VE table ?

[RPO_Z06]

I'm confused about the VE table.
I'm trying to get a grasp as to what this table does and when and how it should be adjusted, I've read several threads about it but most tend to be vauge as to its true function and none have explained what adjustments should be made to it and why or when.

Any help in understanding its function and how to adjust it would be greatly appreciated..

[joecar]

VE table is used by PCM to calculate the mass of air at a given (MAP, RPM) point (and may blend in the MAF airflow value, depending on if the MAF airflow is stable and well behaved (and if the MAF sensor is present and not failed out)).


The air mass per cylinder can be calculated from the VE table using the following formula:
airmass = VE*MAP/IAT

where:
airmass is in g/cyl
VE is in g*K/kPa
MAP is in kPa
IAT is in Kelvin (and may have some ECT blended in)

The PCM needs to know the cylinder airmass so it can compute how much fuelmass is required to achieve a specific AFR.

The AutoVE Tune tutorial shows how to dial in the VE table: http://www.efilive.com/doc/Tutorials/AutoVE%20Tuning%20Tutorial.pdf

[RPO_Z06]

While that is a great tutorial it doesn't do much in the way of explaining what the VE table numbers mean or what changes to it up or down do in relation to fuel intake.
Can someone shine some light on that ?

[ringram]

Numbers basically tell you what % of the cylander is full of air at each point in the table. The PCM then works out how much fuel it needs to put in to arrive at the required air to fuel ratio based on other fueling tables.
So ve table is key it tells the engine how much air is in it at any point in time (SD Mode)

[RPO_Z06]

I'm not using SD mode, so should I just not adjust or smooth the VE table ?
What tables make up fuel, where do I start with leaning or enrichment ?
Thanks for all the information..

Jamie

[dfe1]

Think of the VE table as an indirect "fuel" table. Any number within the table tells the PCM how much air the engine is using, and consequently how much fuel is required. In theory, each number is a percentage of overall engine displacement so a 346 cubic inch engine operating at 100% VE should be processing 346 cubic inches of air every two revolutions. A 90% VE equates to 311 cubic inches of air-- you get the picture. Don't get hung up on the numbers, just make whatever changes are required to get your long term fuel trims between 0 and -5. Even though you're operating with a mass air sensor, changing ve values will have an effect on fuel flow because the LSx control system isn't a pure mass air system. It does a lot of comparisons between predicted and actual air flows and makes adjustments as it sees fit. If you don't believe that, multiply the VE table values by 50% and try to drive the car. Also keep in mind that LTFT values are significant only when the system is in closed loop (idle and part throttle).

[joecar]

As you drive, the PCM uses a blend of MAF sensor airflow and VE table computed airflow;
this is a good reason to do an SD tune, and then re-enable the MAF if you want to run MAF'ed;

also, having your VE table dialed in means you can then use the PCM computed airflow to correctly recalibrate your MAF table (since the MAF sensor seems to be sensitive to any mods Upstream and Downstream).

You don't really have to worry about what the numbers mean (...I can't picture what a g.kPa/K looks like...), just follow the AutoVE Tune tutorial exactly.

Increasing a cell in the VE table tells the PCM that there's more air so it adds more fuel for that cell.

Once your VE table is correct, then tweaking the PE tables is easy since you'll get out what you put in.

Cheers,
Joe

[superls1]

It might help to think about the table in terms of ‘Base Fuel’ rather than Volumetric Efficiency. In a world without a MAF, this table would be the primary means by which the ECM would determine the fueling requirements. I believe many people get tripped up on the concept of ‘how much’ particularly as it relates to Volumetric Efficiency. Most people continue to think in terms of ‘volume’ as in Cubic Feet per Minute, CFM. However, the ECM needs to know ‘how much’ in terms of mass. By calculating the mass of the air, the ECM can then determine how much fuel is needed to achieve a given AFR.

As an example, look at a forced induction setup (assume it is 5.7L engine). If we run enough boost to actually stuff 11.4L of air into the engine, is the VE greater than say a NA engine? I would say yes, but you have to really think about how this was achieved. Did the volume of the engine actually increase? No. Rather the density (weight per unit of volume) of the incoming air changed. The cool thing about air is that it can be compressed. In fact, the density is going to have a direct relationship with pressure and an indirect relationship with temperature.

Most ECM systems have standard or base values that are used as a starting point. It is fairly common to see the standard value for pressure to be 101.3kPa and for temperature to be 20*C. By doing this, we can calculate the mass of the air for a known volume (i.e. the cylinder). Fortunately for us (at least me anyway), somebody a long time ago figured out the unique relationships of gases (like air). Today, we know this as the Ideal Gas Law or

PV=NRT

P = pressure
V = volume
N = mass
R = Gas constant
T = Temperature

If we know P (using a standard/base value like 101.3kPa), T (again using a standard value), V (cylinder volume), and R is a constant for dry air, then we can compute N, which is the mass we are looking for. If we say that at this given temperature and pressure the engine is 100% efficient, then any deviation from these conditions can easily be accounted for as V and R will remain constant.

One of the questions you have to ask is: Do you care what the table was established at as representing 100% VE? Probably not. Although it is good that on this board and others, there are many people always seeking to understand everything. In every system I have ever used, increasing the number adds fuel and decreasing the number removes fuel. However, having the number ‘80’ in a given cell does not inherently mean that the motor is truly 80% efficient or that only 80% of the cylinder was filled. What it does mean is that the density of the intake charge is 80% of some pre-determined value. The EFILive guys didn’t manufacture the number you can see when setting the table to 100% everywhere and then view in g*K/kPa. In fact, I am not for that I can even comprehend that unit. :) The ECM always has to account for P and T. The basic formula for computing density is

2.7 * P / T (where P is in psi and T is in degrees Rankine; this will yeild lbs/cubic feet)


The table is also used (along with the formula) above for comparison against MAF readings. If the MAF value and the calculated value are too different, the ECM sets a DTC and check engine light. It is also important to note that most people attribute the VE table to be very important for fueling during periods of throttle transition. This is due to some of the limitations of the MAF. Therefore, if you have changed your engine’s ability to breathe (H/C, intake, exhaust, etc.) it is important to validate the VE table as it will affect the driving habits of the vehicle.

Just my $.02.

[ringram]

Yeah you need to have VE correct as when in PE mode the car will go more by the VE table as its in open loop.

[FRISKY]

Can the table be adjusted for more than 100% VE?

[ringram]

Yep, check the maximum units. Default is 500% max. Though how you will get there under 105kpa is another matter. A very efficient engine may go a little over 100%, maybe like 110 or so, but 500, never. At least not in the 1 bar table anyway.

[RPO_Z06]

I think I have a better grasp of whats going on now, this was very helpfull information.