Flathead Ford Compression Ratio
© 2003 Roadrunner Engineering - Albuquerque, NM
Head Baron 500  8CM 8BA EAB Offy 425 Edelbrock Offy 400
Engine Dimensions Volume (cc) 92 82 76 71 63 61 54
Displacement Bore x Stroke Relief (Note 4)
cubic inches inches
239 3-3/16 X 3-3/4 no relief 5.9 6.4 6.8 7.2 7.9 8.1 8.9
swept volume 490 cc relieved 5.8 6.3 6.6 7.0 7.6 7.8 8.5
249 3-1/4 X 3-3/4  no relief 6.1 6.7 7.1 7.5 8.2 8.4 9.2
swept volume 510 cc relieved 6.0 6.5 6.9 7.2 7.9 8.1 8.8
255 3-3/16 X 4  no relief 6.2 6.9 7.2 7.6 8.4 8.6 9.4
swept volume 523 cc relieved 6.1 6.6 7.0 7.4 8.1 8.3 9.0
265 3-1/4 X 4  no relief 6.4 7.0 7.5 7.9 8.7 8.9 9.8
swept volume 544 cc relieved 6.3 6.8 7.3 7.6 8.4 8.6 9.4
276 3-5/16 X 4 no relief 6.7 7.3 7.7 8.2 9.0 9.2 10.1
swept volume 565 cc relieved 6.5 7.1 7.5 7.9 8.6 8.8 9.7
284/286 3-3/8 X 4 or 3-5/16 X 4-1/8 no relief 6.8 7.5 8.0 8.4 9.2 9.5 10.4
swept volume 582-586 cc relieved 6.7 7.3 7.7 8.1 8.9 9.1 10.0
293/296  3-5/16 X 4-1/4 or 3-3/8 X 4-1/8 no relief 7.0 7.7 8.2 8.6 9.5 9.7 10.7
swept volume 600-605 cc relieved 6.9 7.5 7.9 8.4 9.1 9.4 10.3
304 3-3/8 X 4-1/4 no relief 7.2 7.9 8.4 8.9 9.8 10.0 11.0
swept volume 623 cc relieved 7.0 7.7 8.2 8.6 9.4 9.7 10.6
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) Relief is 1/8 deep to gasket boundaries.
5) Compression Ratio (no relief) = (SV+HV+8)/(HV+8)
6) Compression Ratio (relieved) = (SV+HV+11)/(HV+11)

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 on the above lookup table or accessing at
www.roadrunnerengineering.com and clicking on Flathead Compression Ratio.

Hopefully this chart will fill a need. Your comments are appreciated..!

 

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