larger fuel injectors - what's the problem?
 

I currently have the stock injectors in my 98 vette A4. I think the 98s have 28# injectors. What problems would I have by swapping in 30# or 36# on the same tune? I'm guessing that if there is an issue it would be at WOT correct since it resorts to a table rather than relying on the sensors.

you need to change the injector flow rate.

I assume that is done from the PCM? What happens if I don't change the injector flow rate? I mean if the PCM thinks the injectors flow 28# and I use 36#, for example, isn't the computer smart enough to back off if the o2 sensor is telling it that its running rich? Of course, once it goes into closed loop (WOT) its going to be burning rich since its not relying on the o2 sensors.

nope, its going to dump in a shitload of fuel and wash your cylinders. remember that the stockers are 28# at 58 psi. most aftermarket injectors are rated at 43.5 psi, so the 36#s will be closer to 40#.

you need to either have it tuned or pick up some tuning software.

Thanks for taking time out and educating me. That would make sense at WOT since it isn't using feedback from the O2 to adjust the AF ratio. However, outside of WOT why would it run rich? That is, right now the computer knows if its running rich or lean and adjusts accordingly so what would be different with larger injectors. Initially, I would imagine that it would be running rich until the computer readjusts..."oh now when I open the injector for this long I get this much fuel and that causes it to run rich, I better not open it up that long now" Or doesn't it work like that? Please educate me.

I'd probably go with 30# (or 36# at 58psi).

It would run rich because the PCM uses the IFR table to calculate final injector pulse width.

Example:

You have a hose with a 1" orifice and you know it takes 5 seconds to fill a one gallon jug. You turn on the water for 5 seconds and it fills the one gallon jug.

Now, you change the orifice size to 2" and turn the hose on for 5 seconds. It now overflows the one gallon jug.

Clear as mud?

Obviously I don't understand how the PCM works :confused:
How does the o2 sensor come into play? Lets say I swap the the 28# for 36# injectors and fire it up. The PCM says, "ok its time to fire the injector..oh lookie here I've done this before I have a handy IFR table that I keep up to date that tells me how much fuel to apply" not realizing that it now has a fire hose in place of its garden hose that was used when it created that entry in the table. The o2 sensor relays to the PCM, "what the "F" are you doing? You just dumped a bucket of fuel into the cylinder!" The PCM responds "hmmm something must have changed, good thing I'm a smart computer and I can learn a few tricks. Let me update my IFR table with a correct value as to not do that again" So the PCM updates the table and doesn't repeat the same mistake.

Doesn't this pretty much summarize the algorithm? If not it should. :)

Nope, the IFR table a static set of variables based on MANVAC. The computer will adjust fueling in percentages, LTFT and STFT, if fueling is off, but it's only to correct the natural drift of sensors, weather, etc.

Bottom line, if you change injector size a retune is required.

That sucks. How is it that the computer can adjust for all kinds of changes like heads, cam, supercharger, etc. (not talking WOT) but is clueless when it comes to fuel injectors. I believe you but it still doesn't make sense. Since the 36# injectors will flow 22% more fuel I would assume that the PCM would adjust it by 22% and be happy.

The flaw in your thinking is bolded. The PCM references the IFR input table which is of fixed, pre-programmed values, it is not a variable output (like fuel trim values).

If you do what you suggest, the PCM will compensate to some degree, by going way negative on the fuel trims. But even a 28 to 36# step (what 30# SVO's flow @58psi) is likely more than what it can compensate for unless you were running very lean to begin with.

Either way, the IFR table is one of the most prominent, core tables of all the PCM logc, and if it is off, the rest of the tune will be off too. Simply put, every fuel-related decision the PCM makes is based on it knowing how much fuel it is delivering...if you jack with the actual flow rate or IFR table, at that point the PCM no longer knows how much fuel is actually being delivered. It's a really REALLY bad way to "tune".

The computer can't adjust by itself for the stuff you listed, they all need a computer tune.

Why do you think you need injectors anyways?

Excessively positive LTFT's also affect WOT fueling. If your fuel trims are positive as you transition into WOT, the PCM will add the LTFT correction to PE fueling. Hence the reason most say you want LTFT's to be between 0 - 5% range.

Also, the computer does not effectively 'adjust' for major engine component changes. For your level of modifications, you need some pretty extensive tuning for it to run optimally. I've never tuned a 98 model PCM, but from what I understand they are more difficult to tune correctly than the 00 - 04 PCMs.

Thanks for all the info guys! Good stuff.

To answer the question why I think I need new injectors check the following link:


https://ls1tech.com/forums/generatio...-wot-help.html

You more than likely have the LS1 injectors which aren't 28lb-ers, they were rated @ 24 or 26lbs IIRC. You would need to verify this, first.

the 99-00's had the 26/lb injectors your thinking of.

the 98's had the 28# injectors.

Flat out you need a tune if you go up in injector size. NO exceptions

When in closed loop your 02's would read rich, and the computer would pull fuel (LTFT). But it is limited in how much it can pull and if you went much larger it would not be able to pull enough to get it back to stoich. It would then throw a code and continue to run rich, eventually destroying the motor. It would ruin the rings, contaminate the oil with gas and lead to bearing failure. When in open loop, it would simply use the existing fuel tables which means it would be extremely rich there as well. You have to tune for larger injectors, as everyone above has already stated.