compression ratio calculator

Discussion in 'Technical' started by JHodges, Mar 20, 2010.

  1. JHodges

    JHodges thumper

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    I will have flattop pistons in a 351w. Summit says the piston head volume is +12cc, I will have 69-73 351w heads. I believe they are around 60cc chamber. I was wondering what the compression ratio would be roughly. I am figuring around 9.5-10 to 1. I have seen the online calculators but I am not sure what to put on some of the things. Thanks for any help
     
    JHodges, Mar 20, 2010
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  2. baddad457

    baddad457 Member

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    To figure the ratio you need to divide the bore by 2, then multiply the answer by that same number. Then multiply by Pi (3.1416), then by the stroke (3.5 for a 351) this will give you the cubic inches of one cylinder. Next to convert to cc's, multiply the cubic inches by 16.387. This is the swept volume of the cylinder. Now, you need to add the chamber volume, the piston top volume (this is the area of the valve reliefs and dish if any) plus the area between the piston and wall down to the top ring. Most flat tops with 4 reliefs this volume will be about 5 cc's. Then you have the head gasket volume (9.5 cc's for a .043 gasket) and any deck clearance. The deck clearance can be figured the same way the swept volume is, except multiply by the distance from the deck to the piston top at TDC. With a zero deck, you can skip this part. 4.03/2 = 2.015 x 2.105 =4.06 x 3.1416= 12.75 x 3.5= 44.625 x 16.387= 731.27. then add the compressed volume to this(swept volume), then divide that by the compressed volume to get the ratio. I don't know what exact pistons you have, or the deck clearance, so you'll have to figure the result on your own.
     
    Last edited: Mar 20, 2010
    baddad457, Mar 20, 2010
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  3. dkstuck

    dkstuck Member

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    dkstuck, Mar 20, 2010
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  4. Mavman72

    Mavman72 Gone backwards but lookin' forward

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    That calculator is pretty spot on as long as you know how far down the bore the pistons are at tdc. It showed 10.3:1 comp ratio for my blue mav with a 306.The ratio IS 10.3:1 on that engine...Pretty good.Wish I knew the dimensios for the 351 in the sprint...I could get a reasonable guesstimate for it.
     
    Mavman72, Mar 20, 2010
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  5. dkstuck

    dkstuck Member

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    Ah,, I know what you mean. The things we think of after we button the engine up!
     
    dkstuck, Mar 20, 2010
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  6. Mavman72

    Mavman72 Gone backwards but lookin' forward

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    I am gonna have to unbutton the Sprint engine some day...I didnt build it but am curious to know if its stock bore...How fat the cam in it actually is, what kind of pistons are in it (I know they are forged,thats all) Its a mystery to me but, it runs stupid strong!!!
     
    Mavman72, Mar 21, 2010
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