Q. Can I do sequential injection with MegaSquirt?
A. There are two common sorts of injection:
Throttle body injection - usually one or two injecotrs for the whole engine
Port injection (aka. Multi-Port) - one injector per cylinder
Then there are three common modes of injection timing:
batch - all injectors fire at once, but not timed to any specific cylinder event,
bank - ½ the injectors fire at once, then the other ½, and so on, but not timed to any specific cylinder event,
sequential - each injector fires at a specific point in the 4-stroke cyle for each cylinder (i.e., 8 independent timing events)
Throttle body injected cars are usually batch or bank fire, simply because of the geometry. Most port injection set-ups before the mid-1990s were bank fire as well (including GM Tuned Port Injection for the 305/350).
Sequential injection requires:
at least as many injectors as you have cylinders, with one dedicated to each cylinder (i.e., not a 4 injector TBI on a 4 cylinder).
as many injector drivers as you have cylinders,
and also requires a camshaft position sensor (a crank sensor is not adequate for a 4-stroke cycle engine).
However, sequential injection does not necessarily mean you are injecting into an open intake valve all the time. The intake valve is only open less than 30% of the time in a typical 4 stroke engine. Once you are trying to produce more than about 25% of maximum HP your injectors are firing for longer than the intake valves are open. If your maximum HP is correctly calibrated to a safe 80% duty cycle, your injectors are injecting well over 50% of the time on closed valves.
MegaSquirt has just two injector drivers (that can handle up to ten injectors each), and no provisions for a cam sensor signal, so it would be difficult to make it into a sequential injection system.
The benefits of sequential injection are that:
you may get slightly better mileage and lower emissions at low engine speeds,
you can tune each cylinder's fuel amount independently (if you know how).
The effect on maximum horsepower is generally negligible.