Please help settle a dispute between my brother and I.

One of us says boring an engine will cause a lower compression ratio because of a bigger cylinder area.

One of us says the compression ratio remains the same because along with the bigger bore is a bigger piston to compress more air.

Which one, if either, of those statements is correct?

 

Static compression ratio is calculated by comparing the volume of a cylinder with the piston at TDC to the volume when at BDC. The displacement of all the components in a cylinder factor into the formula, the cylinder volume, piston dish or dome volume, the gasket thickness, and the combustion chamber volume of the head. Here's an online calculator, try it and see for yourself how changing the cylinder bore or head gasket thickness affects the outcome.
Engine Compression Ratio (CR) Calculator

 

a rough calculation (emphasize rough) showed an increase of compression ratio from 9:1 to 10:1 with a .030" overbore on a 302 (4" bore and 3" stroke vs. 4.030 bore and 3" stroke). This isn't as accurate as it could be since I approximated clearance volume as a cylinder and found a rough value based on a starting compression ratio of 9:1. Holding height constant for both swept volume and clearance volume, I calculated volumes of those cylinders with the increased bore size and it worked out to approximately 10:1.

 

You're both wrong.

Compression ratio is the ratio of cylinder volume with the piston in the bottom of the bore divided by the cylinder volume with the piston at the top of the bore.

When you bore the cylinder it's volume is larger. Your volume increases when the piston is at the bottom of the bore a lot more than it increases with the pistion at the top of the bore. If the piston comes to deck height or higher then the volume with the piston at the top of the bore does not increase, but the volume with the piston at the bottom is larger. That makes the compression ratio higher with a bored cylinder.

 

Well said.

 

Yep, all things remaining the same except bore, compression will be higher, but not that much higher. jd

 

Replacement pistons used to be "destroked", typically they would have the compression height reduced .020" (this was stated in the catalogs) just to retain the original compression ratio. This is in some ways a terrible way to go, since the "squish" area between the piston top and flat part of the combustion chamber is deepened, which can increase detonation.
It's quite possible that economy rebuilder pistons still do this, but I think the higher end ones don't.