated 6 cylinder small truck engines, from various time periods. The blue line is the Toyota 1FZ-FE 4.5 liter I6, the last gasoline inline 6 engine the land cruiser got, the red line is the Toyota 1GR-FE, the dependable but aging workhorse 4.0 liter V6 in the tacoma, the green line is the GM LFX 3.6 liter high feature V6 in the Colorado/Canyon.

I chose these engines because they represent the three main points I want to illustrate when it comes to horsepower, torque and the ways we’ve dealt with it over the years.

Advertisement

1. The 1FZ-FE (blue) is old school; big cubes, cam profiles and head design meant to lower the curve to produce horsepower in the low rpm range. This is what is meant when truck people refer to “stump pullin’ power”. The curve is made to suite a high average, but low peak horsepower figure or a specific part of the rev band that suites high load and low speed. It’s interesting to note that although it has the lowest peak power (212), it makes the most average horsepower (128 hp) in the daily driving range, its in 85% of its peak power over the largest rpm spread (1800 rpm) and its there over the largest percentage of its rev range (44%). Its the “grunt” that old big american V8’s supply. That doesn’t meant that its not slow, cause it is, but its high and consistent average power output makes it perfect for accelerating with high loads at low engine speed or slow speed off roading.

2. The 1GR-FE takes a more moderate approach and tries to balance Torque and Horsepower but runs out of breath at high rpm due to limitations of its cam profile. This engine represents a excellent tune and mission specific profile for a truck within the technology of its time. Providing a linear torque curve that provides good low rpm power, but revs higher to generate higher peak power. Since there is only port injection and variable cam timing on the exhaust side there is a marked drop off in power as the engine looses its ability to breath at high speeds. Although this engine is significantly down on peak power (236 to 301) it has the exact same daily driving range average HP as the much more powerful GM V6 (115 hp), it at 85% or more peak hp over a greater percentage of its rev range (33%) and much lower down in the rev range (4000-5500)

3. The LFX prioritizes horsepower, but because it has variable cam timing on both intake and exhaust, and direct injection, there is still plenty of torque. The “selling point” of the engine, however, is that it keeps revving until a high level of horsepower it achieved. Here, big showroom hp is just a matter of keeping the engine revving, which can be done with cam profiles. Again, if you can rev it out, the LFX will be the more powerful engine, but its no more powerful between idle and 4400 rpm than the “ancient” Toyota V6. It has the same average hp in the daily driving range as the 1GR-FE (115), but has the same rev range that its 85% or higher peak power (1500 rpm) but spends only 26% of its rev band there and its much higher up in the rev range (5500-7000 rpm).

Advertisement

Which is best? Well it depends. The largest and slowest engine is the one with best average power in the part of the curve you drive most often, but its low on peak power and thus is slow. The smallest engine makes the most peak power and generally matches the older, larger Toyota V6 but it has narrower operating parameters and will require more revs to make the most of it.

The way to really live would be to have it both ways; broad and ample torque and enough revs and torque up high to make good hp. Displacement give you this, at the expense of being generally inefficient under low load. Forced induction also gives you this, at the expense of being fuel hungry under high load. Without going into it to much, there just is no such thing as a free lunch; the ICE engine is about as efficient as we can make it (for now) and the relatively slim window of stoichiometry in inherent in the Otto cycle means that making power takes fuel in a (more or less) linear way, no matter what the sales brochure says.

Diesels are much better at wide band stoichiometry but suffer so badly at high RPM operation that its not likely we will ever see a truly high performance diesel without major breakthroughs in the technology.

Advertisement

However, knowing what torque means practically, and how to read its influence on the graph may help you weigh the benefits of an engine.