We love or hate a specific machine based on its character. Now, an engine, among a set of many other things, is probably the most definitive component that defines this so-called character. Whether a motor is a racy, high-revving unit or a lazy lump with oodles of low end torque depends a lot on the engine’s architecture. While combining cylinders in various configurations such as in-line, V, or horizontally opposed, creates a distinctive character in each case, the bore and stroke of the cylinder itself is a very important aspect in defining a motor’s properties. Here in this piece, we will talk about long stroke and short stroke engines, and how the two configurations affect their disposition.
What is bore and stroke?
Before we get into the nitty-gritties of long and short stroke engines, we first need to properly understand the concept of bore and stroke. An engine’s cylinder is like a well finished circular hole carved out of a metal block, which makes for its volume, cubic capacity or displacement. Here, bore is the diameter of the circular opening at its end. The stroke, on the other hand, is the depth of the hole. Look at the following image to understand how a typical cylinder looks like. We have clearly marked out its bore and stroke for better understanding.
Bore to stroke ratio
As you would understand, carving out a cylinder inside a metal block can be done in various ways. For any given cubic capacity, one can make the circular opening too big, with lesser depth, or vice versa. Now an engine with the former cylinder configuration is termed as a short stroke engine and latter a long stroke one. If one makes a cylinder with exactly same bore and stroke measurement, the resulting engine is called a ‘square’ cylinder. That’s the reason why a short stroke engine is also called an over-square engine while a long stroke engine is termed as an under-square engine.
Properties of an over square or short stroke engine
For any given cubic capacity, an over square engine has to move lesser as it has a wider bore. These engines also have lesser inertial stress which allows them to be equipped with faster valve timing. This lends a short stroke engine, in general, the ability to rev higher than its long stroke counterparts. With a bigger bore, these engines also have the space for better intake and exhaust valves to be fitted on the head. As you would imagine, with a high revving engine with bigger valves it’s easier to create relatively more power – displacement being constant.
A short stroke or over square engine typically produces its peak power relatively higher up the rev range. This is a great property to have for high-revving lightweight sports bikes which are meant to accelerate hard and is used preferably for enthusiastic riding on street or track. While the top end power on such engines is higher than their long stroke counterparts, they don’t produce as much torque lower down the rev range, which affects their tractability at low engine speeds, well, relatively speaking.
Properties of an under square or long stroke engine
Since a long stroke engine has a smaller bore, and the piston has to travel longer for any given displacement, by nature, the inertial stress in this variety of engines is relatively higher. To start with, this means that these engines cannot rev as high as their over square counterparts, so the valve timing for this variety of engines is relatively slow. Also, since the bore is not very wide, there is relatively less space for bigger valves which, again, means some restriction on feeding in combustible charge. All this, along with relatively heavy mechanicals, leads to long strokes engines producing less power than short stroke engines of similar displacement.
Now, while the power produced by a typical long stroke engine is relatively less, it has its own advantages. These engines produce a much healthier torque at the lower range of the rev spectrum lending themselves to torquey, lazy revving machines very well. Big, heavy motorcycles, long-legged tourers and cruisers, motorcycles which need load-lugging capability and have an easy going, relaxed demeanor are the ideal products for installing these engines.
Got your own thoughts to share about long VS short stroke engines? Opine. Got a question? Shoot away! We’d be keenly listening, and would be very happy to respond.