Understanding the Specification Sheet

Manufacturer specification sheets are a great way to cross-shop between your shortlisted options while within the comfort of your own home. We all know to look out for things like peak power and the total number of airbags, but if the more technical details leave you head scratching, fear not. Oneshift has prepared a handy guide to understanding how those numbers translate into real world performance. Read on and you could be better clued in to deciphering how your next purchase drives.

The piston heads in your engine compress air and fuel in a cylinder in order to produce an explosive reaction following ignition to generate power. The volume in the cylinder that they displace is measured by the bore and stroke, with bore corresponding to the diameter, and stroke the height of the cylinder.

Just as variable valve timing and turbocharging alter how your engine burns fuel through the rev range, cylinder dimensions alter how that energy is transmitted through to the gearbox.

Oversquare or short-stroke engines are often tuned to develop peak torque at higher engine speeds, since a larger bore allows for larger valves in the head of the cylinder, allowing the engine to consume more air and fuel at high engine speeds when the valves do not have a long time to stay open. More importantly, a short-stroke design also keeps the distance the piston head has to travel as it moves up and down the cylinder per engine revolution short, minimising piston ring speed to achieve higher engine speeds. Imagine it like this: if you need to do the shuttle run in a certain time, to achieve more laps you cut down the distance between the two points, rather than running faster.

On the other end, undersquare or long-stroke engines exhibit peak torque at lower engine speeds, for exactly the opposite reasons. Having a longer crank throw and high piston speed, puts more force behind the cylinder for each engine stroke, translating to more torque at lower engine speeds. Compare the bore and stroke measurements of your engine to where the stated peak torque is generated and shop according to what you favour: rev-happy, spirited driving types will favour the first, while those that prefer to keep their engine lazily humming away will find the latter more suitable.

Honda's K-series engines were notable for their use of a perfectly square cylinder size, at 86mm × 86mm. Clever valve timing technology gave them stratospheric redlines.

Broadly speaking there has been a gradual move towards undersquare designs as a result of increasing pressure to raise efficiency. With smaller piston crowns and a more compact combustion chamber, heat lost to the chamber walls is reduced, and the flame travel needed to consume the fuel-air mix is shortened. On top of this, not only is the broader spread of torque favourable in a road car, but this enhanced flexibility also allows for greater application across car types, allowing for the development of modular engine designs that cut manufacturing costs by sharing component parts.

Another observable trend is the widespread use of 500cc-sized cylinders. With a modular engine architecture, this allows engine designs to be built in one-litre increments, ideal for most tax regimes around the world. But the 500cc cylinder size also strikes a very good balance between minimising heat loss with larger cylinders, and keeping frictional losses from having extra cylinders to a minimum.

BMW Group’s family of engines stick strictly to the 500cc per cylinder template, even down to 1.5 litre 3 cylinder options.

The compression ratio represents the ratio of the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke, to the volume of the combustion chamber when the piston is at the top of its stroke.

Why does this matter? You want to get an engine with a high compression ratio because higher compression allows an engine to extract more mechanical energy from a given mass of air–fuel mixture and thus achieve higher thermal efficiency. A higher compression not only allows the same combustion temperature to be reached with less fuel, but more importantly also gives a longer expansion stroke, giving more time for the hot gasses to expand and push against the piston head, thus creating more mechanical power output and lowering the exhaust temperature.

So why don’t compression ratios increase indefinitely? Unfortunately, many modern engines have already reached a peak whereby further increasing the compression ratio will run the risk of engine knocking or pre-detonation of the fuel-air mixture in the combustion chamber. High performance engines overcome this by demanding the higher-grade fuel you see at the petrol station, while regular engines that are built to run on RON 95 resort to clever technology like direct fuel injection and shaped piston crowns to ensure an even fuel-air mix in the combustion chamber, along with a whole host of acoustic-knock sensors that can alert the engine electronic control unit to adjust ignition timings as necessary. The upshot of all of this as a consumer is that a higher compression ratio is also reassuringly indicative of the presence of better fault detection and electronic management in your engine, important contributors to the general reliability of your engine.

One other thing to note: turbocharged engines are going to run at lower compression ratios next to their naturally-aspirated counterparts, since the intake charge is already “pre-compressed” before entering the cylinder. For some comparative figures, the Mazda 6’s naturally aspirated SkyActiv-G engine runs at a compression ratio of 13.0:1 and is widely considered to be a high compression engine for its fuel grade. Meanwhile, a more typical turbocharged engine, such as that in the Honda Civic 1.5 Turbo, runs a compression ratio of 10.6:1.

SkyActiv-G engines well known for their high compression. Next generation SkyActiv-X seeks to further raise compression still.

If you have a narrow driveway or park in an old multi-storey, you will know not to get a large car. Vehicle dimensions like length and width are obvious, so why is the vehicle wheelbase always included here in the specification sheet?

The wheelbase of a car differs from the total length since it measures only the distance from the centre of the front wheel to the centre of the rear.

A vehicle's wheelbase is actually widely used within the industry: engineers benchmark platform performance values such as torsional rigidity against other cars with a simillar wheelbase, for example, and emissions standards and market segment are also determined by vehicle wheelbase and interior space in many places around the world.

A consumer shopping for a car with generous legroom for all passengers will want to take note of the wheelbase of their shortlisted options since the passenger cell of the vehicle is always located between the two wheel axles. A longer wheelbase thus translates into more area within the car dedicated to housing the cabin. Do also consider how different suspension types will eat into your interior space. Front-engine, front-wheel drive cars that sport torsion-beam rear suspension systems typically do so to save on complexity and cost, however, they also provide the advantage of being easier to package, giving engineers greater flexibility over seat mountings and allow more trunk or rear legroom.

A longer wheelbase can also aid in improving ride quality, since the front wheels are given more time to settle back into position before the car drives over the same bump with its rear wheels.

The new Mercedes A-class comes with a torsion-beam rear suspension on non-performance models – space saving design meant to accommodate battery packs for future hybrid models.

Drag coefficient is a measure that is used to quantify the drag generated by the air as your car passes through it. There is something important to understand here: It is only meant to clue you into the aerodynamic efficiency of the shape of your car, and you can only derive the actual drag force generated at a given speed only after multiplying this coefficient with your car’s frontal area. A smaller car thus does not necessarily have a lower drag coefficient, even if it may have a lower total drag force acting on it at a given speed.

The lower the drag coefficient, the more efficient your car’s shape is at slicing through the air.

Why does this matter? Beyond just the obvious opportunity to save more fuel, since tyre and wind noise are the largest contributors to vehicle noise at highway speeds, you will want to shop for something with a lower drag coefficient if you are looking for a quiet ride and frequent our expressways daily. Other things you can do to keep drag to a minimum include: removing the roof rails and running boards on your SUV, or opt not to go for the more fussy body kit options when specifying your next car.

One additional thing to note here: while a lower drag coefficient corresponds to greater efficiency, you will find various performance cars with relatively higher drag coefficient numbers. This is because at high speeds, maintaining stability becomes a larger priority for these cars, and one way to do so is through generating aerodynamic downforce which requires a compromise in increased drag.

Fuss-free styling keep the drag coefficient for the Mercedes S-class at a low 0.24. Keeping cabin noise to a minimum is a priority in this segment.

Supplement your newfound knowledge with our extensive reviews so you have a better idea of which options to shortlist and save yourself fruitless trips to the showroom!

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