This creates a high expansion ratio for less compression, reducing intake and exhaust energy losses and converting combustion energy to engine power more effectively. (netcarshow.com)
In four - stroke engines the exhaust ports are generally opened just prior to bottom dead centre piston position, thus giving a maximum expansion ratio. (mechanics.stackexchange.com)
This greater expansion ratio is made more efficient by allowing additional energy to be produced. (greencarcongress.com)