Fuel Injection

General Information

TBI

Throttle Body Injection - usually a single point injector placed before the throttle plate; looks much like a carb. 1 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 - 2.8 V6 style 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 2 firing circuits in the ECM, one for the front bank of 3 cylinders, 1 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 - 1 injector per cylinder with a firing circuit for each injector individually. Results in better emissions and probably more power. Same fuel pressure as the MPFI. 3800's use this set up.

From: Dave Cole

TPI

Tuned Port Injection as used on Chevy 5.0/5.7L V8's is the same system as MPFI, just for more cylinders and a fancier name.

All Chevy MPFI/TPI (2.8V6/5.0V8/5.7V8) injectors are physically interchangable. They do, however, have different fuel flow rates:

From: Sketch

From: Scott Backer

The Fiero V6 injection technique is called Simultaneous Double-Fire. Firing all injectors at the same time, once per revolution (half the needed fuel charge each injection). The injected charge is not generally drawn to other cylinders and sits "suspended" (and vaporizing) in the port, waiting for the intake event on it's port. Actually, the worst case is the cylinders that get their shot while the valve is open or opening.

SFI intentionally fires the injector at a _closed_ intake valve (once for every 2 revolutions, synchronized to precede the intake event on the respective cylinder) - that's what you want. The "hot" intake valve improves vaporization of the fuel as it "waits" to be drawn into the cylinder. An open intake valve allows gas flow that interferes with the spray pattern and does not provide the best fuel vaporization.

From: Jeff Ely

"Can I put the fuel injectors from a 350TPI onto my car to increase power?"

The combustion process inside the cylinder of an engine depends on the presence of two things - oxygen and fuel. For the sake of brevity, let's say air and fuel since the oxygen is a component of the air.

The ratio in which these two substances must be present is called the stoichiometric ratio and it is a function of the chemical make-up of the fuel and the amount of oxygen present in the air.

It has nothing whatever to do with the design of the engine or the size of the engine or the brand of injector nozzle or anything else.

For most fuels and regular garden variety air the stoichiometric air/fuel ratio is about 14.7:1. This is the only ratio in which the fuel will be completely consumed in the combustion process and any extra fuel CANNOT be burned since there isn't enough oxygen present to burn it.

This means that putting more fuel into the engine by way of increasing the injector flow is a waste of time (and fuel) unless you also provide more air to go along with the fuel.

This is how supercharging or turbosupercharging works - you push more air into the cylinders and then you can spray in more fuel. In effect you are "fooling" the engine into thinking that it is in a more oxygen rich atmosphere.

This explains why putting the 5.7l injectors into a 2.8l engine resulted in an overly rich mixture.

I suppose the oxygen sensor would have sorted this out but it may have been pushed into operating outside of its normal range by so much extra, unburned hydrocarbons in the exhaust (and a consequent lack of oxygen). This would cause it to simply close down the injectors until the correct amount of oxygen was left in the exhaust stream but perhaps it couldn't drive them closed sufficiently to do this.

In any case, the engine would not have produced more power. It would likely have run a little cooler since the extra fuel would serve to transfer heat out of the cylinders. As an aside, during the last war, pilots would often richen their fuel mixture if their cooling system had been damaged in combat in order to help cool the aircraft engine.

In order to ensure consistent driveablility and help with engine durability, all engines run slightly richer than the stoiciometric ratio (ie. they waste a little fuel). Modern engines have such sophistacated controls that they shave this "extra" fuel down to a very small amount in order to increase fuel economy.

This leaner mixture is why modern engines run so hot - remember the Chrysler "lean-burn" engines - better mileage shorter life. Mopar tried this before engine controls were ready to handle the job safely.

This also implies that cars are already running a little fuel through which is not burned and so, putting on bigger injectors will only dump in more fuel which will also not be burned and so will not contribute any additional output.

It can be done in several different ways including supercharging, turbo- supercharging, changing the chemistry of the incoming gas stream by adding something like nitrous-oxide (which has extra oxygen "built-in"), or by the old tried and true methods of improving intake tract flow rates through porting, polishing, bigger valves, higher lift/longer duration cams, more efficient manifolds, etc., etc., etc., yadda, yadda, yadda.

That's about all I have to say on the matter. I don't read all of the hotrod magazines, but even they have to follow the same laws of physics and chemistry that the engineers who designed the cars did.

From: Peter Frise