Just found this table, this sound about right then?

Block Head Compression with ITR / CTR pistons
B18C1/5 B18C1 ITR= 11.36:1 / CTR = 12.04:1
B16A B16A ITR = 10.19:1 / CTR = 10.80:1
B18C B16A ITR = 11.11:1 / CTR = 11.75:1