A few lines from Uncle Tony Rudd's book "It Was Fun" , some information on BRM engines:
"It took me a very long while to understand the reason for the power differences from the various so-called 2 litres. The best 1930cc gave 262 hp which is 136.6 hp/litre, the 1998s gave 129 hp per litre, with 258 hp, yet the 2070s gave 287 hp which is 139 hp per litre. Alex Osborn's 997cc F2 engine gave 129 hp per litre...I began to suspect the the ratio of connecting rod length to the stroke was more important than we thought. We cut 0.5 inch from the block of Frank Strake's F2 engine and fitted it with the shorter connecting rods from the F1 engines. fitted with the standard head the power went from 128 hp to 136 hp, which explained the disappointing performance of the F2 engine and the 1998cc V8s. The 2070cc engines had a longer stroke, hence the 139 hp per litre."