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Compression ratio and effective displacement

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    I thought about this last night and then ended up confusing myself, so I thought Id ask.

    So from my understanding, to change the tC's compression ratio, you change the length (and strength) of the connecting rods. But would that also change the displacement of the engine? Because if the rod is longer, on the downstroke, there is less space available for air right?
    So the compression ratio can be explained as such:

    You have to consider the volume of air in the cylinder at the bottom of the stroke. And then consider the volume of the cylinder when the stroke is at it's peak.

    So for example let's say you have 1000cc's of air at the bottom of the stroke. And at the top of the stroke after the compression stroke you now have 100cc's of air. If you break down the fraction that's a 10:1 compression ration.

    So if you want a higher compression ratio you can:

    A- stroke the motor (lengthen the stroke of rod, through crankshaft modification or rod length.)

    B- Change the top of the piston to compress more air at the top of the stroke i.e. dome piston. You'll see a lot of these type of pistons with valve clearance cutouts because of the close tolerance between piston and valves.

    C- Change cylinder bore size to allow more volume of air to enter the cylinder. Machining the cylinder out .090 will allow you to retain the same piston but you can use larger rings to seal the cylinder.

    D- Decking the head, or milling the head surface to change the volume of air that te dished area of the cylinder head.

    I'm sure I've left some info out but this is what I know off the cuff.
    Yes, compression ratio is the difference in volume in the cylinder chamber from TDC (Top Dead Center) and BDC (Bottom Dead Center) of the stroke.

    Brian has it correct with the ability of changing it with either stroking the motor with shorter/longer crank rods, boring the cylinders (this will also increase displacement of the engine or e.g. going from a 2.4L to a 2.5L...), swapping pistons where the geometry of the pistons change the compression ratio, and milling the head.
    Ok, I see. So would increasing the compression ratio decrease the displacement?
    Mayo wrote:
    Ok, I see. So would increasing the compression ratio decrease the displacement?



    Engine displacement is defined as the total volume of air/fuel mixture an engine can draw in during one complete engine cycle; In a piston engine, this is the volume that is swept as the pistons are moved from top dead centre to bottom dead centre.

    Compression ratio will not change the displacement of your engine. When you bore your cylinder walls you increase your displacement and change your compression ratio at the same time.

    Think of it as this;

    Compression ratio is the volume ratio of TDC and BDC.

    Displacement is the total volume of TDC and BDC.

    You can do tweeks to change the compression ratio without hindering displacement. Does that make sense?
    Ahh that makes a lot more sense. Thanks.
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