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Old 03-05-2003, 05:39 PM
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Post compression calculation

I was woundering if you could figure out your compression ratio using this formula.

COMPRESSION = 14.7 * ratio

Lets say a cylinder has 125 psi
could you divid atomspheric (14.7) into your compression in the cylinder at sea level and get a compression ratio?

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Old 03-05-2003, 08:26 PM
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Not sure.........

but I know this.

72 340, stock rebuild with long duration cam, about 8.2:1 compression - 130-140 psi

74 318, completely stock with 8.5:1 compression - 125-145 psi

If you're getting 170's, you've got near 9.5:1, or at least that's a good guess.

Something over 200 I would bet would be in the
neighborhood of a mid-high 10:1, maybe more depending on the cam choice.
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Old 03-06-2003, 06:46 AM
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Here's a thread that might help you out:

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Old 03-06-2003, 07:01 AM
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basically, it depends if you want to know you static compression or dynamic compression.

Static compression is a measurment of the swept area of the cylinder vs. the compressed area, e.g. my engine has a total swept volume of 716.62 CCs and a combustion space of 95.55 CCs so I have a static ratio of 8.5:1

Dynamic compression is the pressure in the cylinder just before combustion vs ambient air pressure. So if your getting a reading of 125 psi then your running a dynamic ratio of 8.5:1

cam and ring seal both play a big part in dynamic compression ratio, among other things, a low compression test reading can indicate all sorts of cylinder / piston / ring wear, or even just a poor cam selection.
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Old 03-06-2003, 03:13 PM
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Compression calculations can be done on a theoretical basis by comparing the compressed volume to the uncompressed volume. As andybird correctly states, dynamic compression takes on all the variables that get into play in the combustion chanber at real time. Probably the bigges issue is valve overlap. Big cams bleed compression off and therefore, a static compresion will be much higher than dynamic measurement for a cam with significant overlap. This is why you can often challenge the rule of thumb that 9.5:1 is the most you can run on street gas. In fact, I run a 10.3:1 at sea level with no detonation. Also, properly building the motor to take advantage of quench area will also let you get away with a higher dynamic compression.

But when building a motor, select your parts and do a static calculation. It is a very good way to avoid a tragic waste of money.
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