The static compression ratio discussed above — calculated solely based on the cylinder and combustion chamber volumes — does not take into account any gasses entering or exiting the cylinder during the compression phase. In most automotive engines, the intake valve closure (which seals the cylinder) takes place during the compression phase (i.e. after bottom dead centre, BDC), which can cause some of the gasses to be pushed back out through the intake valve. On the other hand, intake port tuning and scavenging can cause a greater amount of gas to be trapped in the cylinder than the static volume would suggest. The dynamic compression ratio accounts for these factors.
