Power versus Torque - Part 1

So which do you need more of to go fast - power or torque?
By Dennis Jensen

It is a popular topic of discussion with car enthusiasts - but one that seems to be never-ending and usually unresolved. So, just what is it that best determines a car's acceleration performance - power or torque?

In order to be able to properly deal with the question, it is essential to have an understanding of what the terms really mean, as opposed to the gut level understanding (or misunderstanding) common to enthusiasts.

Torque
Torque is simply a measure of the twisting force that is applied in an attempt to rotate an object. It is a force that is applied to a lever arm, and is measured in Newton Metres (Nm). Newtons are a unit of force, and at the surface of the earth a 1kg object will exert a force on the ground of 9.8N, due to gravitation. Torque is, in effect, the product of the force and the length of the lever arm. Understood in this way, it is clear that there are two ways of increasing torque; either increase the force or increase the length of the lever arm.

Think about a door that you wish to open. You can either apply a small force at the outer edge (where the lever arm is long), or far more force close to the hinge (where the lever arm is short). In this way, you can have the same torque, but it is developed by applying different forces at different lever lengths.

"But what has all of this got to do with cars?" I hear you ask. Well, think about some of the engines you know. You know that increasing the lever length can increase the torque. Now think of those long stroke engines, and how, for their capacity, they seem to generate a lot of torque (such as the Ford 4-litre engine). The capacity, in effect, gives you the force, and the lever multiplied by that force gives the torque. So, to increase torque, increase the stroke, or increase the capacity, or both. Of course, there are negative aspects to increasing the stroke, which we will come to later.

There is one facet that is very important to realise: you can be exerting a lot of torque but not be doing anything. Think of all the time that you have skinned knuckles attempting to loosen a rusted bolt. You have definitely applied a lot of torque, but you have not done any work.

We now come to the aspect that causes a lot of angst among many enthusiasts - power. Most people think that they know what power means; unfortunately, many do not.

Power
Power is defined as the rate of doing work, and has units of Kilowatts (kW - named after James Watt) or horsepower in the old Imperial units. To see what power actually is, let's consider the experiment that James Watt did a couple of hundred years ago. He wanted to know the rate at which draft horses could raise coal from a coal mine. So he measured the mass of coal brought up, the distance that the coal was raised, and divided this by the length of time that it took to do this. He found that the horses would lift 33 000 pounds 1 foot in one minute (or 1 pound 33 000 feet in one minute). He called this unit the horsepower. In metric, the Watt is defined as the power to do one Joule of work per second. One horsepower is equivalent to about 746 Watts.

Now, another aspect to realise is that power and torque are intimately related. Remember how power is the rate of doing work? So, with engines, power is the torque multiplied by the radial velocity. Without getting into the physics in detail (we will forget about radians etc), the power of an engine is given by the following relationship:



Or, in the old Imperial units:



So, as you can see, power and torque are very intimately related... but it is important to realise that they are different.

Remember that you can be applying a lot of torque for no result. Well, looking at the fact that power is the rate of doing work, it is obvious that if you are doing no work, you generate no power! So, no matter how hard you push that spanner, if the bolt doesn't turn, you generate no power.

There are two ways of increasing the amount of torque generated by an engine - either increase the capacity (or, more correctly, capacity times volumetric efficiency), or increase the length of the lever arm (or stroke).

For increased power, you can increase either (or both) the torque, or the revs at which that torque is generated.

Here we revisit the issue of stroke. Increasing stroke will increase torque, so theoretically it would be good to have very long stroke engines. The problem is, if the stroke is too long, the volumetric efficiency decreases, particularly with increasing revs (which is why long stroke engines don't like a big rev, apart from the rotating friction and harmonics). Now this decrease in revvability more than compensates for the torque increase, which is why very high power output engines tend to have very short strokes (once again, engine strength issues ignored).

Okay, now we have all that physics stuff out of the way - we understand the meanings of power and torque in their correct contexts. So next week we can get into the interesting stuff - looking at engines, cars, and how power and torque relate to performance in terms of acceleration and top speed

Power versus Torque - Part 2

Torque is for acceleration and power is for top speed... or is it?
By Dennis Jensen

Last week we looked how power and torque are defined and related. This time we apply some of that knowledge to seeing how hard cars accelerate when endowed with engines with either lots of peak torque, or lots of peak power. Which is required to give the best acceleration?
Let's say that we have three engines:
• Engine A, which is representative of the old Ford 4.1 carby engine.
• Engine B, which has the same torque curve, only it is spread out, so that the torque numbers are at one and a half times the revs of A.
• Engine C, which has the same power curve as A, but spread so that the power figures are one and a half times the revs of engine A, but it only has two-thirds of the torque.

To shows how this data looks graphically, here are the power curves of the three engines:


and the torque curves of the same three engines:


Okay, so now we have some numbers to work with to determine which of these engines will give the most performance in terms of acceleration. If peak torque were the most important characteristic, you would anticipate that engines A and B would perform similarly (they have near identical peak torque), and that engine C would trail.
If peak power were the important factor, you would expect that engine B would be quickest, with engines A and C level pegging.
For argument sake, let's assume that each of the cars has a gearbox with a 2:1 first ratio, and a direct second, and that in first the car does 10 km/h per 1000rpm, and 20 km/h per 1000rpm in second. Also assume for argument's sake that the diff ratio is 1:1. Use the basic equation handed down from old Isaac Newton, F =ma, where F is the force (related in this case to the torque at the driven wheels), m is the mass and a is the acceleration. The force at the driven wheels is related to the torque by considering factors such as wheel radius, but these are constant so we will leave them out, and basically say that the torque at the tyres is directly proportional to the engine torque. So, in turn, the acceleration is directly proportional to the torque at those driven wheels.
Let's have a look at what happens when we compare the cars.


Now suddenly things look very different!
Notice that car B pretty much always has the acceleration advantage, but that car C trades blows, so to speak, with car A. Car C has a lot torque at the wheels to 40 km/h, more from 50 km/h to 60 km/h, less between 70 and 80, and more from 90-100 km/h. On the whole, car C would accelerate roughly equally to car A, except over the first part of the rev range, where car A would have a decisive advantage.

But here is where gearing plays its part. If we put in a diff of ratio 1.5 in car C (now call it car D), you will have the same torque figures at the wheels throughout the range as car A (which is what you would expect, given that the torque of engine C was only 2/3 of that of engine A). So, it is clear that you can compensate for a lack of torque if you have power. Simply change the final drive ratio. The question is, can we change gearing to compensate in the case of an engine that has a relative lack of power?
Let's have a look by comparing engines A and B. The first problem that you are stuck with is that if you attempt to lower the final drive ratio, the car will no longer reach its top speed (it runs out of revs). The second point to realise is that you could quite easily change the gearing in engine B (use a 1.5 diff ratio again) to get significantly more torque at the wheels than engine A. So, in short, you can compensate for a lack of torque in a powerful engine by shortening gearing. But you cannot compensate for a lack of power by gearing.

(An interesting example - by appropriate gearing, you can get a human being to generate a lot of torque at the driven wheels at extremely low (like about 1 km/h!) speeds. The ability to hold this torque at higher revs is power, and the fact that a human being does not generate much power is the reason that a human-powered vehicle are not able to accelerate quickly at any but the very lowest speeds.)
Earlier I mentioned that, by knowing engine power at a certain speed and mass alone, you can determine the acceleration. The equation is:
P = Fv
Where P is the engine power, F is the force and v is the velocity. Now, remembering that F = ma,
P = mav,
a = P/ma
So all you have to do at a certain speed is look at the tacho, look at the power graph and determine how much power you develop at those revs, and you can determine the acceleration at that speed (assuming no drivetrain losses). You will note that you cannot do the same for engine torque without knowing the gearing to determine the actual torque at the wheels, rather than the torque at the engine, which is modified by the gearing. As you'll now realise, gearing is a torque multiplier, not a power multiplier.

It is therefore clear that in order to maximise your acceleration, it is very important to have high power, rather than high torque. You can get high torque at the wheels by using appropriate gearing, but you cannot get high power from gearing. In short:
• for best acceleration, maximise power
• for best driveability, a high torque, non-peaky engine is better
An engine with high torque throughout the rev range will have good low-rev and part throttle response; this is the reason that cars like 4.1-litre Cortinas feel that they are very quick, even though they are not in absolute terms. That 4.1 has a very flat torque curve, with a maximum torque similar to the Commodore Ecotec V6 - but I know which engine I would prefer in outright acceleration terms (the V6 has about 50%more power)!

Conclusion

So power is the critical determining factor for maximum acceleration, and torque is necessary for driveability. This can quite clearly be seen in looking at the types of engines used for certain applications. Trucks and industrial vehicles use engines with large amounts of torque with a fairly flat torque curve, but relatively little power. Racing and sporting vehicles use high revving engines, with high power and relatively little torque in comparison to their power outputs. That is why you have F1 having engines revving to over 18000 rpm. And there's no use doing that if to maximise acceleration all you needed to do was maximise the amount of torque you had!

What is in my cooling system

Your cooling system is what keeps your car from having a meltdown. If you didn't have some way to cool things off, your engine would turn into a solid block of useless metal in no time flat. All of the parts that make up the cooling system have one goal of moving coolant around the engine so it can absorb and dissipate heat. The
basic system is made up of the following components:
1. radiator
2. radiator top hose
3. radiator bottom hose
4. water pump
5. thermostat
6. thermostat housing
7. electric cooling fan
8. thermo-time switch
The numbers correspond with the diagram. Below is a definition of each compenent.
Radiator The radiator is the most prominent part of the system. Coolant that has traveled through the engine is pumped through the tubes of the radiator and is cooled off for another round.
Radiator Hoses Your cooling system has a number of rubber hoses that move the fluid from one place to the other. These need to be replaced before they become brittle and cracked.
Water Pump The water pump does what you think it does - pumps the coolant through the system.The pump is belt driven, except in the case of some race cars that use an electric water pump.
Thermostat Your engine isn't always the same temperature. When you start it on a cold morning, you want it to get warm quickly. If you stop in traffic, you want it to cool itself off. The thermostat controls the flow of coolant so that it cools down more or less depending on the temperature of the coolant. It rests in a housing just after the radiator bottom hose.
Electric Cooling Fan Many cars these days have an electric fan for either primary or added cooling. The fan draws air through the radiator when you aren't moving fast enough to get things cooled down.
Thermo Time Switch Also known as the fan switch, this is the temperature sensor that tells the electric fan when to blow.

From Matthew Wright

Technical Information - Spark Plugs

Spark plugs are one of the most misunderstood components of an engine. Numerous questions have surfaced over the years, leaving many people confused.
This guide was designed to assist the technician, hobbyist, or race mechanic in understanding, using, and troubleshooting spark plugs. The information contained in this guide applies to all types of internal combustion engines: two stroke engines, rotary engines, high performance/racing engines and street vehicles.
Spark plugs are the "window" into your engine (your only eyewitness to the combustion chamber), and can be used as a valuable diagnostic tool.
Like a patient's thermometer, the spark plug displays symptoms and conditions of the engine's performance. The experienced tuner can analyze these symptoms to track down the root cause of many problems, or to determine air/fuel ratios.
SPARK PLUG BASICS: The spark plug has two primary functions:
• To ignite the air/fuel mixture
• To remove heat from the combustion chamber
Spark plugs transmit electrical energy that turns fuel into working energy. A sufficient amount of voltage must be supplied by the ignition system to cause it to spark across the spark plug's gap. This is called "Electrical Performance."
The temperature of the spark plug's firing end must be kept low enough to prevent pre-ignition, but high enough to prevent fouling. This is called "Thermal Performance", and is determined by the heat range selected.
It is important to remember that spark plugs do not create heat, they can only remove heat. The spark plug works as a heat exchanger by pulling unwanted thermal energy away from the combustion chamber, and transferring the heat to the engine's cooling system. The heat range is defined as a plug's ability to dissipate heat.
The rate of heat transfer is determined by:
• The insulator nose length
• Gas volume around the insulator nose
• The materials/construction of the center electrode and porcelain insulator
HEAT RANGE
A spark plug's heat range has no relationship to the actual voltage transferred though the spark plug. Rather, the heat range is a measure of the spark plug's ability to remove heat from the combustion chamber. The heat range measurement is determined by several factors; the length of the ceramic center insulator nose and its' ability to absorb and transfer combustion heat, the material composition of the insulator and center electrode material.

Heat rating and heat flow path of NGK Spark Plugs
The insulator nose length is the distance from the firing tip of the insulator to the point where insulator meets the metal shell. Since the insulator tip is the hottest part of the spark plug, the tip temperature is a primary factor in pre-ignition and fouling.
Whether the spark plugs are fitted in a lawnmower, boat, or a race car, the spark plug tip temperature must remain between 500C-850°C.
If the tip temperature is lower than 500°C, the insulator area surrounding the center electrode will not be hot enough to burn off carbon and combustion chamber deposits. These accumulated deposits can result in spark plug fouling leading to misfire.
If the tip temperature is higher than 850°C the spark plug will overheat which may cause the ceramic around the center electrode to blister and the electrodes to melt. This may lead to pre-ignition/detonation and expensive engine damage.
In identical spark plug types, the difference from one heat range to the next is the ability to remove approximately 70°C to 100°C from the combustion chamber.
A projected style spark plug firing tip temperature is increased by 10°C to 20°C.
Tip Temperature and Firing End Appearance

The firing end appearance also depends on the spark plug tip temperature.
There are three basic diagnostic criteria for spark plugs: good, fouled and overheated.
The borderline between the fouling and optimum operating regions (500&800 deg;C) is called the spark plug self-cleaning temperature. The temperature at this point is where the accumulated carbon and combustion deposits are burned off.
Bearing in mind that the insulator nose length is a determining factor in the heat range of a spark plug, the longer the insulator nose, the less heat is absorbed, and the further the heat must travel into the cylinder head water journals. This means the plug has a higher internal temperature, and is said to be a hot plug.
A hot spark plug maintains a higher internal operating temperature to burn off oil and carbon deposits, and has no relationship to spark quality or intensity.
Conversely, a cold spark plug has a shorter insulator nose and absorbs more combustion chamber heat. This heat travels a shorter distance, and allows the plug to operate at a lower internal temperature.
A colder heat range is necessary when the engine is modified for performance, subjected to heavy loads, or is run at high rpm for a significant period of time.
The colder type removes heat more quickly, and will reduce the chance of pre-ignition/detonation and melting or damage to the firing end.
(Engine temperature can affect the spark plug's operating temperature, but not the spark plugs heat range).
Below is a list of some of the possible external influences on a spark plug's operating temperatures. The following symptoms or conditions may have an effect on the actual temperature of the spark plug.
The spark plug cannot create these conditions, but it must be able to cope with the levels of heat...if not, the performance will suffer and engine damage can occur!!
Air/Fuel Mixtures seriously affect engine performance and spark plug operating temperatures.
• Rich air/fuel mixtures cause tip temperature to drop, causing fouling and poor drivability
• Lean air/fuel mixtures cause plug tip and cylinder temperature to increase, resulting in pre-ignition, detonation, and possibly serious spark plug and engine damage
• It is important to read spark plugs many times during the tuning process to achieve the optimum air/ fuel mixture
Higher Compression Ratios/Forced Induction will elevate spark plug tip and in-cylinder temperatures!
• Compression can be increased by performing any one of the following modifications: a) reducing combustion chamber volume (i.e.: domed pistons, smaller chamber heads, milling heads, etc.) b) adding forced induction (Nitrous, Turbo charging or Supercharging) c) camshaft change
• As compression increases, a colder heat range plug, higher fuel octane, and careful attention to ignition timing and air/fuel ratios are necessary. Failure to select a colder spark plug can lead to spark plug/engine damage!
Advancing Ignition Timing
• Advancing ignition timing by 10° causes tip temperature to increase by approx. 70°-100° C
Engine Speed and Load
Increases in firing-end temperature are proportional to engine speed and load.
When traveling at a consistent high rate of speed, or carrying/pushing very heavy loads, a colder heat range spark plug should be installed
Ambient Air Temperature
• As air temperature falls, air density/air volume becomes greater, resulting in leaner air/fuel mixtures.
• This creates higher cylinder pressures/temperatures and causes an increase in the spark plug's tip temperature. So, fuel delivery should be increased.
• As temperature increases, air density decreases, as does intake volume, and fuel delivery should be decreased
Humidity
• As humidity increases, air intake volume decreases
• Result is lower combustion pressures and temperatures, causing a decrease in the spark plug's temperature and a reduction in available power.
• Air/fuel mixture should be leaner, depending upon ambient temperature.
Barometric Pressure/Altitude
• Also affects the spark plug's tip temperature
• The higher the altitude, the lower cylinder pressure becomes. As the cylinder temperature de-creases, so does the plug tip temperature
• Many mechanics attempt to "chase" tuning by changing spark plug heat ranges
• The real answer is to adjust jetting or air/fuel mixtures in an effort to put more air back into the engine
Types of Abnormal Combustion
Pre-ignition
• Defined as: "..ignition of the air/fuel mixture before the pre-set ignition timing mark"
• Caused by hot spots in the combustion chamber...can be caused (or amplified) by over advanced timing, too hot a spark plug, low octane fuel, lean air/fuel mixture, too high compression, or insufficient engine cooling
• A change to a higher octane fuel, a colder plug, richer fuel mixture, or lower compression may be in order
• You may also need to retard ignition timing, and check vehicle's cooling system
• Pre-ignition usually leads to detonation; pre-ignition an detonation are two separate events
Detonation
• The spark plug's worst enemy! (Besides fouling)
• Can break insulators or break off ground electrodes
• Pre-ignition most often leads to detonation
• Plug tip temperatures can spike to over 3000°F during the combustion process (in a racing engine)
• Most frequently caused by hot spots in the combustion chamber. Hot spots will allow the air/fuel mixture to pre-ignite. As the piston is being forced upward by mechanical action of the connecting rod, the pre-ignited explosion will try to force the piston downward. If the piston can't go up (because of the force of the premature explosion) and it can't go down (because of the upward motion of the connecting rod), the piston will rattle from side to side. The resulting shock wave causes an audible pinging sound. This is detonation.
• Most of the damage than an engine sustains when "detonating" is from excessive heat
• The spark plug is damaged by both the elevated temperatures and the accompanying shock wave, or concussion
Misfires
• A spark plug is said to have misfired when enough voltage has not been delivered to light off all fuel present in the combustion chamber at the proper moment of the power stroke (a few degrees before top dead center)
• A spark plug can deliver a weak spark (or no spark at all) for a variety of reasons...defective coil, too much compression with incorrect plug gap, dry fouled or wet fouled spark plugs, insufficient ignition timing, etc.
• Slight misfires can cause a loss of performance for obvious reasons (if fuel is not lit, no energy is being created)
• Severe misfires will cause poor fuel economy, poor drivability, and can lead to engine damage
Fouling
• Will occur when spark plug tip temperature is insufficient to burn off carbon, fuel, oil or other deposits
• Will cause spark to leach to metal shell...no spark across plug gap will cause a misfire
• Wet-fouled spark plugs must be changed...spark plugs will not fire
• Dry-fouled spark plugs can sometimes be cleaned by bringing engine up to operating temperature
• Before changing fouled spark plugs, be sure to eliminate root cause of fouling