I have an 89 T/A with 305 TPI, and just read something about how TPI works. It said there are "2 types of injection: batch or group fire and sequential fire." It also says that port injection promotes superior fuel atomization and more efficient combustion because the fuel is injected right into the high speed airflow above the intake valve. Tuned port injection systems on 5.0 and 5.7 liter Camaros and Firebirds use the group fire method of fuel injection.
My question is this, if the injectors are fired together in groups rather than each at just the right time, then wouldn't the timing be off a bit for most cylinders? Some cylinders would be getting the fuel a little bit early, before the intake valve opened, and some would get it a little bit late, after the cylinder has begun filling with air, and maybe 1 is timed just right? The book didn't go into these details. I find it hard to believe the system could be as bad as this sounds. I hope I'm missing something.

 

i dont think it really matters, because the amount of time between fires are so short it wont make a difference. think about a 600 rpm idle(to make it easy). every second your engine is turning over 10 times. so every tenth of a second your engine spins one. so the fuel sprayed out of the injector is only in the engine for one tenth of a second before if gets spark and fires. i hope this makes sense and clears thing up for you

 

If i remember right, the injectors fire more than once a revolution. I think they fire twice.

 

Cobrakiller and Astro,
I figured the short length of time the fuel had to wait till an intake valve opened at operating RPM would come into play here. It still seems to me though that some intake valves are going to develop lots of deposits due to gas being sprayed on them before they open, and other cylinders will regularly run a little lean. Maybe changing speed with traffic conditions influences the pulse timing so the effects on individual cylinders gets averaged out. I would think the spark plugs would show an interesting history if certain cylinders constantly had slightly rich or lean conditions compared to others. Or maybe the particular bank of injectors fired changes with driving conditions (I don't really believe this). Maybe someone who has compared all the plugs and knows which injectors are fired together can let us know what he's seen. With all that can be done these days, why didn't GM just make all cars sequentially fired? It doesn't seem like it would be hard.

 

It doesnt make any difference in cylinder conditions that any sane person would notice. You have to remember that port FI is much more accurate than other setups, if you want even more accuracy by going to a sequential setup you are just splitting hairs and nothing else.

I've had numerous TPI's apart. There is no visible difference in carbon or rich/lean conditions unless something is physically wrong with the engine.

 

I still have a lot to learn, and I've never seen any engine taken apart. That's why I come here, I know you guys have many years more experience than I do. Thanks for putting this in perspective for me, I don't feel like my car is running wierd any more.:hail:

P.S. I've always wondered why it sounded like the injectors weren't firing enough for the rpm it was giving. Does anyone know how many injectors are fired together in one "batch"? How about the cylinder number pattern used per batch?

 

If I remember correctly the batch firing sequence is super simple....one bank of cylinders is fired all at once and then the other bank....i might be wrong tho.....they might all fire at once...also i think that each bank fires at every other revolution and then the other bank fires on the alternate revolution

 

The center two on one side will fire with the two end ones on the otherside and vice-versa.

 

All the injectors are wired together on each bank, and those tie together in the ecm. They all fire at the same time.

 

Yep, they all fire at the same time and multiple times per revolution. There is only one injector driver although there are two injectors wires; they tie together in the ecm. This way every cylinder sees the same amount of fuel when the valve is open and when the valve is closed. But think about this if you think that's weird or odd: the engine runs and runs quite well, doesn't it? So, there really is no need to question their methods.

 

Exactly. There is no real power advantage to SFI injection. Years of results prove that. It's big advantage is better emmisions and low end control. SFI only opens the injector once per R, so even with big injectors it has half the minimum pulse width of a batch car. That lower available pulse width is clutch, since 2 openings of relatively short dwell is a lot less 'accurate' then 1 opening of longer time. At WOT it's typical to run the injectors at 80, 90, even up to 100% (although a bad idea) duty cycle though, so sequential doesn't buy you anything.

And with SFI you also need a cam sensor, adding another expensive piece to your engine to leave you on the side of the road..

-watered down explanation of why SFI isn't needed for power.

 

Thanks for the explanations guys. This is all something I'd never heard about before.

 

On a GM vehicle anyway... Some use the distributor reference pulse.

 

Are you saying that you can take the wires from say the #5 injector and put it on #3 or #1 and it won't make a difference?

 

Yep.