I have always been a fan of the various automotive magazines available in the country as they have always fulfilled my desire of quenching my thirst in the automotive industry during the times when the internet was still a novelty for many.
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.
Some examples:
- 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.
Some examples:
- 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
Some Examples:
- 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.
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