With sequential MPI, (SMPI), the computer sprays fuel into each cylinder's
air stream only when the intake valve for that cylinder is open. This way,
the fuel stays in a mist in the air at all times, and doesn't fall out
of suspension the way it can in other systems. It's the most efficient
- it can apparently add about 15 hp without reducing fuel economy - and
it
makes for the cleanest emissions.