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 = Volume @ BDC Volume @ 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 = Volume @ BDC Volume @ 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 Volume @ BDC Compression ratio = Volume @ 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 78 + 8.6 86.6 cu cm Cylinder volume + Clearance Volume Compression ratio = Clearance Volume 740 cu cm + 86.6 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 78 - 12 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 = = 10.35 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 16.387 cu cm 590.75 cu cm x = = 590.75 cu cm 1 1 cu in 1 Cylinder volume + Clearance Volume 590.75 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

cc’ing a cylinder head Burette Cylinder head Plexiglass 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” 0.7536 cu in x 16.387 12.35 cu cm 0.060” 0.045” + 0.015” 0.015” 4.000” Clearance Volume = 58 cu cm + 12.35 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 590.75 cu cm 12.35 cu cm 70.35 cu cm 58 cu cm Cylinder volume 590.75 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