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How to determine Compression Ratio

Discussion in 'The Garage' started by 91GMCSuburban, Nov 17, 2004.

  1. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    I've got a TBI motor of which I will slaping some vortec heads on. How do I determine the compression ratio for this combo?
     
  2. folkenheath

    folkenheath Worthless Trash Premium Member GMOTM Winner

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    If you know all of the specs I have an excel spreadsheet I made that you can input the variables and it gives you compression ratio, as well as dynamic compression ratio and other info.
     
  3. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    would you mind getting me a copy of this file?
     
  4. heavy4x4

    heavy4x4 1/2 ton status

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    Compression ratio is: volume at bottom-dead-center divided by volume at top-dead-center.

    Essentially, you'll need to know your bore size, stroke length, distance from top of piston to bottom of head at top-dead-center, and head combustion chamber size.

    That's sounds difficult to do, but it's pretty simple. A standard Chevy small block 350 has a bore size of 4.00 inches and a stroke of 3.48 inches. This computes to a single-cylinder volume of 716.62 cubic centimeters. Now, for the total combustion chamber size (volume at top dead center), you need to add the head cc's to the volume between the top of the piston and the bottom of the head at top-dead-center. This is a combination of top-of-piston to top-of-cylinder height at TDC and head gasket thickness. For the purpose of carrying out these calculations, let's call that distance 0.030 inches.

    If I remember correctly, the combustion chamber size of a vortec head is 68 cubic centimeters (might be 72...I'm drawing a blank right now). Adding in the volume stated above, we get a total combustion chamber size of 74.17 cc's.

    So, volume at bottom-dead-center divided by volume at top-dead-center = 716.62/74.17 = 9.66.

    These are all general calculations and will vary based on your head size, whether or not you've bored out your cylinders, and how much clearance you have from the top of the piston to the bottom of the head at TDC.
     
  5. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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    thanks,

    I like how you use inches and centimeters together so easily /forums/images/graemlins/bow.gif

    I understand now.
     
  6. folkenheath

    folkenheath Worthless Trash Premium Member GMOTM Winner

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    If you give me your e-mail address I can send you a copy of the spreadsheet I made. It accounts for all of the variables. The inputs it uses are..

    Number of Cylinders
    Bore (in)
    Stroke (in)
    Connecting Rod Length (in)
    Anticipated Redline (RPM)
    Gasket Volume (cc)
    Piston Volume (cc)
    Comb Chamber Volume (cc)
    Deck Height (in)
    Extra Quench Height (in)
    Intake Closing(deg)
    Boost Pressure (lbs/in2(psig))
    Volumetric Efficiency (%)

    And it outputs these outputs:

    Cylinder Volume (in3)
    Swept Volume (in3)
    Engine C.I.D.
    Engine Liters
    Connecting Rod Ratio
    Compression Height (in)
    CFM Flow
    Compression Ratio
    Dynamic Compression Ratio

    Of course you don't have to fill in all of the inputs just to get compression ratio. It is fairly simple once you look at the spreadsheet format. /forums/images/graemlins/wink.gif
     
  7. folkenheath

    folkenheath Worthless Trash Premium Member GMOTM Winner

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    I tried to send you the file with all the info to the e-mail address on your file, it didn't work. Is that old or am I just having problems? Please let me know if you would like the file still. Also, the Vortec heads are 64 cc, which if you have standard flat top pistons you're compression will be 9.85:1, assuming everything else is stock. /forums/images/graemlins/thumb.gif

    Inputs A
    Number of Cylinders 8
    Bore (in) 4.000
    Stroke (in) 3.480
    Connecting Rod Length (in) 5.700
    Anticipated Redline (RPM) 5500
    Gasket Volume (cc) 8.9
    Piston Volume (cc)1 -5
    Comb Chamber Volume (cc) 64
    Deck Height (in) 9.000
    Extra Quench Height (in) 0.015
    Intake Closing(deg)5 66
    Boost Pressure (lbs/in2(psig))2 0
    Volumetric Efficiency (%)3 90

    Outputs A
    Cylinder Volume (in3) 43.73
    Swept Volume (in3) 4.94
    Engine C.I.D. 349.8
    Engine Liters 5.7
    Connecting Rod Ratio 1.64
    Compression Height (in) 1.560
    CFM Flow4 501
    Compression Ratio 9.85
    Dynamic Compression Ratio 7.80
     
  8. 91GMCSuburban

    91GMCSuburban 1/2 ton status

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