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Old 07-09-2005, 12:13 PM
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stroke and compression question

I've heard stroke does not affect compression ratio but I can't see how it wouldn't. Could someone explain it to me? thanks bf

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Old 07-09-2005, 12:14 PM
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It does effect compression.
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Old 07-09-2005, 12:16 PM
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Here is a compression calculator. Note how stroke is one of the variables.

http://kb-silvolite.com/calc.php?action=comp
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Old 07-09-2005, 01:56 PM
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Size does matter

Given two engines with the same bore all else being equal, the one with the longer stroke will have a higher compression ratio.
Example;
4 inch bore X 3.5 stroke vs. 4 inch bore X 4 inch stroke.
75cc Combustion chamber. 0 deck to piston height clearance, 4.06 diameter gasket bore,.040 head gasket thickness = 83.5 CC. No allowance given here for valve reliefs or a piston dish or dome, in this equation.
Total unswept area is 83.5 CC.
4 inch X 3.5 inch = 750.5 CC cylinder displacement
4 inch X 4 inch = 823.4 CC cylinder displacement

750.5 + 83.5 = 834 CC, Divided by 83.5= 9.99:1 compression ratio.

823.4+ 83.5= 906.9 CC, Divided by 83.5= 10.86:1 compression ratio.

Stroke does matter in compression ratio, as does the bore.
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Old 07-09-2005, 02:00 PM
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An engine's displacement, which if you think of the word displacement, is how much air is actually moved or displaced when the piston moves downward on the intake stroke.

Simple geometry is all we need to know how much volume a cylinder holds, which is pie times the radius of the bore squared times the stroke (how far the piston acually travels). This forms an imaginary cylinder in the engine that has length, the top would be the top of the piston at TDC, and the bottom would be the top of the piston at BDC.

The compression ratio of an engine is determined by how much that cylinder's volume is squeezed into the combustion chamber, this is expressed in terms of a compression ratio. If the air of the cylinder is squeezed into a space five times as small as the actual displaced volume, the ratio would be 5:1.

If you increase the cylinder's volume by either making it longer (more stroke), or by making the bore size a larger diameter (bore), you have made the cylinder larger, making it ingest more air, so if that amount of air is squeezed into the same sized combustion chamber on the cylinder head, your compression ratio is going to increase.

So, if all other factors are equal, making the stroke longer will increase displacement, and it will also increase compression ratio.
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Old 07-09-2005, 03:54 PM
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so if I bored a motor and kept the stroke and all the other factors the same then the compression would drop, right? If this is true, then if I bored and stroke a motor to the right combination I could end up with the same compression I started with before boring and stroking? thanks bf
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Old 07-09-2005, 04:13 PM
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"so if I bored a motor and kept the stroke and all the other factors the same then the compression would drop, right? If this is true, then if I bored and stroke a motor to the right combination I could end up with the same compression I started with before boring and stroking? thanks bf"

No, That's not right...If you increase the bore and leave everything else the same the compression ratio would increase...
JA
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Old 07-09-2005, 04:34 PM
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If you bore the engine, the displacement will increase or, in other words the engine will be larger. If you increase the length of the crankshaft throw, which is the same as increasing the stroke, the displacement will increase.


In either situation Brian, you will wind up with an engine that has more cubic inch displacement. If you increase cubic inches and compress it into the same size combustion chamber as it had before the compression ratio will increase.

You need to think and study about what CID means.

Learn to compute the volume of a cylinder.

Once you can do that, you can compute the displacement of any piston engine.

Try to think of it this way: If you were to take a syringe, and you pulled the syringe down to let's say, one inch down from the very top. If the needle end of the syring is open, it's going to pull in a certain amount of air. If you pull the syringe down one inch and let the air come in, then you cap the top of the syringe and push it back up, you will compress that air.

Now, if you do the same thing, but pull the syringe down 2 inches, cap the top and compress it, you have increased the compression, because the you have increased the stroke of the syringe piston from 1 inch to two inches.

As you pull the syringe down, you are pulling air in with it. You are creating an area of low pressure inside the syringe, the air pressure outside that has higher pressure will rush inside and equalize, so you have displaced the air, which is to say, you have caused it to move from outside the syringe to inside the syringe cylinder by creating a pressure drop.

Think of the syringe body as a cylinder, and the other moving portion of the syringe as a piston. If you increase the diameter of the syringe, it's going to pull in more air also.
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Old 07-09-2005, 04:50 PM
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increasing displacement

Whether you bore or stroke an engine, with all other factors being the same, it will boost the compression.
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