BlowDown - Measured in degrees of crankshaft rotation from Exhaust Port opening to the Transfer Ports opening.
BlowDown TA - Must allow the cylinder pressure to drop below the pressure of the fuel air mixture at time of transfer ports opening. If the Blowdown pressure is to high when transfer ports open, it will stall or reverse, the incoming charge of fuel and air.
To add to this also, and putting any other 2 stroke theory aside for now - you need to get the exhaust out of the engine before (or at the same time as)the new charge can come in. That goes for 4 strokes too!
So if you wait until the piston is very low to get rid of the exhaust, you will be left with very little time for the intake charge to come in. Once the piston starts coming back up, and especially once it's past your low exhaust port, the cylinder pressure is rising, and no more intake charge is going to enter.
Remember, we have one revolution for the engine to do it's entire job.
