VinceTrifecta

I have some questions about fuel injector pulse synchronization on GM. There are really two aspects to fuel injector timing - the first is the pulse width, or, how long the fuel injector's coil is energized to allow fuel through the injector, and the other relates to at what point the injector is energized with respect to the crankshaft position.

I'm looking at this on an LT1 (my Impala SS w/ HOT cam), but the question applies to LS1 as well. The tuning tools seem to have many tables for modifying the pulse width and the minimum pulse width but I do not see any way to control the timing of the injector firing.

Now, there is the injector offset vs. battery voltage table. From what I've read, the point of this table is to give the PCM information regarding how long the fuel injectors take to respond at various system voltage levels.

Is the value in this table simply tacked onto the calculated pulsewidth? Or does this value also affect pulse timing? It would seem that the PCM would want to add this value on at the beginning of the calculated pulsewidth, and furthermore, it seems the PCM would want to subtract this offset from the actual point in time the PCM wants the fuel sprayed, because it is supposed to represent the delay in the fuel injectors.

If those assumptions are true, it would seem you could affect injector pulse timing with this table as well. If you needed to / wanted to time the injectors to spray earlier then whatever the factory wanted, simply adding offset should cause this to happen, right?

SSpdDmon

iTrader: (10)

There's an injector timing table or two in EFI Live that will allow you to adjust the injector timing with regards to the reference pulses and coolant temps. There's a range of 0~8mS for adjustment. From the factory, the injector timing ranges from 2.55~5.55mS.

I would assume this is the table you're looking for.

jimmyblue

iTrader: (11)

I've seen it asserted that the injector offset table also
affects firing-point but that was by someone I consider
an unreliable blow-hard, and no confirmation.

What you'd like is access to the start-point value and
the injector offset has the delivered-pulse-width effect
for sure, so you'd rather not mess that around (for the
squirt-timing purpose) even if true that it pushes that.

VinceTrifecta

Yes. I have read on other fuel injection systems - like Ford EEC-IV with sequential fire (like LT1/LS1), that matching injector timing to the camshaft intake lobe profile is critical to getting the engine running properly.

The theory is that once you start messing with when the intake valve opens, you also need to mess with when the fuel injector fires. From the article I read on this:

"It has been recommended to fire earlier rather then later. Firing the injectors too late can cause the fuel to wash down the cylinder wall, causing excessive wear and oil contamination. While firing the injector early will puddle fuel on top of the valve, this does allow the fuel to absorb heat from the intake track and valve. This heat will keep the fuel close to vaporization..."

My Impala SS with the LT4 conversion (and HOT cam) has a stumbling problem in the 1000-2000 RPM range, and does exhibit some of the symptoms of late injector firing (oil gets dirty *really* quick). I have no idea if injector timing has anything to do with these problems I'm working on, but the article got me thinking about it. And, I would guess that switching to the HOT cam changed the intake lobe profile such that the injector timing may be less optimal than it was with the factory cam shaft, because I'm pretty sure the intake valve opens sooner with the LT4 HOT cam then a factory LT1 cam.

Also, I am running Ford SVO 30# injectors which are supposedly slower-responding then OEM Delphi injectors. This could be making it worse.

I've never seen injector timing discussed in GM tuning articles (that I've read anyway). I did actually try playing with the injector offset tonight, but it didn't seem to dramatically affect the stumbling issue I am dealing with. I'd simply guess the problem is not related to injector timing or changing the injector offset does not change the start-point of the spray (or the PCM's compensation for increased richness due to the offset nullified the effect).

Silver02Z28

iTrader: (2)

What are your fuel trims? I just went to 39# SVO injectors and my trims were very high. Bumping the offset vs. volts vs. kPa brought them back to normal. The SVOs are slower to open and close.

VinceTrifecta

Did you write about the cobra blue-tops? It was your post that got me thinking about this problem in my car.

Idle LTFTs were in -4% range. Pretty close. I did find bumping the offset vs. volts richened the mixture. Bumping this is going to have the largest affect at idle and other low pulse width conditions because the amount of time the pulse width is being increased is a larger percentage of the actual calculated pulse width. But it didn't solve the stumbling problem.

My car is behaving a lot like another car I worked on recently. It was a 2002 Corvette with a supercharger and 60# injectors. It would stumble at low pulse-width conditions, and the root problem ended up being that with those large injectors, when the calculated pulsewidth fell below a certain threshold, the injectors wouldn't operate effectively anymore and would occassionally misfire. The fix was to increase the default minimum injector pulsewidths, but no such table seems to exist in the LT1 PCM (or, at least, the tuning tools available don't expose the table).

Under the stumbling conditions, I get split BLMs (left side is rich, right side is lean). Maybe I'll try swapping injectors from side to side and see if that changes anything.