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8 phase motor how 4 strokes really work part three 7 exhaust blowdown at around 40 degrees BBDC Before Bottom Dead Center while the piston is still moving down the exhaust valve opens allowing the still burning mixture to expand outwards out the exhaust port if the piston were allowed to move all the way down the cylinder before the exhaust valve opened the piston would have to fight against the still expanding exhaust gases as it came back up while the exhaust valve opened by opening the exhaust valve before the piston has moved all the way down the exhaust can blow itself out the exhaust port 8 exhaust return as the piston returns to the top of the cylinder it pushes the remaining exhaust gases out past the exhaust valve however because of the previous exhaust blowdown phase the escaping exhaust gases now create a negative pressure pulse which can actually pull the remaining exhaust gases out of the cylinder while the piston merely follows them upward as they exit the exhaust port this eliminates some pumping action energy losses on the part of the piston