From MopedWiki

Blowdown is the number of degrees between the opening of the exhaust port and the opening of the transfer ports. Blowdown can be calculated by using this formula

B=(T-E)/2

Where

• B=Blowdown (in degrees)
• T=Transfer timing (in degrees)
• E=Exhaust timing (in degrees)

See calculating timing for explanation of different blowdown usages.