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Old 02-14-2009, 12:58 AM
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figuring out compression ratio?

how do i figure out what my compression ratio is? im going to be switching parts from one block the the next. the block im going to be putting the heads onto has flat top pistons and the motor its coming off of doesnt have flat tops. how would i go about figuring out cylinder head volume? i must be missing something what else goes into the equation for figuring out comp ratio?

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Old 02-14-2009, 03:17 PM
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Old 02-14-2009, 03:36 PM
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compression ratio is.......

the biggest volume possible divided by the smallest volume possible

the biggest volume possible is when the piston is at the bottom of the stroke and the smallest volume possible is when the piston is at the top of the stroke.


the biggest volume is a sum of the following:
1) One cylinder volume from the bore and stroke
2) volume from the piston (+dish, - dome)
3) volume from the head gasket
4) volume from the deck height (how much the piston is below or above the deck)

The smallish volume possible is the same as the biggest volume except for the cylinder volume which would be zero.

So if you have a 350 (717cc/cylinder) with a 0.040" head gasket (9cc), 64cc heads, flat top piston with 2 valve reliefs (6cc), and a deck height of -0.015" (4.5cc) then the compression would be 9.6:1. see math below.

(717+ 9 + 64 + 6 + 4.5) / (9 + 64 + 6 + 4.5) = (717 + 83.5)/83.5 = 9.6

Last edited by 454C10; 02-14-2009 at 03:45 PM.
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