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psinuse 06-24-2011 09:49 PM

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....................;)

imitationpro 08-07-2016 11:03 PM

[quote=psinuse;263200]More to come I need some sleep....................;)[/quote]

Said in 2011....

mosc 08-11-2016 08:39 AM

@psinuse finish this so I can sticky it

psinuse 09-26-2016 08:28 PM

[quote=mosc;576506] @psinuse finish this so I can sticky it[/quote]

I'll have some down time here soon now that track season is almost over. I'll get on this soon.

titter 09-27-2016 05:35 AM

welcome back buddy! how was the season?

psinuse 09-30-2016 06:39 PM

[quote=titter;581185]welcome back buddy! how was the season?[/quote]

Rather hit and miss. Had some suspension issues and tuning with the car, tires, brake upgrade, etc etc. I also built a car for Race Lab Rally School in Canada, up near your neck of the woods in Toronto, I have been busy doing both rally and road courses. Managed to get hooked up with Crazy Leo and and he started this school, so needless to say, I've been going non stop for months...

titter 10-01-2016 07:12 AM

thats awesome! i've heard good things about those guys down there toronto. havent had a chance to check em out though. havent had a change to do anything this summer actually lol, been too busy prepping for a drap strip that im never going to hit this season lol :(

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