# Applied Math Compression Ratio.

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Applied Math Compression Ratio

Compression

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

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

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

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

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?

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

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).

Changing the Piston Shape Dome
How would the compression ratio change if domed pistons are used instead? Ford 351 Cleveland Dome =12 cc

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

Compute Compression Ratio Piston Shape: Dome
Conclusion: Pistons that are domed will raise the compression ratio (if everything else stays the same).

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

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

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

Pictures from Auto Math Handbook by John Lawlor Published by HP Books

Combustion Chamber Volume
58 cubic centimeters CompressedGasket Thickness Deck Height

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

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

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

Practice Compression Ratio Worksheet