For a low-rated engine, with a bmep of 10.4 bar compared with 5.5
bar for a naturally aspirated engine, a boost pressure of some 0.5 bar
is required, corresponding to a compressor pressure ratio of 1.5:1. As
average pressure ratios today are between 2.5 and 4, considerable
boost pressures are being carried on modern high bmep medium
speed four-stroke engines.