by Dyno Dave & 6000 SSEi

TBI: Throttle Body Injection
Usually a single point injector placed before the throttle plate; looks much like a carburetor. One injector feeds all the cylinders. In some applications, two injectors may be used side by side. Usually TBI systems run low pressure (15 psi) fuel systems.

EFI: Electronic Fuel Injection
Generic name for electronically fired injectors versus continuous mechanical injection (older Bosch). This does not refer to any more specific injection set up than that as far as I know.

MPFI: Multi point Fuel Injection
Where an injector is mounted in the intake plenum pointed more or less at each cylinder's intake valve. 6 cylinders = 6 injectors. There are two firing circuits in the ECM, one for the front bank of 3 cylinders and one for the rear bank of 3 cylinders. Example: when fired, the front three injectors simultaneously spray fuel toward their respective intake valves. Fuel Pressure is approximately 45 psi. An additional injector for added fuel during cold starts is often present.

SFI: Sequential Fuel Injection
One injector per cylinder with a firing circuit for each injector individually. Results in better emissions and more power. Same fuel pressure as the MPFI.

TPI: Tuned Port Injection
Is the same system as MPFI, just for more cylinders and a fancier name.

Each basic system has one or more sub systems and that can get confusing.

If you want to be really thorough, and talk GM injection, you could mention the Cross Fire injection system, which was 2 TBI systems on a cross ram intake. Kinda cool really.

Also, as far as Tuned Port goes, from a purely "injector" point of view, it is a stright forward batch fire system. But that somewhat ignores the beauty of the TPI system. Namely, it's tuned length runners, which helped accentuate low end torque.

Also, in the truck world, there is CPI (Central Port Injection). It's sort of a hybrid of TBI and MPFI. It uses cental injector, like a TBI, but this time mounted down inside the intake manifold. Attached to the business end of the injectors is a platic tube assembly, that somewhat resembles a spider. Each leg of that spider runs right to a port where a mechanical poppet style injector is located. When an electrical (central) injector fires, it send pressurized fuel through the spider to the poppets, which open at a specified pressure. On a Vortec 350, there are 2 central injectors, and 8 poppets. It's a neat system that works quite nicely. There was also a varient of this system in the early '90s used on 4.3L V6s that used a tuned intake with an active valve, to change the runner lenghts.

Information courtesy of.. and DynoDave.