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Old 03-26-2001, 02:57 AM   #23
texan
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for JD and anyone else interested...

this is an excerpt from an older article of mine on compression science, it should answer the static vs. dynamic compression ratio question.



"Static compression ratio: the total amount of volume in the chamber at BDC vs. the amount at TDC (that's Bottom Dead Center and Top Dead Center, respectively). Now I gave some simplified versions of this in the last discussion to aid in explaining what I was talking about, but here's the actual formula...

Static Compression Ratio = (S+C)/ C
Where S= total swept volume and C= chamber volume, and...

Swept volume= (x² × pi) × stroke
Where X= radius of the bore (bore/2), and...

Chamber volume= cylinder head volume + gasket volume + piston volume + crevice volume

Now it's not important to be able to actually calculate these numbers yourself, as either Honda Motor Co. or your piston provider will have already done this.
What's important is to note that static compression ratio is entirely mechanical, being based solely on swept volume and chamber volume. Meaning that the amount of air flowing into the engine has no effect on static compression ratio. Now, for a quick explanation of VE...

VE (Volumetric Efficiency) is an expression of how much air is actually flowing into an engine vs. how much total air it can displace, and is always expressed as a percentage. For example, a Honda B16A motor has a displacement of 1.6L, so if you had 1.6L of air flowing into and out-of the motor at every 2 RPM (being that these motors are 4 strokes, it takes 2 revolutions of the crank to completely pump air in and out), you would have a VE of 100%. It's VERY important to know here that VE varies with RPM, as the engine's efficiency at flowing air is continually changing. In fact, VE is one of the most important factors to engine performance.

The entire torque curve can be indirectly expressed as a result of VE. The only reasons why it isn't directly proportional to VE are the air/fuel ratio, ignition timing, overall mixture homogeny (uniformity) and frictional losses. Other than that, the VE percentage is basically what makes the torque curve rise and fall on any dyno chart. In fact, it's basically a given to state that maximum torque output of any engine also comes at maximum VE. So OK, as VE rises and drops in an engine, so does power output. But what does that mean?

Well that question leads us to a concept called dynamic compression ratio, which is in truth, probably the most basic of all engine parameters. Th dynamic compression ratio is, simply...

Dynamic CR= static CR x VE

To illustrate, if you had a static CR of 10:1, and had a VE of 90%, you would have...
Dynamic CR= 10 x .9 (90% stated in decimal form), or 9:1 dynamic CR"




Now obviously the explanation as to what this means to performance and combustion dynamics has been left out, but this should give you all the math and background needed to understand each term. Hope this helps, peace.
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