Another_User

Stupid trivia of the day. Multiple scenarios:
1) You modify the VE table to compensate for a cam. The car is always going to try to reach what it considers the ideal mixture. Does it really make a difference.
2) You descreen your MAF. The car tries for the ideal mixture. It should just compensate.

So if you are within the capability of the engine to compensate for rich and/or lean mixtures, and it is always attempting to reach what it considers the ideal mixture at that rpm, are you really getting the car richer or leaner? Why?

Another stupid question. You change your Inject Fuel Rate (or whatever IFR is), the PCM computes your fuel mixture. Is this mixture messed up because it is injecting fuel that it is not really aware of?

These are just things that have been bothering me that i have never really seen a concrete answer on. Anyone please?

HumpinSS

iTrader: (1)

1. In closed loop the computer is trying to acheive stioch (you know this) and based on trims it tries to compenstate +-. If you make a mod and push the trims positve they wont lock at 0 when going WOT. And all we are trying to achieve is either 0 or slightly negative trims.

2. Ideally but since you have now modified a calibrated sensor you are now misreporting air to the PCM. It does compensate via trims.

You are in essense you are just making less work for the PCM and having it lock at 0 when you go WOT

Yes because the injectors are stock and calibrated for such. By changing the IFR table you are lying (again) to the PCM telling it, it has bigger or smaller injectors.

In closed loop no matter what you do it will always try and maintain 14.7 what you are changing is how far or close it stays to the ideal fuel ratio and this is represented by your trims

Me thinks I got that right or tried to explain

Another_User

Sooo....why do people change their VE table at low rpms....
I know that the VE table is used during transients and all, but the trims are still being applied right? Don't the trims "stick" while you are in open loop? I don't understand why it helps...

Another stupid question: will shifting your LTRIMs one way or another affect if you run on the rich side or lean side of the desired mixture? Maybe even reverse as the vehicle tries to pull fuel to compensate for a rich mixture, etc?

HumpinSS

iTrader: (1)

With a cam the reason to change the ve at low rpm is because the cam idles at higher map values. The stock PCM sees these higher values and starts dumping fuel. Remember higher map = higher load, higher load = more fuel, so you modify the ve table at lower rpm to lean it out and to cure rich surging and all that other stuff


If in closed loop it will only get you close to 14.7 the trims take care of this in open loop when the trims lock at 0 you are running off the pe table and the open loop f/a table which has a commanded ratio the computer is trying to acheive

Bink

A big cam will result in predicted airflow values out of range of the MAF values. Usually this will result in a P101. FWIW.

Another_User

But does it really lean it out? It saves the trims right? So wouldn't it just add fuel once it figured it out?

If you are working off your closed loop while in PE, your VE will affect that too right?

HumpinSS

iTrader: (1)

Yes it should the o2 should sense that the car is running real rich and you get super negative trims at idle you have to lean it out to get them closer to 0 is my understanding

Yes to my knowledge

P Mack

iTrader: (6)

Here is an example of how the fuel trim's won't keep you at the right a/f ratio:

You get an aftermarket maf that's not calibrated right. It overreports the airflow, so while your ltrims were around zero before, now they are negative. When you go wide open throttle you aren't getting as much air as the pcm thinks, and it doesn't adjust because it won't pull fuel even when the ltrims are negative. Now you're rich at wot.

Maybe an overreporting maf is unrealistic, i don't know, but you get the idea.

P Mack

iTrader: (6)

Another reason to modify your ve table rather than relying on fuel trims is because the ve table has much finer resolution than the fuel trim cells. Where there's only 22 ftc's to cover all driving conditions, there's 400 ve cells.

HumpinSS

iTrader: (1)

I am also leaning towqards the ve table for tuning. Instead of older way such as the MAF and IFR table

Another_User

Well...then here is my problem with the entire thing. When my car is warming up it looks like it is running pig rich (900ish steady O2 readings at idle), but after it gets warm the trims at idle are just about 0, and they are cycling like you would normally expect. If the VE table were off, wouldn't my trims be out of whack? Or am I thinking about this wrong? (Which is completely possible...)

P Mack

iTrader: (6)

There was another thread about this. Your target a/f ratio when warming up is richer than normal, it's under the open loop F/A vs ECT vs Map table. You really can't trust your o2 readings when it's in open loop warming up anyway.

Colonel

"You really can't trust your o2 readings when it's in open loop warming up anyway."

Very true. The O2 sensors read differently at different temperatures. That's why they have to warm up before going to closed loop.

Another_User

So I am probably dead-on, it's just way rich at startup...so can I just lean it out a tad...