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  • CHR Technical Library

    by Published on 04-28-2011 11:41 PM  Number of Views: 13536 

    Static compression ratio, domed pistons, how to figure the math.
    We'll use a 454 Big Block Chevy for this explanation. Let's say we're using the stock bore and stroke of 4.250" and 4.000" and production cast iron heads with 122cc chambers. The pistons are down in the bore 0.025" (piston deck height) and have a dome volume of 28.7 cc's (get this info from the piston mfg or figure the dome volume yourself: See Determining Piston Dome Volume). To figure static compression ratio, you need 5 different values...

    1. cylinder volume in cc's
    2. combustion chamber volume in cc's
    3. piston crown volume in cc's (+ or - or flat with eyebrows)
    4. piston deck height volume in cc's (volume above the piston crown up to level with the block deck with the piston at TDC)
    5. head gasket volume in cc's

    Let's take these one by one so that you and others will know how to figure SCR with a domed piston....

    1. We know that the bore is 4.250" and the stroke is 4.000", so we multiply (.7854 times 4.250" times 4.250" times 4.000" times 16.387) and find that there are 929.88 cc's in the cylinder.

    2. Anyone building a motor should use a burette and colored alcohol to "pour" the chambers and determine the exact volume in each and every chamber (yes, they differ one to the other, but can be equalized prior to assembly), but I doubt that many of you fellows do that. These heads are advertised at around 119cc's, but production heads will usually pour a little bigger, so let's call them 122 for this exercise.

    3. OK, we'll do a little play acting here. We will visualize taking a sharp instrument and slicing the dome off the piston, leaving only a flat plane crown with no eyebrows. So, here we are holding a lump of aluminum that displaces 28.7 cc's. We are now going to jam that lump up into the combustion chamber to reduce the chamber volume to 93.3 cc's. This will result in a piston crown value of zero.

    4. We have no idea what the piston deck height is on this motor, but let's just use a theoretical value of 0.025". So, we have a bore of 4.250" and the piston is 0.025" in the hole at TDC. Remember, the piston is now a flat-top, so we will simply multiply (.7854 X 4.250" X 4.250" X 0.025" X 16.387) and find 5.81 cc's.

    5. I'm going to assume that the decks of the block and heads are flat enough to use a Mr. Gasket shim head gasket, #1131, 4.370" X 0.020". This will put our squish at 0.045". Again, figuring cc's just like anything else that's round and has a thickness......(.7854 X 4.370" X 4.370" X 0.020" X 16.387) and find 4.92 cc's in the head gasket.

    Now, we add all our values together....
    929.88 + 93.3 + 0 + 5.81 + 4.92 and find a total volume of 1033.91 cc's.

    Now, we will drop out the cylinder volume and add the other 4 values together....
    93.3 + 0 + 5.81 + 4.92 and find a compressed volume of 104.03 cc's.

    Now, we will divide the larger volume by the smaller volume (1033.91 / 104.03) and find 9.94:1 static compression ratio.

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