Power under the curve. 

"HammerheadFistpunch" (hammerheadfistpunch) 07/25/2015 at 10:00 • Filed to: Tech, Power, HHFP

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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].

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 !!!error: Indecipherable SUB-paragraph formatting!!! . 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.

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)

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.

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.

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 ).

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 !!!error: Indecipherable SUB-paragraph formatting!!! 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.

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

LongbowMkII > HammerheadFistpunch 07/25/2015 at 10:21

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You saved this for the weekend on purpose. FP ahoy!

Tim (Fractal Footwork) > HammerheadFistpunch 07/25/2015 at 10:40

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I though I understood HP and torque, then I realized I didn’t.

Berang > HammerheadFistpunch 07/25/2015 at 10:42

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Ok. Do tractive effort next.

LongbowMkII > HammerheadFistpunch 07/25/2015 at 10:56

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Suggestion for the next piece, or appendix to this, discuss why low-end torque motors are generally paired with tall gear ratios and smaller high revving motors paired with shorter gears. Matching gears to motors is often overlooked.

Nauraushaun > HammerheadFistpunch 07/25/2015 at 11:02

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This is excellent.

Does this mean....say you have two hypothetical (weird) engines:

- Quad-turbo 3-cylinder 800cc petrol making a peak of 500hp at 4000rpm

- NA 12-cylinder 12 litre diesel making a peak of 500hp at 4000rpm

These have the same torque at 4000 rpm. Correct? And possibly the same peak torque figure.

This means that getting useful figures for a car is difficult. Some would tell you to ignore the HP and look at the torque figure, but in this case that wouldn’t tell you anything. The truth lies in the power curve - if the diesel can make that 500hp at 1000rpm as well, there’s your usable power/torque/shove/awesomeness.

Peak torque is about as useful as peak horsepower: it’s still only describing a single point in the rev range.

...right?

davedave1111 > HammerheadFistpunch 07/25/2015 at 11:14

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A good in-depth look, very interesting to see the torque/power figures compared. One addition, there’s a simple way to understand the difference between torque and power: torque is how fast you accelerate, power is the speed you accelerate to.

Imagine cranking a grindstone by hand:

The force you put in is the torque. The harder you can push the handle around, the faster the stone gets up to speed.

At a certain p0int, though, you can’t turn the handle any faster - all your energy just goes into moving your hand around to keep up with the handle. That’s the limit of the power you can generate.

Rusty Vandura - www.tinyurl.com/keepoppo > davedave1111 07/25/2015 at 11:34

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I like this analogy, but I don’t see it explaining the idea of a moment of torque. For that, I like the analogy of the guy standing on the end of the breaker bar when the axle nut won’t budge. Torque applied, but no work done. And rather than power , shouldn’t we be talking about work ?

Rusty Vandura - www.tinyurl.com/keepoppo > Nauraushaun 07/25/2015 at 11:36

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I’m no physicist, but I do understand that we refer to a bending moment of torque . So a torque measurement is a single point, an instant, a moment.

I think you’re onto something here.

langadamd > HammerheadFistpunch 07/25/2015 at 11:40

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Great article!

brianbrannon > HammerheadFistpunch 07/25/2015 at 11:43

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The confounding factor is the curves only represent full throttle runs. Most folks daily driving range does not include full throttle from idle to 4k. Good write up though. 

macanamera > HammerheadFistpunch 07/25/2015 at 11:54

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How did I not see this post? This is spectacular, dude. This is excellent Oppo.

HammerheadFistpunch > brianbrannon 07/25/2015 at 11:56

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Good point, that’s a topic for research on another slow day at work, and thanks

LongbowMkII > LongbowMkII 07/25/2015 at 12:04

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I'm going to go ahead and +1 myself for calling it.

PardonMyFlemish16 > HammerheadFistpunch 07/25/2015 at 12:08

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Good analysis. Average power is definitely key, and on the street torque wins. For a street car I’m way more concerned about what the engine is like at 3000 RPM than at 6000 RPM as that’s where I’m at 90% of the time. This is also a big reason why, while I do love the geekiness and ingenuity behind them, I hate high revving high HP/L 4 bangers. Numerically they get the job done but on the street they just don’t feel as fast as their HP would suggest. I am OK with a well sized turbo engine now (at least .75cc displacement for each lb of car it has to move).

Baeromez > HammerheadFistpunch 07/25/2015 at 12:22

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The freaky part, and the way to win a lot of bar bets, is that because of the conversion factor, the horsepower and torque curves on every dyno chart ever cross at 5252 rpm. Even my very automotively minded friends have a hard time grasping this simple concept.

HammerheadFistpunch > Rusty Vandura - www.tinyurl.com/keepoppo 07/25/2015 at 12:56

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Power is just work over time

Rufant v1.0 > HammerheadFistpunch 07/25/2015 at 13:50

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As ever, good job.

The car that brought this concept home to me was the Nissan R33 GT-R. Remember when that came out? It was blitzing everything and yet it ‘only’ made something like 480hp peak power. However it was the power it made over the entire rev range (and a mega fast trans, 4wd, launch control, etc, etc) that made it so ballistic.

shpuker > HammerheadFistpunch 07/26/2015 at 21:46

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As far as explaining in general terminology the impacts of torque vs HP I’d say this is close enough and quite well put.

However. A much more accurate way to make this argument and to properly bring this point across is that HP is a result of torque being applied over time, or in other words that the two don’t exist in some unique dimension away from one another (which I know isn’t the point you were making but rather the implications of how you made it). Or in other words, as torque is applied to a shaft, if the shaft maintains a constant inertia then it will accelerate at a constant rate, forever, so long as the torque applied stays the same. This concept is what separates electric motors from combustion engines as well actually. A combustion engine is limited not only by the forces of its internal friction and external friction, but also by its ability to fire its cylinders at set rates. While electric motors essentially eliminate the restriction due to their rate of firing (though you do bring in other restrictions). Back to the point, HP itself is not a measure of something meaningful to your power source, torque and rpm are. HP exists as a quantifiable way to try and merge the two into a more general understanding. Yet to rebuke my own point slightly, HP still hold a very meaningful value as it better quantifies what the occupants of the vehicle will experience. If you really wanted a proper term for this however look at the derivative of HP.

HammerheadFistpunch > shpuker 07/26/2015 at 21:54

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yeah this is basically what I was getting at...only with more graphs and such.

shpuker > HammerheadFistpunch 07/27/2015 at 01:39

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Fair. I probably got the wrong implication from what you wrote but it seemed to still imply that torque and HP were still two unique phenomena.

YouAreWrongSir > HammerheadFistpunch 07/27/2015 at 12:42

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I was excited to read this article but then you said

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].

which shows us that your fundamental understanding is flawed. Torque is NOT a force. Force is a component of torque. That’s like saying pie is sugar [in practical terms] when we all know sugar is a component of pie.

The reason why your lug nut doesn’t move when a certain torque is applied is becauase there is an equal magnitude of torque opposing it.

Snuze: Needs another Swede > HammerheadFistpunch 07/27/2015 at 13:10

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I know I’m a bit late to the party, but this is a really good write-up. Well done!

One thing I’d like to mention, in your closing you mentioned that we will probably never see a truly high performance diesel without breakthroughs in technology. To be honest, you probably never will, and the reason is fuel chemistry. Something that I had never thought of before taking an ICE theory course in college was that fuel takes a finite amount of time to burn. We all imagine it as an “explosion,” like a gun shot, but in reality it’s a fast burn, and however fast, still takes time.

As you’ve alluded to, high performance engines burn gasoline, or alcohol (methanol or ethanol) because these fuels burn fast. And as you’ve said, you can relatively easily make more power by moving to a higher RPM band. So what limits the RPM potential in a gasoline engine are mechanical factors, typically valve springs. At high RPMs you approach the natural frequency of the spring-valve assembly (it’s an undamped spring-mass system). As you approach this natural frequency, the springs begin to resonate on their own, skipping off the cams, making valve control difficult, and in and interference engine leading to piston-valve contract and FIREY DEATH!

Smaller, lighter valve trains can wind higher, which is why most OHC engines can rev higher than OHV engines - no heavy push-rods. Going smaller still, you can push the envelope higher with lighter weight components - street bikes have tiny, light weight springs and valves with higher natural frequencies. At the super exotic end you can use air-springs, such as in F1 and MotoGP, which have a significantly higher natural frequency than a metal spring, and allow for positive valve control at engine speeds exceeding 20,000rpm.

So where does that leave the poor diesel? Well, massive industrial diesels, such as in stationary gen sets, or on ships, typically max out at a few hundred RPMs. One reason is the massive components are so heavy it’s hard to swing them any faster - diesels rely on compression ignition - detonation in a gasoline engine, which is a violent and unpredictable affair compared to a controlled burn induced by a spark plug. As such, all of the components are massively overbuilt to take the punishment. But the other reason is the flame front takes so long to propagate and burn the air-fuel mixture.

In a car or truck, the engine displacements are smaller, relatively, so the combustion chamber and cylinder volume are less, meaning a flame front moving at a fixed speed will cross the chamber and complete the burn in a relatively shorted amount of time. This is why cars and trucks and rev to a couple thousand RPMs vs. a couple hundred. Interesting fact - my parents have a 1998 GMC with a mechanical, indirect injected, 2 valve 6.5L turbo diesel. They also have a 2006 GMC with the Duramax diesel which features a high pressure common rail direct injection system, and 4 valves per cylinder. The newer truck makes 360HP and 560 ft-lbs to the old trucks 190HP and 440 ft-lbs. But, both engines redline at 3500 rpms.

luvMeSome142 & some Lincoln! > HammerheadFistpunch 10/07/2015 at 14:45

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Nice!

V12 Jake- Hittin' Switches > HammerheadFistpunch 11/19/2015 at 18:02

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Awesome write up ! But I have a question. How come my s600 feels faster than a s55 if both have the same hp outputs? I have a big torque advantage though .

HammerheadFistpunch > V12 Jake- Hittin' Switches 11/19/2015 at 18:09

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you have a big torque advantage, which means you make more hp more places in the curve, which means you are accelerating harder than the S55 more of the time.

V12 Jake- Hittin' Switches > HammerheadFistpunch 11/19/2015 at 18:11

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Thanks ! Again awesome article.

TheRealBicycleBuck > HammerheadFistpunch 02/15/2016 at 16:27

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I think what everyone misses in the torque/horsepower discussion is applying it to the ground. Imagine two engines producing the same torque and horsepower at two different RPMs. The engine producing the high-RPM power requires more slippage in turning parts in order to get the power to the ground. The slippage will occur in the torque converter in an automatic, the clutch in a manual, or in the tires. Too much torque at low RPMs results in excessive wear.

One-Wheel-Peel > HammerheadFistpunch 02/16/2016 at 10:08

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Interesting. I’ve been approaching this topic with many people over the years. you’ve taken an interesting approach by showing power available at engine speed. I find a good way to illustrate the difference is to “linearize” the rotational relationship that is RPM. In this case, torque would be force, and power would still be power but moreover; force-at-speed.

Another way is to think of power as energy “production”. the more power you make, the more energy you can put into the system that is the car (in the form of kinetic energy, ergo speed). Think of body-checking a big block across a floor. every time you ram into it, it moves a little. obviously to move the block more quickly you can do two things - hit it harder or hit it faster (keeping the other constant). Hitting it harder is like applying more torque. Hitting it faster is like raising RPM and thus power, releasing the energy from that explosion more frequently -> more energy.

BUT, you said “horsepower is all that matters”. Riddle me this:

Lets suppose there is a car in two situations, 20mph and 40mph. it is geared in such a way that it turns 3k RPM at 20mph, and 6k RPM at 40MPH. Lets also assume that the engine has a perfectly flat torque curve and torque at both engine speeds is identical. Math says at 6k RPM the car will make twice the power than it did at 3K. Does the car accelerate faster at 40mph? (neglect losses for rolling resistance, wind, friction, etc).

The answer is the acceleration is identical. when we write programming for vehicle dynamics simulations torque is always the variable. Acceleration is a function of engine speed (vehicle speed / gears) since engine speed determines available force. Horsepower, however, is required to balance top speed. power into the system - power out of the system.

With torque you’re evaluating a force balance (FBD), with power you’re evaluating an Energy difference. Apples and Oranges - strung together with engine components.

Race car design almost always favours highest average torque across the expected powerband. A sacrifice in peak power for higher average torque nets a faster lap time.

GranTourer > HammerheadFistpunch 02/16/2016 at 10:13

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Aren’t almost all the endurance prototype cars diesels now?

One-Wheel-Peel > shpuker 02/16/2016 at 11:00

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I see the point you’re trying to make with cylinders firing. But in all reality it happens so fast that the pressure curves melt into one rather linear curve. The overlap of those valves is what makes more cylinders feel “smoother”. In any case, the torque variation across the RPM range in an ICE is due to the fact that almost all engine geometry is fixed and designed to be optimal at one point. ie the torque fluctuation is mainly a result of the engines flow characteristics. if it could flow the same at all speeds, you would have a nearly-perfectly-flat curve.

But, the two do exist in a dimensions removed from one another. torque is a measure of force, whereas horsepower is a measure of energy-rate. the mix-up happens when we translate the energy burned in a combustion to actual force due to expansion of the gasses. The explosion has a set amount of energy, and the rate of explosions would translate to power (energy/time). But, remember that at different engine speeds peak pressure happens at different points along the pistons travel (combated by ign timing) - ergo different crank-conrod geometry and thus different TORQUE resultant from the same cylinder pressure. Its the translation from chemical energy to physical force that links torque and hp and confuses people. But that is the different dimensions within which they live. Torque is a force balance (F=ma and FBD’s) and Power is an Energy balance (net energy / conservation of energy principle). think of it as approaching a simple projectile problem and solving by separate methods. force balance or energy balance.

AeroNerdPorsche > TheRealBicycleBuck 02/16/2016 at 20:38

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Why would it need more slipping? You’d need different gear ratios (assuming the same vehicle speed), but if your clutch is slipping any more at 6krpm than it is at 3krpm (and if that slip is anything other than zero), you need a new clutch.

TheRealBicycleBuck > AeroNerdPorsche 02/17/2016 at 08:16

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It is impossible to go from a dead stop to moving without some form of slippage, no matter what gear ratio you use. Engines idle around 800 rpm. When the wheels are moving at zero, the only option is to transfer that energy in a gradual manner. If you try to instantly transfer the energy (like dumping the clutch), either the engine stops turning or the tires lose traction and slip.

The greater the disparity between the rpm of the engine and the drive wheels, the more slippage required to get them to speed.

AeroNerdPorsche > TheRealBicycleBuck 02/17/2016 at 15:28

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Yes, and just because an engine has a powerband in the upper RPM range doesn’t mean that you have to dump the clutch at 6k. In both the high torque and low torque engine, you can slip the clutch at 1-1.5krpm to get the car moving, and then get it up into its power band.

TheRealBicycleBuck > AeroNerdPorsche 02/17/2016 at 16:33

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My point is that people talk about max torque -vs- horsepower but they ignore when that power is being applied. They want lots of torque for getting off the line, but forget that too much torque will result in a vehicle that is difficult to launch smoothly, excessive wear on the parts designed to slip (clutch) or excessive wear on the parts designed to grip (tires).

Anyone who has driven a tractor will understand. Tractors tend to have grabby clutches. Most of the slip occurs when the massive torque is applied to the rear wheels and they tear up the ground and the tractor lurches into motion.