Torker II MPFI Build

I used to do research in liquid sprays. At one conference in the early 1990's, a Phd student working on a GM project presented research on spraying far upstream in the intake runners, for more time to vaporize the gasoline. I recall no noticeable improvement in performance or emissions.

Not sure how or why they started spraying at the back of the intake valve. It likely started with the earliest mechanical fuel injection systems, perhaps for reasons unrelated to vaporization, and now lost to history. I doubt they try to spray thru an open intake valve since the valve is open for only a small part of the spray's duty cycle, except at idle. Though, minimizing idle emissions is important. Spraying at the hot intake valve helps vaporize fuel, but fuel drops would likely hit the valve backside anyway on their way to the cylinder. My guess is there hasn't been a lot of research and it doesn't make much difference how you spray the fuel into the intake runner. The most important thing is getting the fuel evenly distributed between the cylinders, which is what MPFI excels at, as long as all the injectors stay matched in flow.