Why can fuel injected engines use larger intake manifolds and still operate at low engine speed?

Answer 1

Intake runner size determines not only how much air is flowed but at what speed. The larger the runner, the more air can flow but at a slower speed. Keep in mind,

Proper Mixing of fuel and holding it in suspension with the air until delivered to the cyl is maintained by air speed or velocity, as used with carburation.

With direct fuel injection Fuel is injected directly into the cylinder rather than need to mix with incoming air before entering the cylinder, so air velocity is not as important as volume. As air has already been drawn into the cyl and then fuel injected under pressure as a mist, is introduced into the cyl and ignited.

The mixing of fuel /air and fuel suspension is bypassed when using a direct injection system. A faux injection system using carb type body to inject fuel into the manifold must use smaller runners than a true direct injection system. Fuel directed by on board computer/electronic sensors according to temp, timing, rpm and air volume. No pre-mixing of fuel/air required.