If you want a good estimate of your flathead compression ratio, you run into some difficulties. It is very difficult to actually measure combustion chamber volume in an assembled flathead.

Also, the compression ratio calculators found on many sites for overhead valve engines are not applicable to a flathead because the combustion chamber perimeter (head gasket combustion chamber cutout) is not a circle (the cylinder bore) like it is for most overheads. Knowing the compression ratio is not vital for many applications but it is extremely important for supercharged engine design.

Now you can estimate your compression ratio for a variety of heads and displacements by going to the attached lookup table or accessing at www.roadrunnerengineering.com and clicking on Flathead Compression Ratio.


 

 

Flathead Ford Compression Ratio
2003 Roadrunner Engineering, .. Albuquerque, NM

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Head

Baron 500

8CM

8BA

EAB

Offy 425

Edelbrock

Offy 400

 

Dimensions

 

 

 

 

HV (cc)

92

82

76

71

63

61

54

 

 

 

 

 

Displacement

Relief

 

 

 

 

 

 

 

 

Bore x Stroke

 

 

 

Cu. In.

 

 

 

 

 

 

 

 

 

 

 

 

 

1)

239

no relief

5.9

6.4

6.8

7.2

7.9

8.1

8.9

 

3-3/16 X 3-3/4

 

 

 

 

relieved

5.8

6.3

6.6

7.0

7.6

7.8

8.5

 

swept volume 490 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

2)

249

no relief

6.1

6.7

7.1

7.5

8.2

8.4

9.2

 

3-1/4 X 3-3/4

 

 

 

 

relieved

6.0

6.5

6.9

7.2

7.9

8.1

8.8

 

swept volume 510 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

3)

255

no relief

6.2

6.9

7.2

7.6

8.4

8.6

9.4

 

3-3/16 X 4

 

 

 

 

relieved

6.1

6.6

7.0

7.4

8.1

8.3

9.0

 

swept volume 523 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

4)

265

no relief

6.4

7.0

7.5

7.9

8.7

8.9

9.8

 

3-1/4 X 4

 

 

 

 

 

relieved

6.3

6.8

7.3

7.6

8.4

8.6

9.4

 

swept volume 544 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

5)

276

no relief

6.7

7.3

7.7

8.2

9.0

9.2

10.1

 

3-5/16 X 4

 

 

 

 

 

relieved

6.5

7.1

7.5

7.9

8.6

8.8

9.7

 

swept volume 565 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

6)

284/286

no relief

6.8

7.5

8.0

8.4

9.2

9.5

10.4

 

3-3/8 X 4 or 3-5/16 X 4-1/8

 

 

 

relieved

6.7

7.3

7.7

8.1

8.9

9.1

10.0

 

swept volume 582-586 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

7)

293/296

no relief

7.0

7.7

8.2

8.6

9.5

9.7

10.7

 

3-5/16 X 4-1/4 or 3-3/8 X 4-1/8

 

 

 

relieved

6.9

7.5

7.9

8.4

9.1

9.4

10.3

 

swept volume 600-605 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

8)

304

no relief

7.2

7.9

8.4

8.9

9.8

10.0

11.0

 

3-3/8 X 4-1/4

 

 

 

 

relieved

7.0

7.7

8.2

8.6

9.4

9.7

10.6

 

swept volume 623 cc

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Notes:

 

 

 

 

 

 

 

 

 

 

 

 

 

 

1) See Blown Flathead, Appendix B for more information on calculating flathead compression ratio.

 

 

 

 

 

2) All heads should be checked for actual combustion chamber volume of the head only. Values listed are typical.

 

 

 

 

3) Compression ratios listed are for domed (3/16 dome) pistons. Flat top pistons should not be used with listed heads.

 

 

 

 

4) Compression Ratio(no relief) = (SV+HV+8)/(HV+8)

 

 

 

 

 

 

 

 

 

 

5) Compression Ratio(relieved) = (SV+HV+11)/(HV+11)

 

 

 

 

 

 

 

 

 

 

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