[jpx7777]

Hi, I was doing some research about going with a 4.020 bore vs. a 4.030 on a rebuild. I came across the following paragraph, and I can't wrap my mind about how increasing the bore size increases the compression ratio.

Perhaps someone could break it down barney-style for me?


"Although quite a few builders push the 289/302/351W to 4.060-inches, this is strongly discouraged
because the lightweight gray-wall iron cylinder walls are quite thin. Taking bore size to 4.060 inches
is courting trouble, but doing it without sonic-checking the block is foolish. By taking the bore size to
4.060 inches, you drive the compression ratio higher, which raises operating temperatures and
pressures. With a larger bore, we drive compression higher by increasing the volume we squeeze
into the existing combustion chamber. Because cylinder wall thickness is marginal at best at this
oversize, you also risk getting into the water jackets. So, take it from us – do not go above 4.040-
inches on bore size with any small-block Ford."

 

[jking2010]

When you bore the block you get a bigger cylinder hole no difference in the size of the combustion chamber...If you have a bigger bore you get more air/fuel in the cylinder and it compresses in the same size combustion chamber you had before the machine work equaling more compression

 

[jpx7777]

Thanks, right on. In my mind I wasn't using a cylinder head, I was just putting an imaginary flat top on the cylinder. Makes more sense now, Thanks.
-J

 

[Mark O'Neal]

In a 347 with a flat top you go from 10.25 at 4.030 to 10.37 with a 4.060.


The guy was just filling out a paragraph......