target A/F.. vs.. base VE table

[quickstrike]

can someone expalin the relationship between the A/F ratio table and the VE table. What I mean is while running in lower RPMs in open loop I want to bring the A/F ratio up or down a little, I can obviously change the Base VE table. But according to the instructions even in open loop the ECU will use the Target A/F ratio table to calculate some portion of the Inj pulse width.

So my question is what is the realtionship to the 2 tables and can I change either one to get to my desired condition or is there a reason I should be doing it with only the VE table?

Is that confusing? I think I confused myself!

Randy

 

[stevec]

AF Table

The target AF table is used in closed loop only. And does what it names implies tries to target the AF ratio set in the table.

If your running in open loop all the time this table is not being used and the ecu using the VE table to determine fuel requirements. Rasie the VE numbers and the inj duty cycle increase and the actual AF ratio decrease because you have richen the mixture.

Most people run in closed loop when the exceed a certain rpm. This way the ECU is using the the target AF and the Wide band O2 is adjusting the inj duty cycle automatically, and with in a certain limit (the correction limits table) .

Given your Target AF table is correct, running in closed loop is pefferred. Keeps you from grenading the motor.

You then adjust your VE table so that there is minimal correction.

Here is the tricky part if you want to richen/lean the car up you need to change your target AF ratio table at the desired RPM/Pressure.

Do a search of this section there is a unoffical manual that explains what each table does.



SteveC

 

[TurboJim]

Re: AF Table

Originally posted by stevec

If your running in open loop all the time this table is not being used and the ecu using the VE table to determine fuel requirements. Rasie the VE numbers and the inj duty cycle increase and the actual AF ratio decrease because you have richen the mixture.

Actually Steve, I spoke to Lance a few days ago and the target AF table *IS* used in even open loop. Its used for the speed density calculations.

Directly from the C-Com Manual:
"Even if you are running in open loop mode, the ECU will use the information in this table as part of the base injector pulsewidth calculation in Speed/Density mode. When operating in open loop mode and in Alpha-N mode, this table is not used. "

I doubt this guy is running Alpha-N mode.

I was told to run my target real rich, and pull back the desired fuel from the VE tables. I honestly dont understand why. I didnt really get into it with Lance OR Harry as they were both kinda busy, but suffice to say, they both said it (not the same target, but the same theory) so their thinking must be right.

Personally, in my car, changing the target didnt change driveability. Only when I set it to idle in open loop did the low op driveability come around.

As I understand it, you tune the VE tables to true up the ENGINE. This will be the personality of the engine. Once corrections are minimal, you will change the targets as needed, and the VE tables should need little if any tweaking until you change something that effects the VE of the engine. (different cam, heads etc)

Now to get rid if this darned tip-in

 

[quickstrike]

Yeah That makes sense. Tune the VE and leave it alone for the most part untill an engine component is changed requiring a new number. I guess it would make sense to tune the Taget A/F numbers to the rich side and back it out a little with the VE table, that way when and if the car goes into closed loop it will be erroring on the rich side. There is probably some other programming reason why as well. Either way this is what I will do.
Thank you for the replys.

Randy

 

[Craig Smith]

OK, here it is straight from the horse's mouth...

When in speed/density mode, the target a/f table is ALWAYS used. Closed loop simply means that the computer will adjust the injector pulsewidth according to a difference between the measured a/f ratio and the targeted a/f ratio. But the target a/f ratio is part of the speed/density calculation and is always used.

The VE table should be adjusted so that the measured a/f ratios and the target a/f ratios match as closely as possible. If you are running closed loop, a simpler way of looking at that is that the O2 correction should be as close to zero as you can get it throughout the map. The VE table is basically the fudge factor that you program into the ECU that allows the speed/density calculation to properly calculate the amount of fuel needed for your engine under any condition. After the VE table is calibrated as well as you can do it, adjustments to the a/f ratio should be made in the target a/f ratio table and the VE table can be left alone.

Everyone have a swell day... almost time to go eat dead chicken now.

 

[TurboJim]

Yea, what he said!

Hey, thats what I said!

Least I'm gettin it!

Good idea, time for some dead something.......work sucked....

 

[quickstrike]

Excellent! Thats how I will approach it! (The car not the dead chicken).

Thanks for the help guys
Randy

 

[bobc455]

In a nutshell-

If your VE table is correct, then your actual A/F will equal your target A/F.


More detail- The VE table is a calibration table, not a tuning table. If you are running to lean or rich, change your target A/F ratio (tuning). If your target A/F does not equal your actual A/F, then change your VE table (calibrating the FAST).

The VE table tells FAST how much air is entering the engine. The FAST then calculates the injector PW to obtain your desired A/F ratio.

If the VE table is off, the FAST can step in and make a correction based on what it sees from the O2 sensor.

Clear as mud?

-Bob Cunningham
bobc@gnttype.org

 

[quickstrike]

Clear as mud!!

So far its working like a screen door on a submarine!

No actually its going pretty well. I have learned from one visit to the track, that I am only having to concern myself with a few cells. The run centers right around the same basic area.

Thanks everyone for the input

Randy