Applied Math

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Applied Math

• Compression Ratio

2
Compression
3
Compression Ratio
4
Compression Ratio
90 cu in
10 cu in
Compression Ration 9 to 1
5
Compute
Swept Volume of Piston Clearance Volume
Clearance Volume
45 cu in 7 cu in
Compression ratio
7 cu in
Compression ratio
7.4 to 1
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Compute Compression Ratio Intro Problem

• Determine the compression ratio
• Swept Volume 740 cu cm
• Clearance Volume 78 cu cm

Volume _at_ BDC
Cylinder volume Clearance Volume
Compression ratio
Clearance Volume
Volume _at_ TDC
740 cu cm 78 cu cm
10.5 to 1
Compression ratio
78 cu cm
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Changing the Piston Shape

• General assumption
• Flat top pistons.
• How would the compression ratio change if dished
  pistons or pistons with valve relief cutouts were
  used instead?

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Compute Compression Ratio Piston Shape dished

• Determine the compression ratio
• Swept Volume 740 cu cm
• Clearance Volume 78 cu cm

86.6 cu cm
78 8.6
Cylinder volume Clearance Volume
Compression ratio
Clearance Volume
740 cu cm 86.6 cu cm
9.5 to 1
Compression ratio
86.6 cu cm
It was 10.5 to 1 before the piston change
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Compute Compression Ratio Piston Shape dished

• Conclusion

Pistons that are dished or have valve relief
cutouts will lower compression ratio (if
everything else stays the same).
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Changing the Piston Shape Dome

• How would the compression ratio change if domed
  pistons are used instead?

Ford 351 Cleveland Dome 12 cc
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Compute Compression Ratio Piston Shape Dome

• Determine the compression ratio
• Swept Volume 740 cu cm
• Clearance Volume 78 cu cm

78 - 12
66 cu cm
Cylinder volume Clearance Volume
Compression ratio
Clearance Volume
740 cu cm 66 cu cm
12.2 to 1
Compression ratio
66 cu cm
It was 10.5 to 1 before the piston change
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Compute Compression Ratio Piston Shape Dome

• Conclusion

Pistons that are domed will raise the compression
ratio (if everything else stays the same).
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Compute Compression Ratio Problem 2

• Bore 3.985
• Stroke 3.000
• Clearance Volume 4 cu in

Cylinder volume Clearance Volume
37.40 cu in Clearance Volume
37.40 cu in 4 cu in
10.35 to 1
Compression ratio
Clearance Volume
4 cu in
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Compute Compression Ratio Problem 3

• Bore 4.000
• Stroke 2.870
• Deck Height 0.015
• Head Gasket Thickness 0.045
• Combustion Chamber 58 cu cm

590.75 cu cm
36.05 cu in
16.387 cu cm
x

590.75 cu cm

1 cu in
1
1
590.75 cu cm
Cylinder volume Clearance Volume
Compression ratio
Clearance Volume
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Clearance Volume
Volume of combustion chamber
CompressedGasketThickness
Deck Height

• Bore 4.000
• Stroke 2.870
• Deck Height 0.015
• Head Gasket Thickness 0.045
• Combustion Chamber Volume 58 cu cm

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ccing a cylinder head
Burette
Cylinder head
Plexiglass
Pictures from Auto Math Handbook by John
Lawlor Published by HP Books
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Combustion Chamber Volume
58 cubic centimeters
CompressedGasketThickness
Deck Height
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Combustion Chamber Volume
58 cu cm
CompressedGasketThickness
Deck Height
0.045
4.000
12.35 cu cm
0.7536 cu in x 16.387
0.045 0.015
0.060
0.015
4.000
Clearance Volume
58 cu cm 12.35 cu cm
70.35 cu cm
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Compute Compression Ratio

• Bore 4.000
• Stroke 2.870
• Deck Height 0.015
• Head Gasket Thickness 0.045
• Combustion Chamber Volume 58 cu cm

590.75 cu cm
12.35 cu cm
70.35 cu cm
58 cu cm
590.75 cu cm
Cylinder volume

Clearance Volume
70.35 cu cm
Compression ratio
9.40 to 1
Clearance Volume
70.35 cu cm
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Other Things to Consider Piston Shape

• Dished Piston
• Or flat top piston with valve relief cut outs
• Affect on Overall Clearance Volume
• Domed Piston
• Affect on Overall Clearance Volume

Adds to Overall Volume
Takes away from the overall volume
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Practice

• Compression Ratio Worksheet