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Applied Math Compression Ratio

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Compression

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**Compression Ratio Volume @ BDC Compression ratio = Volume @ TDC TDC**

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**Compression Ratio Volume @ BDC Compression ratio = Volume @ TDC**

10 cu in 90 cu in TDC Compression ratio = BDC TDC 90 cu in BDC 10 cu in Compression Ration = 9 to 1

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**Swept Volume of Piston + Clearance Volume**

Compute Swept Volume of Piston + Clearance Volume Compression ratio = BDC TDC 7 cu in 45 cu in 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 Cylinder volume + Clearance Volume BDC Compression ratio = TDC Clearance Volume 740 cu cm + 78 cu cm Compression ratio = =10.5 to 1 78 cu cm

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**Changing the Piston Shape**

Valve Relief 8.6 cc 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**

8.6 cc valve relief Determine the compression ratio: Swept Volume = 740 cu cm Clearance Volume = 78 cu cm 86.6 cu cm Cylinder volume + Clearance Volume Compression ratio = Clearance Volume 740 cu cm cu cm Compression ratio = = 9.5 to 1 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**

12 cc dome Determine the compression ratio: Swept Volume = 740 cu cm Clearance Volume = 78 cu cm 66 cu cm Cylinder volume + Clearance Volume Compression ratio = Clearance Volume 740 cu cm + 66 cu cm Compression ratio = = 12.2 to 1 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 + 4 cu in 37.40 cu in + Clearance Volume Compression ratio = = to 1 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 36.05 cu in cu cm cu cm x = = cu cm 1 1 cu in 1 Cylinder volume + Clearance Volume cu cm Compression ratio = Clearance Volume

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**Volume of combustion chamber**

Clearance Volume Volume of combustion chamber CompressedGasket Thickness 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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**cc’ing 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 CompressedGasket Thickness Deck Height

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**Combustion Chamber Volume**

58 cu cm CompressedGasket Thickness Deck Height 0.045” 4.000” cu in x 12.35 cu cm 0.060” 0.045” ” 0.015” 4.000” Clearance Volume = 58 cu cm 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 cu cm 12.35 cu cm 70.35 cu cm 58 cu cm Cylinder volume cu cm + 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 Adds to Overall Volume Takes away from the overall volume

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Practice Compression Ratio Worksheet

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