Re: What exactly does the VE table do?
Do a search on here and ls1tech under the pcm section. You'll find a lot of good info. A quick summary: while we do have maf's, the ve table is referenced during open loop. It's also referenced in closed loop at lower rpm and on throttle transients b/c the maf is slow to react to the sudden change in airflow. At 4000 rpm and up the maf is the source of airflow calculation. Below this, it's a mix of the maf and ve table. It's a good idea to tune the ve so that you don't have lean spots when you hit the throttle and it will also give you better throttle response.
Re: What exactly does the VE table do?
Great explanation luv2spd :cheers: thanks!!
Re: What exactly does the VE table do?
the VE table is used for throttle transient airflow estimation. The concept is simple.
1. you are running along at part throttle, with good vacuum in the inlet manifold, MAF is reading the amount of air that is entering the engine.
2. you suddenly go WOT and you get a big rush of air into the manifold to fill the vacuum. ie. this air enters the manifold and is measured by the MAF but it doesn't actually go into the cylinders.
3. At this point the MAF is reading far more air than the cylinders actually have entering and the result would be a rich condition, more measured air = more fuel. If you look at your MAF logs you can see this "spiking" during positive throttle transients. The opposite is true negative transients where you are snapping the throttle shut (eg. MAF stops reading airflow while the manifold builds vacuum, would result in a lean condition).
4. Now this is where the VE table steps in and says "the maf is almost right guys, but here's my take on the situation"
5. the two results are filtered together, also taking into account TPS values into quite a long predictive airflow filter routine, with varying coefficients depending what "zone" of operation the engine is in.
6. The net result is an airmass value that the PCM actually uses for the fueling and spark calculations.
If you disconnect the MAF you basically bypass all this predicitive filtering and essentially the PCM uses the airmass value that the VE table calculates at all times. You can think of this as being in transient mode all the time.
I like to think in analogies so here's something that may make it a little clearer. You can think of this whole thing like a sponge when washing your car.
Hold the dry sponge in your hand over your car and turn the hose on it. For a while there you see a constant rate of water going onto the sponge from the hose but nothing is coming off the sponge onto your car (the sponge is absorbing it). After a while as the sponge saturates you will see the same amount of water running off the sponge as you are applying with the hose.
Now think of the MAF as a flowmeter on the end of the hose, and the engine cylinders as receiving the water coming off the sponge. In this dry sponge example the flowmeter (MAF) doesn't accurately reflect the amount of water running off the sponge until it saturates (reaches steady state), and depending on the size of the sponge it may read high for quite some time.
Now imagine you turned off the hose, for a while there water still flows off the sponge as it slowly dries out. But now the flowmeter (MAF) is reading zero (cos you turned the hose off completely) but your eyes can plainly see that water is still running off the sponge.
This is exctly what happens to the MAF when you have a positive or negative throttle transient, only the sponge is now your intake manifold.
The, VE table on the other hand is a "calculation" thats tries to guess how much water is coming off the sponge at any given time - note the big difference here is "guess" rather than "measure" as is the case for the flowmeter. In our sponge case it might weigh the sponge to seehow much water is on it at the current time and look up a table that is outflow vs. mass. The point here is it measures a bunch of other things and does a calculation to guess the answer. In the VE table case it looks at MAP and RPM and looks up a human defined table that says for this MAP and RPM this is how much air is in the cylinder.
So you can see that while things are at steady state (or close to it) the flowmeter gives a very accurate indication of the water running off the sponge onto the car. But as soon as you get into a transient condition it can be wildly inaccurate.
So why bother with the flowmeter at all? Good question. The thing here is that the flowmeter is a universal device that "measures" flow very accurately in the case of water, and reasonably accurately in the case of air. You put one on an engine and you don't need to spend a whole bunch of time calibrating and refining your "guessing" calculation as most flowmeters are temperature independant devices (ie. they measure accurately at all temps). The carmaker can be assured that for the majority of cases across wide operating conditions and applications the engine is performing to the standards required. And if optionally backed up by a decent transient model the whole thing can perform even better giving accurate transient contions (again with very little calibration work). ie. transients are short and generally so long as the transient "guess" is close to the mark once steady state returns (usually very quickly) the flowmeter takes over again (with a smoothed blending in and out).
For those electrically minded replace the water with a variable current source, the sponge with a capacitor and the engine with a light bulb. ANyone who send me a VE table in volts vs. load impedance wins a prize...
For those physicists out there, this is an area known as "sponge dynamics" and there is a new show on Nickelodeon called Spongebob that discusses this in great detail ::)
Chris...
Re: What exactly does the VE table do?
Chris....excellent explanation/analogy
Now we know how the VE works can we move onto the profile of the VE table....
My understanding and experience says a smooth VE gives the car a smooth feel but as you can see below this is not always the way things come out from the manufacture.
The first VE is from a standard 04 Monaro and the second is from a HSV GTS which is a standard MAF Less car.
Would I be correct in thinking that as the GTS only uses the VE table it needs to have very accurate VE cells at the expense of smooth transitions OR is a smooth VE not the best for power?
http://www.hsvdriversclub.co.uk/tuning/m-2004.jpg
http://www.hsvdriversclub.co.uk/tuning/gts-2001.jpg
Re: What exactly does the VE table do?
the GTS comes from the factory running solely on the VE table. Some poor calibration engineer is given the task of sitting on the engine dyno, stepping through every cell in the VE table and tweaking the number until the measured AFR matches the commanded AFR. A stocky on the other hand would have very little cal work done on the VE (near enough is good enough), this is evident if you've ever had a look at your fuel trims on a stocky with the MAF unplugged.
Re: What exactly does the VE table do?
Chris..... if I end up with a spikey VE table (like the GTS) after tuning the table I should not try and smooth the table out?
Re: What exactly does the VE table do?
as long as it is correct, i wouldn't bother. But be aware that a rough VE table will be more succeptable to tripping burst knock unless you tune that out.
The ultimate measure here is wahever it takes to eliminate the error between commanded and measured AFR. We have added some new features to the scanner histogram and editor for 1.6 to make tuning your VE a dream with the WBO2 and EIO interface.
Chris...
Re: What exactly does the VE table do?
ahhhh you said the magic words "Burst Knock"....
I have been suffering with burst knock when my VE table was spot on (spikey)and by smoothing the table the problem reduced..........
I have just sent my black box off for the upgrade to W/B and I am getting a W/B plug fitted to the exhaust to feed the correct W/B data as I drive.
so v1.6 is due around Christmas ? and it will make VE tuning easy?
Re: What exactly does the VE table do?
if your VE is correct then increase the burst knock delta cylair threshold. You'll notice GTS cals increase this as well.
CHris...
Re: What exactly does the VE table do?
Chris..............when tuning the VE table to zero I asume this is approx an A/F of 14.7 as this is the figure best for emissions. So the PCM does not trim the fuel....
now when I tune with my wideband should I aim for 12.8 A/F (closedloop turned off) at then let the PCM bring it back in line when closed loop is re enabled.....
As I see it when you go WOT you need a VE optimised for 12.8 (I use this as a safe power A/F) and you want to enter PE with the correct A/F?
Re: What exactly does the VE table do?
When tuning the VE you want to aim for your measured AFR (with the WB) to match your commanded AFR (whatever that may be).
Re: What exactly does the VE table do?
I figure this discussion is as good as any to ask my question. I installed a LSX intake on a C5 and now he's getting +7's on his LTFT's cells. when you get more air flowin into the cube do you need to increase or decrease the VE table? At first I tried scaling the IFR table with the idea of telling the PCM that it had smaller injectors so that it would add more fuel but didn't feel it was the right way to approach this. What should I be messing with?
Re: What exactly does the VE table do?
If you have positive trims, add to the ve table to richen it up.
Re: What exactly does the VE table do?
dam i should have read this days ago!!! :banghead:
Re: What exactly does the VE table do?
If the MAF is already compensating for air flow, what benefit is gained from doing this type of tuning on a supercharged car (ie. better power output, mpg, less hesitation, etc.) or any car for that matter?
Re: What exactly does the VE table do?
The maf is only accurate/used in steady-state conditions. Tuning the ve will help throttle response and overall smoothness on airflow transitions.
Re: What exactly does the VE table do?
How much should we change the VE table if your seeing +7, +9 LTFT?
Also, should the 'ENTIRE' primary VE table (Airflow-> Main VE
Primary VE vs RPM vs MAP) be increased?
I'm not sure what is a significant change is.
400 rpm
15 kPa 36
20 kPa 39
Change the above to 37 and 40?
I started seeing higher positive LTFT after installing a ported stock TB.
Thanks so much in advance.
Re: What exactly does the VE table do?
Quote:
Originally Posted by 98A4LS1
How much should we change the VE table if your seeing +7, +9 LTFT?
Also, should the 'ENTIRE' primary VE table (Airflow-> Main VE
Primary VE vs RPM vs MAP) be increased?
I'm not sure what is a significant change is.
400 rpm
15 kPa 36
20 kPa 39
Change the above to 37 and 40?
I started seeing higher positive LTFT after installing a ported stock TB.
Thanks so much in advance.
Modify the value in the appropriate cell in the VE table by HALF the FT value. Use the copy/paste special, Multiply by % - Half option for these operations.
If you are seeing value's of 10 and less than I would recommend using STFT's instead of LTFT's.
Re: What exactly does the VE table do?
405HP-Z06, thanks for the reply. Iwill give your suggestion a try and see if that helps the lean condition I have.