What's the point of tuning VE?
 

Ok so I have a question as to why there's a need to tune the VE table (with STFTs only for this example) to perfection, or close to it (between -5 and +5).

The way I understand it, the o2 sensors always try to keep the car at stoich, so it doesn't matter if the STFTs are +10 or -10, the car engine always sees ~14.7 AFR at part throttle. It's just a different way of getting to that value.

This is just an example of units, not something real, with 100 units of fuel being perfect for 14.7 AFR:

What's the difference in commanding 90 'units' of fuel and having the o2 sensors add in +10 (from STFT) to achieve 100 'units' of fuel vs. commanding 98 'units' of fuel and having the o2 sensors add in +2 (from STFT)?

I've been battling with trying to get my car to be in the small range of -3 to +3 for STFTs on a drive, but it's tough. When I think about it, why bother since no matter what my trims are (whether -10 or +2), the car will always see 14.7 AFR or close to it. My friend tells me that -10 trims makes the car run rich and will wash the cylinder walls with extra fuel and foul my plugs, but how can that be...-10 trims means 14.7 AFR just as much as -2 trims?

It is just to get things close, It never has to be perfect.. The closer the better. Basically you are just trying to bring your base up as close as possible, so all the other things work smoother.


Also, the problems associated with having a wrong ve table are, during throttle transisitions, fuel may be wrong before strims and ltrims fix them, Or, if you have a maf failure.

If you have an untuned ve, you may be off in a CELL BLOCK different from the one next to it, so if you are driving in between 2 cell blocks, that could also cause a fuel issue.

A good ve table helps make the car feel smoother.

That is a very good question and I look forward to some expert answers.

Could it be something like a matter of just everything working smoother and more precise?

For instance, say you are your engine and your tv is your VE map. If all the programs and commercials are at the same or near the same volume you can keep the volume at a pleasant listening level and everything is in harmony. But if the sound shifts and fades randomly and severly you will not only have to work harder to adjust it but the end result volume will be less precise and there's a chance of annoying your neighbor in between spikes (say your pissed off neighbor = washed cyl wall).

That's definitely true, and it does feel smoother especially when the whole table is smoothed out and not have huge discrepancies between adjoining cells. And I'm not saying having a completely un-tuned VE, I'm saying how important is it to get it close?

I've been battling with this thing for a while now, it's come a far way from stock in terms of smoothness and values. But I can never get it to close to perfect, and it's bugging me after reading all the tuning guides.

Just trying to understand if there's a difference between having a nice smooth VE table (but at -6 across the board) vs. having the same smooth VE table but at -1 across the board.

I can't remember the last time I even looked at STFT's.

As W4M said it's pretty much a base file for other adders ect to work off of. We are talking closed loop of course.

I was assuming that he was using our our custom os, where we make the strims update super fast, and turn off the ltrim system. That way it is always closer to correct..

This is my answer to the question as well.

I get that now, thanks guys. Again, without having a wideband available to check myself, what really is the difference between say -8 trims accross the board (whether they be long or short) and -1 trims? Is there a difference, or do they both get you to ~14.7 so it's all moot?

There is no real difference in A/F at -1% or -8%. The % is a correction. The less correction that needs to be made (less "swing") the better. The more efficient the car runs. The smoother it runs because it doesn't have the large "swings" in percentage of fuel change.

To answer the question directly, in the GM strategy, they use a combination of MAF and VE. The MAF is used primarily as a steady state reference for fueling. During throttle transitions, the VE is referenced. So, if your MAF is spot on, but your VE is off 20%, what is going to happen when you're moving the pedal, and the computer decides to switch from MAF to VE for it's fueling decisions? The fuel trims will go wacky, and the result will be some sort of driveability issue. A hiccup, or a stutter, or a hesitation, or whatever.

The whole premise of tuning, is to get the PCM to the point of having to correct as least as possible. This goes for fuel trims, and idle trims, and timing swings, and retard due to knock. Everything.

If the MAF is calling for 100 units of fuel, but the VE is only calling for 50 units of fuel and the engine needs that 100 units when the two are combines the engine only gets 75 units or 25 percent less than it needs and is very lean causing the motor to stumble or jerk. 10 percent may not seem like a lot, but it could be the difference of 12:1 air fuel and 13:1 air fuel.

This is a greatly over simplified example, but hopefully illustrates the point.

That make sense, though in both instances the 'swing' was similar.

When I had -8 accross the board, high and low trims were between -4 and -11 so a 7 swing. When I was -1 accross the board, high and low trims were +3 and -4 so again a 7 swing. That's the one thing about the car that seems to be consistent, the trim swing is always just about 7, but sometimes I get way positive trims, sometimes I get way negative trims.

I think I finally got it down to a good VE table last night, though in the low load cells it's still adding a bit of fuel.

Thanks for all the help, I can now sleep at night knowing that as long as I have a smooth table and keep my swings in check I'm ok.

Thats the answer of what a tuner should be doing.