Sequential injection attempts to only spray fuel while the intake valve is open for the cylinder that is about to fire, which in most cases is only possible at low engine speeds between idle and low speed cruising. Benefits can be seen in emissions, drivability, and fuel economy at these low engine speeds.
Batch injection (which is what the MegaSquirt-I and and most MegaSquirt-II versions do) does not attempt to spray only when the valve is open. It’s a fact that at higher engine speeds you won’t have enough time to spray all fuel while the valve is open, making this somewhat pointless, particularly for a performance oriented application. At these higher engine speeds 99% of sequential systems (exceptions being race only systems with super massive injectors) cannot spray all of the fuel needed while the valve is open anyways, and sprays it both while the valve is opened and closed, very much like a batch system such as the MegaSquirt I and II. We’ve dyno tested several cars with using batch fire and sequential injection on the same motor. Peak power was the same without using individual cylinder tuning. However, the sequential fire setup did run a bit more smoothly at idle and at light throttle angles.
The MegaSquirt III system with the MS3X Expansion board or MS3-Pro will do full 8 cylinder sequential fuel and ignition. If you’re looking for the absolute best possible emissions and fuel economy, as well as the ability to fine tune each cylinder individually, then the MS3 system is the way to go.
There is generally no significant performance difference with a sequential system over a batch system unless you use individual cylinder tuning. There can be minor emissions and fuel economy benefits at very low speeds though. If your tuning budget allows for individual cylinder tuning, it may be possible to pick up 3-4% more power depending on how much cylinder to cylinder variation your engine has.