Motoring enthusiasts sometimes get roped into impromptu debates about whether engines should have long strokes or short strokes. Some people would be tempted to say: “Who cares”, but it is an important dimension. Why?
The values chosen for these two dimensions have an influence on the engine’s stress levels, rev-ability, combustion characteristics, maximum power output, overall size and mass, as we shall see. Note that in the following discussion comparisons are made between engines with the same cubic capacity. This implies that a shorter stroke must always result in a larger bore, and vice versa.
Let’s look at the choices:
A SQUARE ENGINE
This is one where the bore and stroke are equal in length. This layout has no particular merit apart from the stresses and other parameters that depend on the particular sizes chosen.
A LONG-STROKE ENGINE
This is also known as an under-square engine. It’s not really a long-stroke engine unless the stroke is at least 25 per cent longer than the bore. The stroke/bore ratio is thus 1,25.
The inertial (due to movement) stress levels in an engine are directly related to the stroke length. This means that a longer stroke implies higher inertial stress levels, which is why long-stroke engines are usually not designed to rev to high levels. Mild valve timing is usually employed, resulting in the good low-down torque delivery that this kind of engine is famous for.
The long stroke will result in a tall engine, but the small bore will keep the block narrow and short, and put a limit on valve sizes. This will restrict breathing at higher speeds. This is another reason why such an engine is known for good low-down torque.
A SHORT-STROKE ENGINE
Also called an over-square engine. To really qualify, the bore must be at least 25 per cent bigger than the stroke. The stroke/bore ratio is thus 0,8.
The short stroke will reduce inertial stress and this means that such an engine can be given valve timing that will make high-speed revving possible. The bigger bore that goes with the short stroke will enable larger valves to be fitted, and this factor, combined with the higher revs, usually means that short-stroke engines can be made more powerful than long-stroke engines. The short stroke will reduce cylinder block height, but the big bore will make the block wide and long.
It is interesting to note that during the last 100 years the stroke/bore ratios have at various times been more influenced by engineering fashion or even legal reasons rather than common sense. In the first 25 years after the car was invented practically all power increases were obtained by increasing the bore and stroke. This resulted in bores as much as 200 mm and strokes of over 300 mm. Four-cylinder engines of ten or twelve litres were quite common. By 1910 engineers woke up to the advantages of a short stroke, but then the British government brought out a motoring tax using a formula that punished the bore size. This resulted in English designers going back to long strokes with the result that that country’s engine development was set back compared to Europe until 1947 when this particular formula was abolished.
