Adjusting Main VE Table, When and By How Much?

[n8dogg]

Probably a dumb question...

When do you adjust the VE Table?
When you modify after the MAF?

What determines how much to adjust the VE Table?
By the LTFT %?
Headers tend to give - LTFTs, you don't adjust the VE table negative do you?

Thanks!

[WeathermanShawn]

If you are a believer in the interplay of MAF and VE this formula gives the relationships between the two: {SAE.MAF.gps}*({GM.DYNAIRTMP_DMA.C}+273.15)/({SAE.RPM}*{SAE.MAP.kPa})*3445.2/displacement()".."{CALC.SELBEN}*{CALC.VEN.%}".

The MAF and VE Tables are simply Airflow Models. In a MAF-Enabled vehicle the MAF-derived airflow appears to be the dominate model. The exact amount of airflow correction from the VE table is not totally known.

When you properly tune the MAF, the best way to test if the VE Table is correct is to fail the MAF and then compare the two models. You will find that any time you adjust the MAF you must also adjust the VE Table. Normally if your MAF Airflow increase, the VE Table values also increase. But the RPM and MAP it changes is also critical.

What determines how much to adjust the VE Table. It is a combination of Airflow, charge temperature, RPM, MAP, displacement and fuel correction. It is a complicated formula. There is not a 1:1 ratio between LTFT's and VE Values. If adding headers give you -LTFTS, then by fuel correction alone, your actual VE values would decrease. But it is not that simple. Your Airflow will most likely increase at various RPMs and MAPs. So that would increase your VE Values in those areas alone. At other RPMs and MAPs it may decrease.

So the simple answer is that while your questions are very good, the answers are not that simple or intuitive. If you run enough logs using the formula it all becomes very clear.

Hope that helps..:).

[joecar]

Hi n8dogg,

What mods do you have...?

You adjust both VE and MAF tables when engine mods cause PCM's airflow/airmass model to not match the engine;

in the meantime PCM applies closed loop trimming when commanding stoichiometric, and makes a trend-based guestimate for WOT.

[n8dogg]

Hi n8dogg,

What mods do you have...?

You adjust both VE and MAF tables when engine mods cause PCM's airflow/airmass model to not match the engine;

in the meantime PCM applies closed loop trimming when commanding stoichiometric, and makes a trend-based guestimate for WOT.

It's not necessarily for my car, although I will need to know for when I install LTHs or other mods. I've been doing some light tuning on Corvettes and a few have had long tubes installed.

I have a customer who has a 2004 Z06 and has long tube headers, he had his car tuned by a local tune shop. I pulled his tune to pick at and try to understand the adjustments. What I gathered is the tune shop simply increased the VE table by 5%. Here are a couple of screen shots.

This is the stock VE table for a 2004 Z06
http://i224.photobucket.com/albums/dd238/n8chandler/Random/Stock.jpg

Here is the modified
http://i224.photobucket.com/albums/dd238/n8chandler/Random/Modified.jpg

Does it make any sense to just do a complete % increase on the VE table? What's a better way to tune for headers?

[WeathermanShawn]

If the customer has a MAF and closed-loop..the adjustments should be made first through the MAF..and then the VE Table (CALC.VET does it all at once..:).)..

Headers may shift the MAF Airflow/VE table slightly. Airflow or VE efficiency may stay the same (or slightly less at lower Rpms), and then slightly-moderately increase as you get near 4000 Rpms..then come on strong until redline.

Basically it mimics what a dyno curve with headers looks like..

[Mr. P.]

...Basically it mimics what a dyno curve with headers looks like..

+1. In theory, it should precisely mimic the torque curve of the motor.

Not dumb Q's at all, they're very good EFI101 questions! Elaborating on what joecar and WeathermanShawn have said -

VE Table: The VE table is a description (from us humans!) to the PCM of how much airmass the motor should physically 'process' under any given condition; the PCM has no actual way of double-checking that the VE table is right or wrong, the PCM just operates on good faith that the numbers in the table actually match the engine it is commanding when making its fueling decisions - the VE table is our way of telling the PCM how the motor actually breathes.

MAF: the mass airflow sensor is a bit different than the VE table in that it is an actual sensor (not a theroetical model, or description of airflow) - SO, if you can get a calibrated MAF sensor then the PCM will know exactly how much air is being ingested by the intake. The issues with the MAF are (1) they are not super-accurate from the factory, (2) they can only read steady airflow, (3) on Gen-III motors, they are only useful up to 512-g/sec.

CLOSED-LOOP AND FUEL-TRIMS: Because we don't live in a perfect world and sensors have production tolerances, and 'identical' engines from the same assembly line can vary in their VE enough to affect fueling, and engine components wear over time enough to affect fueling, and gasoline is different from pump to pump, and ..... GM added a system of narrow-band oxygen sensors and 'fuel trims' so that the PCM can adapt to real world operating conditions. Simplest words, fuel trims are the PCMs best guess at how far off its fueling calculations are, in percent; in our PCM tuning efforts we desire trims (LTFT) and wideband readings (BENs) to be as close to zero as achievable, usually dialed-in within +- 1%.

Answering your original questions:

IMO you calibrate a MAF as soon as you buy a car; to whatever degree it is inaccurate, it will directly affect the fueling/economy of your vehicle. The Closed-Loop logic should be seeing the inaccuracy of the MAF and adjusting for this, it will show as non-zero fuel trims.

You must update the VE table anytime you physically make an alteration to the engine that changes how it breathes (exhaust, induction, camming, turbo/blower), or how it fuels (injectors, regulator, fuel pump). VE table inaccuracies will also affect fuel trims. And little changes matter, on my own truck a 2% change means the difference between 15+ MPG and 12-MPG.

Your long-tube modding example: usually (!) when adding headers, the motor will breathe a tad less airflow below peak torque, then show a gain 4-5% more airflow at peak torque, and hold those gains to just after peak HP - or so we hope! All of this actual airflow behavior will depend on the specific combination of induction, camming, headers, exhaust, etc but I would not expect the entire VE table to go up 5% because in reality the new headers will not make the motor breathe 5% more airflow 'everywhere', that's just physically not realistic; after fully tuning the vehicle using a wideband you will see the VE table change in those exact cells where the motor actually processes more (or less) airflow. In fact you can read the VE table and see where and how much your 'mods' have helped or hurt the engine, because the cells where VE had to be adjusted are the places where airflow was directly improved or hindered, and to what degree - if you make a 'mod' and your VE table doesn't change (throttle-body spacers anyone?!) then you did not change the behavior (performance or economy) of the engine at all!

The accepted best practice on how to evaluate and update the VE table is to drive the vehicle logging PCM parameters in combination with feedback from a wideband sensor (which tells you exactly how rich/lean the exhaust really is); one widely accepted method of doing this is published as the EFILive AutoVE Tuning Tutorial; WeathermanShawn and joecar have also recently developed an alternative method they call CALC.VET, it additionally considers LTFTs and this process is receiving many positive reviews and compliling a successful track record.

Mr. P. :)

[WeathermanShawn]

Wow, very good Mr. P..

You know your stuff!:)

[joecar]

+1 good write up :cheers:

[Mr. P.]

Wow, very good Mr. P..

You know your stuff!:)

LOL if you were to see my latest logs you would know I have a looong way to go in filling my tuning knowledge! I know enough to be dangerous, now I want to learn how to be REALLY dangerous!

Mr. P. :)

[n8dogg]

Great write up Mr. P, thanks.

The next question is;

How is this tune shop getting away with tuning cars with out a wide-band and by modifying the VE table as a complete with a percentage?

[Mr. P.]

Their customers don't know any better! And in all fairness to the tuner, the customers don't come back, so they must be fine with the work they have purchased.

One thing I am learning, tuning is very time-consuming. And when time is money, you look for 'efficiencies'. The marvelous thing about these PCMs is that they do adapt, to a degree - I would expect that in your observed example the car will be showing LTRIMs in the lower-most areas of engine operation. Remember that on a MAF-equipped car, above 4000-RPM the PCM uses the input from the MAF to entirely make its fueling decisions, the VE table can be off a pretty big degree and the car will still not hurt itself. But I bet if you interview the driver, they will tell you that at WOT their engine runs a lot different under 4000-RPM (below the MAF RPM set-point) versus above 4000-RPM - when I bought my first mail-order tune my truck acted like this, it was a rough-running dog until 4000-RPM then would just smoothly blast to redline, because the MAF was closer to correct than the VE table I purchased over the internet.

Mr. P.

I'll add more - most people have never been in a well-tuned car (think, i.e. a race car). Most people have never experienced what it is like when the speedometer needle acts like there is literally a string between it and your right toe! A well-tuned car, especially a light one, is explosive to drive, the car just does what you want it to do, when you ask for it; it's responsive. If you've had the chance to be in a vehicle with a well-dialed tune, you'll know the difference and won't take less than that; but most people haven't experienced that, they have no idea what they are supposed to be "buying".