I remember buying a magazine every month to satisfy my urge of knowing more and more about automobiles and I also loved reading reviews about the latest bikes being launched.
There were always some or the other terms that were mentioned but I never understood what it actually meant. And one such term was over square engine and its similar terms like under square engine. I never knew what it really meant, but I assumed it must be something related to the engine.
So, here I am, trying to write down what exactly these terms mean and by what way it benefits the rider. The engine size or capacity is generally the amount of the space the combustion chamber displaces and is measured in cc’s as we all are familiar with 220cc, 150cc engines mounted in various bikes.
The engine capacity is measured as Bore x Stroke. Here, Bore is the diameter of the cylindrical block and Stroke is the depth of the cylindrical block, as shown in the image.
The ratio of Bore and Stroke categorizes the engine. There are three different types of configuration available depending on the ratio
Short Stroke/Over square Engine: As the name signifies the Stroke is shorter than the Bore. Due to the relatively shorter stroke, the engine revs fast and is more suitable where a quick build of power is important, more importantly where the bikes make power at higher rpms.
- RTR 180 (62.5mm X 57.8 mm) Bore x Stroke
- Pulsar 220 (67mm X 62.4 mm), 180(63.5mm X 56.4mm) Bore x Stroke
Long Stroke/ Under square Engine: As the name suggests the Stroke is longer than the Bore. Due to the longer stroke, the engine makes good torque at relatively low rpms. It is important where the bikes’ pulling more (torque) at relatively low rpms is more important than the top speed.
- Royal Enfield Classic 500 (84mm X 90mm), Classic 350 (70mm X 90 mm)
- Hero Honda Karizma (65.5mm X 68.2mm)
The one exception to the above rule is the pocket rocket Yamaha R15 (57mm X 58.7mm) which is strange as R15 is a track oriented bike.
Square Engine: As a square has both the sides equal, here also the Bore and Stroke are of almost same size. The engine tries to strike a balance between torque and top speed
- Yamaha FZ-16 (58mm X 57.9mm)
- Honda Unicorn (57.3mm X 57.8mm)
All these engines have their own advantages and disadvantages and it depends on the rider to decide what kind of motorcycle he/she needs.