Some entries on our road test data page supply information that is immediately useful; other entries only become meaningful if the reader has some background knowledge that enables him or her to put the numbers into proper perspective. Here, we take a look at those entries that may require further explanation, starting with the top left-hand corner of the specifications panel.
SPECIFICATIONS
Cylinders
This lists the way the cylinders are arranged, as well as the orientation of the engine in the vehicle. The decision to use an in-line, flat (horizontally opposed) or vee engine configuration depends on tradition, the tooling available, and the space utilisation in the vehicle. Most front-wheel-driven cars have transverse engines, but Audi usually employs longitudinally mounted engines. Rear-wheel drive cars are either mid-engined or front-engined, and the engines are usually mounted longitudinally – the exception is the Porsche 911, which uses a rear-mounted horizontally-opposed engine driving the rear wheels. The engine orientation is chosen to optimise interior space or vehicle balance. Sometimes tradition also plays a role. For example, upmarket BMWs and Mercedes-Benzes have always had front longitudinally-mounted engines coupled with rear wheel drive.
Fuel supply
This is usually by means of electronically-controlled fuel injection. The carburettor is just about dead, but may be found on very old designs such as the Nissan 1400 half-tonner. Fuel injection gives an engine easier starting, a very steady idle speed, and quick warm-up from cold. It also ensures the mixture is close to being chemically correct for most of the time, so that a catalytic converter can reduce harmful exhaust by-products. The output may also be boosted by means of a supercharger (mechanically-driven) or turbocharger (driven by the exhaust gas), and sometimes with a charge air intercooler.
Bore and stroke
The bore is the diameter of the cylinder bore in the cylinder block, while stroke is the distance the piston travels from the top to the bottom of its movement. These points are known as the top and bottom dead centres, respectively. Quite a lot can be deduced from the ratio between the bore and the stroke. If the bore is narrower than the stroke is long, the engine is said to be undersquare. If the bore and stroke are equal in dimension then the engine is square, but if the bore is bigger than the stroke the engine is oversquare.
Considerably undersquare engines normally have their breathing restricted by the small valve sizes that go hand in hand with a small bore, as well as by deliberate valve timing design, so that they run at fairly low engine speeds to keep the bearing loads within acceptable limits. This follows because crankshaft bearing loads intensify with an increase in stroke length as well as an increase in engine revs.
Vastly oversquare engines are usually designed to be happy at high engine revs. Not only will the bigger bore make it possible to fit larger valves, but the shorter stroke will reduce the bearing loads at speed. This explains why Formula One engines are grossly oversquare. In fact, a modern 3,0-litre V10 Formula One engine has a bore and stroke of about 104,4×35 mm to cope with the volume of mixture to be inhaled, as well as the bearing loads that arise from running at more than 18 000 r/min.
Square engines are a good compromise between the limits imposed by the engine dimensions, and most modern engines are close to being square.
Cubic capacity This is also called the displacement volume, and is the theoretical total volume the pistons have displaced while moving from top- to bottom-dead-centre. This can be calculated from the bore, the stroke, and the number of cylinders. Compression ratio Actually a ratio of volumes. It is the total volume of air (or mixture) above the piston at bottom-dead-centre divided by the total volume of air above the piston at top-dead-centre. An engine’s efficiency increases as the compression ratio increases, but there is a practical limit of about 11:1 for petrol engines because above this ratio the tendency for detonation (non-uniform combustion) to take place becomes very strong. Diesel engines need compression ratios as high as 22:1 to make starting easier, because such an engine relies on the heat from compression to ignite the fuel. Valvegear The valves can be operated in a number of different ways. On most modern engines, the intake and exhaust valves are operated by separate camshafts, mounted above the valves, so that they open and close the valves via inverted cups or fingers. This is known as a double overhead camshaft layout. It is the most expensive arrangement, and is best able to cope with high revolutions. Some designs employ a single overhead camshaft, which is mounted between the two rows of valves, so that it operates the valves via rocker arms. The intake and exhaust valves will then share the same camshaft. This is less expensive, but less efficient at high revs than the ideal. A few engines, especially of American origin, employ a pushrod overhead valve layout. Here the camshaft is mounted in the engine block, and the valves are operated by vertical rods that are moved up by the cam lobes, and down by the valve springs. The upper ends of the pushrods operate the valves by means of rocker arms. This layout is not effective at high revs, because the valvetrain inertia due to the many moving parts means that above a certain speed the valves no longer follow the cam profiles precisely. It is a very cost-effective layout, and has the major advantage that the cylinder head can be removed without disturbing the valve timing. Ignition timing Spark generation and timing is mostly taken care of by an integrated electronic system. Some of the less expensive engines still employ a distributor and coil arrangement, but often without points. Instead, an electronic trigger is mounted inside the distributor. Main bearings These are usually found on each side of a big-end journal, especially in the case of in-line engines. This means that a four-cylinder can be expected to have five main bearings, and a six should have seven main bearings. In V-engines the con-rods belonging to cylinders opposite each other usually share the same big-end journal, so that a V8 should have five main bearings and a V6 should have four. However, some of the older four-cylinder engines have only three main bearings. This makes for a rougher engine, but there should be less internal friction, and the engine will perhaps use slightly less fuel. Fuel requirement Unless otherwise stated, all our tests are done on 95-octane unleaded fuel, or diesel fuel. Max power The maximum power output figure at maximum throttle opening is obtained from the manufacturer. This is usually measured under ISO (International Standards Organisation) or SABS conditions of temperature and pressure. The reference pressure and temperature is 99 kPa and 25 degrees celsius. This means that differen- ces between identical models arise from the effects of temperature and altitude. The accepted 18 per cent drop of power in Gauteng for normally aspirated engines is approximately correct. A kilowatt is a measure of how fast the torque is being delivered. Its value at any speed can be calculated by multiplying the torque by the revs, and then dividing by 9549,3 to get the correct units. Power peak The engine’s output varies with engine speed, and the engine speed where the maximum output occurs is the power peak speed. Max. usable r/min This is either the beginning of the red zone on the rev counter, or the maximum engine speed recommended by the manufacturer. Max. torque When a force causes rotation, the size of the force multiplied by the perpendicular distance from the line of action of the force to the axis of rotation is called the torque. In the case of an engine, the torque is measured at the flywheel by a dynamometer. This is a machine that enables the engine to work against an electrical, or frictional, resistance. The results are used to calculate the power output. For any particular engine, the torque is proportional to the average force that acts downwards on the pistons. This means an engine’s torque delivery is also a measure of how well it breathes. Torque peak This is the engine speed at which the engine delivers maximum torque. This could be anything between one-quarter to three-quarters the speed at which the maximum power is being delivered, depending on the valve timing. Transmission The value of knowing the gearbox and final drive ratios is that it makes it possible to calculate the available torque at the rear wheels in each gear. This is a far better way of comparing cars than just looking at the output figures, because gear ratios differ from vehicle to vehicle. (See separate panel.) Drive wheels The choice is between front-, rear- or all-wheel drive. Front-wheel drive is the most popular, because it leaves more interior space for the occupants and promotes a safe understeering cornering mode. Rear-wheel drive makes it possible for an expert driver to utilise both under- or oversteer during cornering, so that it is more common on very fast cars. All-wheel drive is just another name for four-wheel drive, and is generally found on off-road vehicles as well as cars intended for use on icy roads. Its use improves grip between the tyre and the road because each wheel gets only one-quarter of the engine’s output. Road wheels The first number shown is the wheel diameter in inches (one inch = 2,54 cm) measured from the base that the tyre sits in. The second number and letter refers to the tyre base width and rim profile. The possible letters are B, C or J, but most automotive wheels are of J design. For example, a 6,5J rim has a width of 6,5 inches (165,1 mm), but a 5,5J rim has a width of only 5,5 inches (139,7 mm). Both are of J-type, so the radii and smaller dimensions will be the same. Tyre dealers usually have charts on which these codes are explained.
Tyre size
The code on the side of the tyre gives a lot of information that can best be explained by picking a particular tyre. For example, a 235/45 tyre has a width, measured across the widest part, of 235 mm, and a height measured from the ground to the base of the wheel rim that the tyre sits in, of 45 per cent of this figure, which is 105,75 mm. The figure 45 represents the tyre’s aspect ratio, or profile. Percentages above 60 are known as high profile, whereas the lower percentages are called low profile tyres. Some modern profiles are as low as 35, because this increases road grip, but can be at the expense of ride comfort. All these measurements are taken when the tyres are installed on the car at the correct pressure.
The speed rating is given by a code, and ZR means “over 240 km/h”. The last number is the wheel diameter in inches, the measure still used for wheels. A chart showing all the above information is available for consultation at tyre dealers.
Brakes
Disc brakes are now the norm at the front on virtually all passenger vehicles, because they fade less when you brake hard and give more controllable braking. Some of the cheaper cars and most LCVs soldier on with drum brakes at the rear. This is not as bad as some journalists would lead you to believe, because the harder you brake the more braking effort is taken over by the front wheels, so that the rear brakes have less to do.
Hydraulics
This would include all the modern braking enhancements such as ABS, EBD, BAS, ESP, CBC and more. ABS (antiblockier-system) prevents wheel locking under braking. EBD (electronic brake- force distribution) distributes the braking effort to each wheel in proportion to the available grip. BAS (brake assist) monitors the brake pedal speed so that you may brake hard without it coming to your aid, but if you brake in a hurry it will call up full braking effort. ESP (electronic stability program) brakes one wheel at a time to prevent the car skidding out of control on a corner. CBC (cornering brake control) allows you to brake in a corner without the car veering to the outside.
Steering
Most modern cars have rack and pinion steering. The rack moves horizontally, and is connected to the road wheels. The pinion is a small gear wheel attached to the steering wheel shaft, and it engages with teeth cut on the upper side of the rack. In this way, steering wheel movement is transmitted to the front wheels. This system is reversible, so that some road wheel shocks will be transmitted to the steering wheel. This is nowadays considered to be an advantage because it gives more road feel to the steering. Older systems, such as ball and nut or worm and peg, are not reversible, and most road shocks do not reach the steering wheel, resulting in very little steering feel – ie the driver cannot feel how much grip the wheels have.
Lock to lock
This is a count of how many times the steering wheel has to be turned to go from full left lock to full right lock. It is a measure of how direct the steering is. Cars fitted with power steering seldom require more than three turns, whereas older American cars without power steering often needed up to five turns from lock to lock. This makes it very difficult to correct a skid.
Suspension
There are a number of suspension designs, each with strong points and weaknesses, but the main quality to look for is whether the suspension is independent or non-independent. Non-independent systems usually have a beam axle, whereas independent systems have suspension arms with struts, coil springs or torsion bars. Many front-wheel driven cars have a torsion beam at the rear; this is a semi-independent system. Independence implies that the wheels are free to move up and down on their own, but dependent systems force the left- and right-hand wheels to move as a unit.
Fuel tank
The useful fuel tank capacity is less than the figure you would get if you filled an empty tank until the automatic stop on the fuel hose kicked in. This is because when the engine comes to a stop through lack of fuel there will still be some fuel in the tank, but the fuel intake pipe is deliberately positioned so that the last litre or so cannot be used, since it is usually full of dirt. This may or may not be part of the rated fuel capacity, depending on the manufacturer’s measuring procedure.
It is also not the maximum that the tank can take, because most tanks have an air gap equal to about 13 per cent of total volume to prevent the tank bursting in an accident. When a pump attendant keeps trickling more fuel in after the automatic stop on the hose has been activated, this fuel goes into the air gap, and so makes a burst tank more likely. This dangerous habit is very widespread, and should be discouraged whenever you fill up.
Boot space
This is the space measured with ISO-standard 8 dm³ blocks and, where applicable (eg hatchbacks), with the luggage cover in place.
Utility space
Here the luggage cover has been removed and the seats folded forward. This gives the largest space that can be utilised.
CALCULATIONS USING ROAD TEST DATA
Gear ratios and wheel sizes can be used to compare vehicles more accurately than is the case when simply looking at power outputs.
Any gear ratio is a torque multiplier as well as a speed divider, but the ratio leaves the power output unchanged because power is equal to torque times revs, so that any gain in torque is offset by an equal loss of revs.
The Toyota Tazz engine has a maximum output of 103 N.m at 4 200 r/min, and is coupled with a gearbox having a top gear ratio of 0,82:1 and a final drive ratio of 4,06:1. At this speed, the driven wheels will get 103 x 0,82 x 4,06 = 342,9 N.m. However, the driven wheel revs will be 4 200 divided by (0,82 x 4,06), which is 1 261,6 r/min. Note that this calculation has not taken any transmission losses into account. This could be as much as ten per cent.
The rolling radius for various tyre sizes can be obtained from the tyre chart mentioned in the text. This enables maximum torque at the driven wheels to be converted to the force between the tyres and the road. For example, the Toyota Tazz is fitted with a 155/80 R13 tyres, with radius of 262 mm. The torque at the driven wheels is equal to the force between the tyre and the road, multiplied by the wheel radius, and this was calculated above to be 342,9 N.m. It follows that the torque divided by the wheel radius in metres will give the force between the tyre and the road. This is 342,9/0,262 = 1 308,8 newtons. In practice, this answer will be about 22 percent lower because of frictional losses in the transmission, and between the tyres and the road. On its own, this answer does not mean much, but when these calculations are used to compare vehicles, the true winner will emerge.
