larger fuel injectors - what's the problem?
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On The Tree
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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.
Thread Starter
On The Tree
Joined: Jun 2007
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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#.
Thread Starter
On The Tree
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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?
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?
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Thread Starter
On The Tree
Joined: Jun 2007
Posts: 177
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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?
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?
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.
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Brett Foote Obviously I don't understand how the PCM works
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.
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.
Bottom line, if you change injector size a retune is required.
Thread Starter
On The Tree
Joined: Jun 2007
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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.
Bottom line, if you change injector size a retune is required.
Obviously I don't understand how the PCM works
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.
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.
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".
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.
Why do you think you need injectors anyways?
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.
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.
Thread Starter
On The Tree
Joined: Jun 2007
Posts: 177
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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
To answer the question why I think I need new injectors check the following link:
https://ls1tech.com/forums/generatio...-wot-help.html
Launching!
Joined: Feb 2004
Posts: 260
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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.
TECH Fanatic
Joined: Feb 2004
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From: Lexington, KY
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.