Your analysis ignores the unswept volume, Diesel911. You will lose a lot more than 1.334 psi.

Say you have 600 swept volume and 21:1 compression. This means that (600+u)/u=21/1 or 600=20u giving u=30. This is with the plug in place.

Say that the plug displaces 2cc. Then we compare (600+30)/30=21 with (600+32)/32=19.75. You have lost 1.25 bar, or about 17.5psi.