All those numbers, what do they mean?

Car buffs like us are no strangers to reading spec-sheets from the manufacturers and magazines. Some of us can even write them too. There are people out there with an encyclopedic knowledge of the torque and horsepower figure of almost every car you can think of. I have seen car spec-sheets with more numbers written on it than balance sheets – top speed, 0-100, displacement, fuel consumption, gear ratios et. al.

These numbers provide us with a useful means to rate and compare the abilities of different cars. The performance oriented freaks will examine the power ratings, the torque output, the top speed and the 0-100 times. Those with fuel economy in mind will look at consumption figures. Those who specifically buy cars based on size will examine the dimensions, but these are rare. Usually, the figures that matter most to people are all the drivetrain related figures.

However, there is a little inconvenient reality to all this. For one, manufacturers often quote figures with different units. How do you compare the power output of a car whose manufacturer quoted 100kW against another quoted at 139bhp? How do you see which car uses less fuel? 7.6l/100km or 27mpg? The other question is, what really is the meaning behind all these figures? It’s all good to know that your BMW M5 produces 500bhp, but what is a brake-horsepower?

Let’s look at a shortened spec-sheet of the Peugeot 407 and see what each of these figures mean.

Displacement, Bore & Stroke.

Or engine capacity, as some would prefer. This is the total volume of that’s being swept by the pistons as they move from their top dead centre position to bottom dead centre. To obtain this figure, one needs to calculate the volume swept by the piston of one cylinder, and then multiply that figure by the number of cylinders. As such, we have

But, how do you calculate the volume swept by a cylinder? That’s where the next set of figures, the bore and stroke come in. The bore is quite simply the diameter of the combustion chamber, hence it’s usual measurement in terms of the millimetre. The stroke meanwhile, is the length of the straight-line distance between the piston’s top dead centre and bottom dead centre positions as illustrated below.

Going back to Form 5 mathematics, the volume of a cylinder is pi multiplied by the square of its radius(r) multplied by its height(h). In the context of our combustion chamber, the radius of the cylinder is half the bore, while the height is the stroke, allowing us to calculate the volume of one combustion chamber with the following formula:

As such (pardon the maths lesson), working all the algebraic thingamagics, we have,

As you can see from the formula, the cylinder’s bore contributes a square term to the equation, and as such, reboring your cylinder by even half a millimetre each can lead to a significant increase in engine capacity compared to an equivalent amount of restroking.

Taking the figures of the 4-cylinder 407 with its 85mm bore and 88mm stroke, you will arrive at a figure of 1,997,425mm³, which is not a figure used by anyone on this planet to measure engine capacity. I am sure you all know that everyone measures engine capacity by way of cc or litres. How much is one cc? How much is one litre? First, cc stands for cubic centimetre, and if you want to follow the SI way of writing it should be cm³.

So, in order to get the 1,997cc figure in the brochure, all you have to do is to convert your bore and stroke figures to cm, and you’ll get your answer. Then, we all know the conversion from cc to litres is just dividing the former number by 1,000. Of course, we always see people mixing these units up and telling us their Proton Wira’s engine is 1.5cc.

There is another unit used to measure engine capacity, but only in America, which is the cubic inch (abbreviated cu. in.) and it’s obtained (obviously) when you calculate your displacement using the bore and stroke values measured in inches. An inch is about 2.54cm, so a cubic inch is roughly equivalent to 16.4cc. The diagram below should help your conversions.

So, going back to our example of the 407, the bore, stroke and displacement figures in the various units are listed below.

As noted earlier most American cars quote their engine displacement using the cubic inch unit. It was a great nuisance to me when I first started out reading about their muscle cars being so used to comparing in terms of either cc or litres, the cubic inch unit was completely meaningless to me. Now, knowing the 2.0-litre 407 displaces a mere 122 cu. in., it puts the size of motors like the 426 cu. in. Hemi nicely into perspective.

Power

The most basic definition of the term power is the rate of work done, but that’s a definition used by physicists, and while it’s also a correct definition in relation to our cars, it’s not a very useful one for us to understand in too much detail. Suffice for us to understand that higher power output would roughly translate to a higher top speed.

The most commonly used unit to measure power in our part of the world is the horsepower, which although popular, isn’t actually the best unit of choice. This is because the term horsepower alone consists of several definitions and measurement methods. Relevant to us petrolheads are the metric and mechanical horsepowers. The mechanical horsepower was the original definition of the horsepower unit, as coined by James Watt, and roughly equals 0.746kW

The Germans introduced the metric horsepower in the 19th century and it is defined as being equivalent to 0.735kW, roughly 98.6% of the mechanical horsepower. The metric horsepower is often indicated using the abbreviation PS, which stands for pferdestarke, the German word for horsepower. The unit brake-horsepower (bhp) meanwhile, simply indicates that the figure is the engine’s horsepower on crank.

Among my very first car reading materials was the owner’s manual of my parents’ Proton Wira, which quotes power output in terms of kilowatts (kW), making the kilowatt the first unit of power that I was familiar with. I also know for a fact that Australian publications all use kilowatts in their specs sheets. Honda and Nissan prefers to use PS, while everyone else here favours horsepower.

The presence two different, but yet very close horsepower units creates quite a bit of confusion among readers. As a personal rule, for all my future writings, hp and bhp will refer to mechanical horsepower, while PS while refer to the metric horsepower. Personally, with the ambiguities surrounding the horsepower unit, I am in favour of using the kilowatt although it doesn’t sound as cool saying, “My car produces 220kW” compared to “I’ve got 300 horses under the hood.”

Once again, using the 407 as an example..

Torque

Going into the definition of torque given by the physicists will leave most in a daze, so suffice to summarize it laymen’s terms that torque is equivalent to what you would call a twisting force. Where a force causes an object to accelerate in a straight line, torque would twist it about a certain axis. In the case of cars, the torque from the engine, transmitted via the gearbox and driveshafts, would twist the wheels about the axles.

The quantity torque is obtained as a product of force multiplied by distance, hence the units to measure torque would consist of a force unit multiplied by a distance unit. Three units are commonly used: Newton-metre (Nm), kilogram(force)-metre (kg.m), and pound-foot (lb.ft). My personal preference, along with most car manufacturers here, is to quote torque figures in Nm.

It must be said that the unit kilogram(force)-metre unit to measure torque isn’t a fully acceptable one, as in this case, the force term is measured by kilograms, which is a measurement of mass, rather than by Newtons, the universally accepted unit to measure force. Some manufacturers also quote in this unit using the abbreviation kgf.m, but they are exactly the same. Fortunately, the conversion from this unit to Nm is a simple matter of multiplying with 9.81, the value of the acceleration due to gravity.

A more complicated one is the pound-foot unit preferred by the Americans and the Brits. If done manually, you need to consider two seperate conversion factors, one to convert the force term, another to convert the distance term. Converting from imperial to metric units was never straight forward to begin with, and to convert a product of two quantities is quite a headache. Factoring in both conversions, 1 lb.ft is roughly equivalent to 1.36Nm.

As far as this writer is concerned, all of my personal future writings will quote torque figures using Nm without exception. Once again, bringing out the 407…

Speed

I am sure we all know what speed is – it’s the rate of covering distance. Quite simply put saying your car is travelling at 100km/h is saying that ‘if we maintain this speed indefinitely, the car would travel 100km every 1 hour.’ Some writers, me include, also like to use the abbreviation kph to denote the same unit. It’s not a scientifically acceptable abbreviation, mind you, but it is understood.

The other speed unit commonly used of course, is miles per hour (mph). This unit is another one favoured by the Americans and the Brits, while the rest of the world talk in terms of kilometres per hour. The conversion between these two units is straightforward, though not necessarily accurate. One kilometre is roughly equal to 0.621 miles, and the conversion between kph and mph would follow the same factor.

The 407’s top speed of 206km/h would translate to 128mph, and in case if you’re wondering, the DeLorean needs to hit 142km/h in order to activate its flux capacitor.

Fuel consumption

For the purposes of this article, consumption in terms of money per unit distance will be ignored, but even then, there’s still quite a bit to talk about, as not only are there several different units, some are quoted as volume of fuel per distance travelled, and some are quoted as distance travelled per volume of fuel.

My personal spreadsheet to monitor my own car’s consumption has four columns quoting FCs at different units: RM/km, l/km, km/l, and km/60l. For my personal use, the most useful number is actually the km/60l figure which gives me the full range for one tank in my Waja’s 60l tank.

In Malaysia, the commonly used unit is litres per 100km travelled. The reason for using this in opposed to just litre/km is probably to shift the numbers to the 5-15 range, which is more useful for our visualisation purposes. Equally popular in this part of the world is the reverse kilometres per litre unit.

There is also the famous miles per gallon (mpg), and here’s where you need to be careful, as the US and UK gallons differ by 20%. The mpg is actually of no consequence to us, but since many of us here watch shows like Top Gear and Fifth Gear, and not to mention read magazines with Top Gear Malaysia, Autocar Asean and Evo with British roots, a passing familiarity with this unit might be useful.

Our good old friend, the 407, has the following consumption figures:

Tyre Size

To the uninitiated, the tyre size code can be an incomprehensible series of numbers. The important dimensions in a tyre are the width of the tyre, the thickness of the sidewall, and the diameter of the rim that will fit in it. There is one number denoting each of these dimensions, and they are all of different units, as illustrated below.

So, for the last time, using the 407 as an example, the 407 comes in tyres that measure 215mm in width. The tyre sidewall thickness, is 55% of 215mm, which is 118.25mm. Then finally, in the middle is a rim that measures 17″ in diameter. It is fortunate that the industry has managed to adopt a single system worldwide to codify tyre dimensions, or it would be quite a riot trying to understand each system.

Finally

This post was written as an attempt on my part to make some sense of all the myriad of units out there you’ll find when reading car materials. It’s especially confusing sometimes when some parties decide to use SI units, and some want to stick with the old Imperial units. I personally prefer to use SI units as much as possible, being educated in them. Also, converting units in SI is a far simpler affair as all multiplications and divisions are done by factors of 10.

The most important use for all these numbers, as far as us plain simple motorists are concerned, are simply to provide a means to compare the abilities of various cars side-by-side. It would be good if everyone in the industry agree on the same common set of units, but until that happens, we’ll need to know how to convert between them.