1. Applied Math
Compression Ratio

2. Compression

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

4. 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

5. 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

6. 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

7. 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?

8. 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

9. 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).

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

11. 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

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

13. 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

14. 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

15. 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

16. cc’ing a cylinder head Burette Cylinder head Plexiglass
Pictures from Auto Math Handbook by John Lawlor
Published by HP Books

17. Combustion Chamber Volume
58 cubic centimeters
CompressedGasket Thickness
Deck Height

18. 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

19. 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

20. 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

21. Practice
Compression Ratio Worksheet