21 What is a Long Stroke / Short Stroke Engine? Which is better?

What is actually a Long/Short (Under/Over Square) Engine? How do they differ in engine characteristics?

I clearly remember that when the TVS Apache RTR 160 was launched in 2007, TVS made sure to highlight (at least in the print media) that the engine of the RTR was an "Over Square" one. But there was never a proper explanation of what the term "over square" engine meant or what exactly was it's benefit for the rider.

Let me try to explain these terms in as simple English as possible.

The engine size along with the cubic capacity (cc) is also specified as "Bore x Stroke" dimensions of its cylinder.

Bore: The Diameter of the cylinder block

Stroke: The Depth of the cylinder block







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What is a "Short Stroke Engine" (also called as Over Square Engine): The Stroke is "shorter" than the Bore.

Engine Character of a Short Stroke Engine: Due to the relatively shorter stroke, the engine revs fast and is good for bikes where fast build up of power is essential (eg. Race/track bikes, Supersports bikes).

[Update 29,Oct 2010: I have recently learnt that for an engine to be termed as "Over Square", the ratio obtained by dividing the numerical value of the Bore by its stroke, should be around 1.2 (plus - minus a few decimal points is considered o.k)

In which case the Apache RTR 180, Pulsar 150/180/220 would not qualify as ones with over square engines, as posted previously.]

"Short Stroke Engine" bikes in India:

• Apache RTR 160 (62mm x 52.9mm)

What is a "Long Stroke Engine" (over called as Under Square Engine): The Stroke is "longer" than the Bore.

Engine Character of a Long Stroke Engine: Due to the longer stroke, the engine makes good torque at relatively lower engine revs. It is good for bikes where torque (pulling capability) is essential at lower/relaxed engine revs (Eg. commuter bikes, touring bikes)

Some "Long Stroke Engine" bikes in India:

• Royal Enfield Thunderbird Twinspark (70mm x 90mm)

Square Engine: Engines where the "Bore" and "Stroke" are almost of same size. These engines theoritically seem to balance the best of both worlds.

[Update 29,Oct 2010: I have recently learnt that an engine to be termed as "Square", the ratio obtained by diving the numerical value of the Bore by its stroke should be around 1 (plus-minus by a few decimals is o.k).

In which case the Apache RTR 180, Pulsar 150/180/220 qualifies as ones with "Square" engines, and not as "Over Square" engines as posted previously.]

Some "Square Engine" bikes in India:

• Hero Honda CBZ Xtreme/Hunk & Honda Unicorn (57.3mm x 57.8 mm)



• Yamaha FZ-16/FZ-6/Fazer (58mm × 57.9mm)



• Apache RTR 180 (62.5mm x 57.8mm)



• Pulsar 150 (58mm x 56.4mm)/180 (63.5mm x 56.4mm)/220 (67mm x 62.4mm)



• Hero Honda Karizma (65.5mm x 68.2mm)



• Yamaha R15 (57mm × 58.7mm)

Which kind of engine one is better?

Well, it depends on what type of rider you are.

If you are of the sporty tyre and like to rev the engine hard, "Over Square / Short Stroke" engines should rock your boat.

If you like your bike to have good pulling ability at low engine revs or if you have to carry heavy loads (maybe a heavy pillion), then "Under Square / Long Stroke" engines should be ideal for your needs.

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• What is Engine Knocking? Do you really know?



• What is a "Superbike"? What is a "Super Sports" bike?

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