Engine size: Why bigger isn’t always better

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09 Oct 2017 by smartleasing

Choosing the right engine size used to be a straightforward proposition: if you wanted to go fast, you got a car with a big engine; if you wanted to save fuel, you went for the opposite. Today it’s not quite so simple. Varied and rapidly advancing technology means similar cars that are direct competitors can have vastly different sized engines.

Enter the turbo

The turbo is a piece of technology behind much of today’s engine-capacity complexity. A turbocharger is a compressor that forces more fuel and air into an engine, allowing it to burn more fuel and produce more power than a non-turbo engine of the same capacity.

As a result, a small engine can be tuned to crank out the power of a much bigger engine. Buyers can find the power they might expect to get from a bigger engine through a smaller engine with a turbocharger. This is the engine downsizing you’re hearing about.

However, because the turbo is essentially a switch that only comes on when needed, the fuel-burning potential of a turbo engine isn’t always bigger than its regular capacity through its range of operation. This is how a turbo-charged engine offers potential fuel-economy benefits over a bigger engine.

Apples and oranges

If you’re lucky enough to be comparing apples with apples (that is, two different-sized turbo engines or two different-sized non-turbo engines) the traditional rules still apply. Bigger means more power, smaller means better economy.

But comparing apples with oranges is the more common showroom-floor dilemma. In the small-car class you’ll find some entry-level contenders with 2.0-litre non-turbo engines, some with 1.4 turbos and even some with teensy-weensy 1.2 turbos. Some models, such as Honda’s Civic, cover both sides of the engine-size spectrum within their own range. Which means it’s all about the numbers.

Understanding the numbers

The only way to determine the merits of different engines is to stack up their performances on the power, torque, drivability and economy charts.

Take the Honda Civic’s two engines, for example. One is a 1.8 litre, the other is 1.5 litre. Logic would suggest the 1.8 is the ‘hero’ engine but in fact it’s the other way round.

The 1.5, which is equipped with a turbocharger, direct injection and other contemporary technology, doesn’t just outpunch its more dated, non-turbo sibling for power (127kW vs 104kW) but produces more peak torque (the engine’s rotational force) lower in the rev range (220Nm between 1750-5500rpm vs 174Nm at 4300rpm), which essentially means it doesn’t have to work so hard to deliver its stronger response.

It also uses less fuel (6.1L/100km vs 6.4L/100km).

Exceptions to the rule

So a smaller capacity turbo engine is always preferable to its bigger, non-turbo equivalent, right? In theory, probably. In reality, mostly – but not exclusively.

A small-capacity turbo can drink just as much as its bigger, non-turbo equivalents when worked hard, and some engines ask for premium, high-octane fuel (which is more expensive), where a non-turbo might operate on regular fuel.

In heavy-duty applications such as towing, a larger capacity engine might deliver better off-boost (non-turbo) drivability because a smaller proportion of its power is derived from turbo assistance. All turbos suffer from some degree of delay (known as lag) between asking for and getting full acceleration, so many keen drivers prefer a non-turbo engine’s sharper, more linear throttle response.

In essence, determining what sort of engine and which size will suit you turns on the kind of driving you do. It might take a little research and reading before you come to grips with the seemingly arcane numbers associated with each engine but the potential fuel savings and lower annoyance make it worth it in the long run.

Did you know…

Smartleasing can organise your test drives for you, putting you in touch with our trusted dealer network to ensure your experience is low on sales push and high on service. In fact, we do most of the legwork for you.

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