engine basics 101 - Subaru WRX Forum
 
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#1 Old 06-24-2011, 09:49 PM
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engine basics 101

It has been suggested that I discuss engine basics in a way, so as to help others understand how an engines parts make up how it performs. I will talk directly about engine bore and stroke here.
Any questions or corrections please PM me so as not to clutter up this thread for ease of reading by others.

Bore is the diameter measurement of the cylinders in a piston engine.

Stroke can refer to the distance the piston travels as well as the cycle the cylinder is on, i.e. intake stroke, compression stroke, power stroke and exhaust stroke for a typical 4 cycle, or 4 stroke engine.

The ratio between bore and stroke can determine the general characteristics of an engine and how it behaves.

The bore and stroke of the engine decides the size or displacement of the engine. The larger the bore and stroke, the bigger the displacement and thus a more powerful engine.

An engine’s bore and stroke are generally chosen to keep the piston speed to a reasonable level for a street driven car to obtain longevity which is usually around 66 ft/s.
This would be on any basic gas powered engine.

A big-bore, short-stroke engine will have the piston traveling a shorter distance than one with a longer stroke but smaller bore, and this will help keep the piston speed down. In addition, the bigger the bore, the larger the valves that can be fitted inside the combustion chamber, leading to potentially more air into and out of the engine.

Because of the shorter distances a big-bore engine’s pistons have to travel, such an engine will tend to be quite free-revving and will make power higher up the rev range than an equivalent long-stroke engine. However, the long-stroke engine will normally be tuned to give more torque, for a given cylinder pressure, at lower engine speeds, since it will not perform as well as the engine speed rises. This can be shown in the 2.0L and 2.5L engines in Subarus. The smaller bore shorter stroke of the 2.0L gives it more ability to rev higher and faster than a 2.5L, but because of the displacement, or bigger bore and stroke size, of the 2.5L it will have more low end power or torque than the 2.0L because of it's ability to take in more air. Now there are other factors that come into play, but I am just using these as a reference and not taking into account forced induction or any type of power adders.

An engine that has a wider bore than stroke is referred to as oversquare or shortstroke engine. An engine that has a shorter bore than stroke is referred to as undersquare or longstroke engine. An engine that has an equal bore and stroke is referred to as square engine. Usually engines that have a bore/stroke ratio of 0.95 to 1.04 are referred as square engines.

So a 2.0L has a bore of 92mm and a stroke of 75mm which makes it oversquare. The 2.5L is the same but it has a bigger 99.5mm bore and a larger 79mm stroke.

More to come I need some sleep....................

Resident Grumpy Old Man(Keith)
03 SRP 2.34L
08' LGT Gears...
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