What exactly are you supposed to do with all the technical info that usually accompanies a road test in a motoring magazine? Is it even worth perusing? In his next two blogs Jake Venter will explain why this data can be very useful.
The specification sheet that is part of a road test, or a vehicle brochure, contains a lot of information that may not be meaningful to some people. In some cases one might even ask; “why is there a need to know this?”
While it is true that some information is not really necessary, many of the facts are worth knowing, as they can help you to make up your mind about what the vehicle is like, especially if you’ve never driven it.
In this article and the next, I’m going to explain what most of the terms mean and why they’re important. Let’s start with the engine.
Number of cylinders
The more cylinders an engine has the smoother it becomes, but the less torque it will have at low engine speeds. For maximum economy and the least amount of maintenance costs a small number of cylinders are better, which is why some the latest Fiats only have two cylinders. Many other manufacturers have recently changed from four to three cylinders on their economy models.
Bore and stroke
These numbers are always quoted but for most people they are of little use. An automotive historian, however, might find the way the stroke/bore ratios have changed over the years quite fascinating.
This number tells you how much air the engine is capable of sucking in over two revolutions and it is fairly closely related to the maximum torque values but tells you very little about the maximum power. A high revving small engine can quite easily develop as much power, if not more, than a low revving big engine.
This is the volume of mixture above the piston at bottom dead centre divided by the volume above the piston at top dead centre, and is a measure of the maximum extent to which the mixture has been compressed. An engine’s efficiency increases as the compression ratio increases, but petrol engine compression ratios are limited to approximately 12:1. Above this value harmful detonation (non-uniform combustion) is likely to occur. Diesel engines compress air only, and require ratios sometimes as high as 22:1. These high ratios generate enough heat to ignite the fuel a fraction of a second after it is injected.
The location and operation of the valves is very important because power can be lost if the intake ports are long with many bends in it, or if the camshafts are so far away from the valves that the push rods and the rockers have to be used to transmit the motion. For this reason most modern engines have either single overhead camshafts or twin overhead camshafts. However, some American engines are still equipped with camshafts in the cylinder block and pushrod operated valve gear.
Most people tend to look only at the maximum kilowatts delivered by an engine, but the maximum torque is just as important because it is closely related to overall engine efficiency. It is also important when considering the aspects of an engine’s performance that cannot be measured with a stopwatch, such as the engine’s pulling power when pulling up a hill, especially when towing a caravan. On the other hand, a car’s maximum speed is determined by the maximum power output in relation to the total aerodynamic drag.
It’s also worth knowing that the kilowatt developed by an engine under any set of circumstances is equal to the torque times the revs, divided by a constant number to get the units right. This explains why the gearbox, (which changes the engine revs in the relation to the rear wheel revs) is a torque multiplier.
This can best be explained by means of an example. 100 Nm delivered at the flywheel by an engine at 2 000 revs becomes 200 Nm at 1 000 revs when going through a 2:1 gear ratio. If this is transmitted through a 4:1 final drive ratio the rear wheels will receive 800 Nm at 250 r/min. In all three cases the power output remains the same (except for a small frictional loss) because the torque times the revs gives the same number.
Engine output graphs
CAR magazine always publishes these graphs, but very few other magazines do. They’re measured at the factory by means of an engine dynamometer that is coupled to the engine at the flywheel. The readings are always taken at full throttle and the important points to notice are the maximum values of the power and torque delivery as well as the engine speeds where this occurs. The shape of the torque curve is particularly important; if it rises to a high value really quickly and stays there the vehicle will not need frequentl gear-changes, but if it struggles to get to a maximum value and then falls rapidly constant gear-changing will be required.
* We’ll post Jake’s follow-up to this article soon…