| 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..!