I always hit the accel when the "2" becomes big. So i guess 1/2 a sec from when 2 appears is about right lor

If you are first most of the time in the GPs, surely you would have got hit by it at least once O_O

Yeah, the lock on is better compared to the red shell but there are ways to dodge/cancel the effect. The warp tunnel in Waluigi Pinball and the cannon in Airship Fortress is one. (The shell blows up at the entrance) You can also run into a banana just as the shell hits (spinning out > flipped up)

There's also the Blue Shell dodge guide posted by Pr1me somewhere in the forums...at least the link, which works, but difficult to execute

Have fun