I would approximate a very low intake manifold vacuum at 25hg. The pistons of a 454 are 4.25 inches in diameter with a surface of 14.18 sq. in. x 8 = 113.49 sq. in. at 25hg equals 12.28 PSI x 113.49 = 1393.66 LB of force that would drop relatively with momentu... Login to read more.