Technical Guide: How to tune the VAFC

[shiroitenshi]

Here's some more to add to the peningness.. got a question via PM.. and here's the current news.. hahaha... my memory about VAFC tuning really rusty lah... So posting in forums will help me spot my own mistakes if any.. :P

Thing is.. everything seems so clear when you don't type it out.. but you mess up when re-reading and stuff.

Okay let me understand here i think i am a bit confuse . I have been asking many people who done dyno and tune with VAFC . They are all telling me on the upper side of the graph means MORE FUEL and lower half of the graph means less FUEL . But from you explaination it seem to be the opposite .

upper side means all the position numbers are and lower side means all the negative numbers are .

Hmm.. I might have mixed up a bit.. because it's been a long time since I touched a VAFC.. and I usually have a A/F meter on hand to know if I 'removed' or 'added' fuel.

But as far as I can remember.. The VAFC % numbers aren't fuelling numbers... They're AIR CORRECTION NUMBERS.. so the -1-3% actually tweaks the manifold pressure reading... not control the injectors for more fuelling. And as far as I can remember.. the positive numbers decrease fuelling.. But best if you check with a proper A/F meter... or post in the VAFC thread and see...

On wide Throttle does it mean we adjust more on the side after the Vtec have engage on the upper side ?
and leave evertyhing b4 the Vtec engage point at the lower side of the graph ?

Still.. I can't help but think that positive air correction results in less fuelling... but as for the actual pattern.... depends on your fuel rail pressure. (if using adjustable regulator).. again.. if unsure about how the air correction rates adjust fuelling..... use a A/F meter to check. (a proper one from innovate or other brands, not the APEXI Turbo timer A/F reading)

And on narrow throttle , before the vtec engage point should we just adjust at the lower half of the graph at negative side and at the after Vtec engagement point it should be on the possition side ?

Don't think of the graph as a pattern.. think of the graph as how the air correction increases or decreases.. This air correction is what influences the fuelling. This is key in learning how to use the VAFC.

This way aint it the same setting for both Narrow n Wide Throttle ?

Yup... nearly right.. The air correction rates should be more near the end of the RPM range at wide throttle (unless if you're not running more than normal higher fuel pressures).. except you can be more conservative at narrow throttle to save fuel.. :P

Or... you can set them to 0% for all.. (which means you're running off the stock ECU map/setting/open loop) <- provided your engine is not too crazily modded that running the stock ECU results in (detonation inducing) lean conditions.

Does the ECu only read Wide Thro setting when u floor at the Wide throttle % ? And NArrow throttle setting when you re in the range of narrow throttle ?

Yes.. the range of Wide Thr and Narrow Thr. is set under the T/H point setting.

Ask more in the forum thread.. maybe some other people have the same questions you have. I want to post this in the vafc thread so others can share this questions and answers as well... Do you have any problems with this?

no problem to share the question .

But for my understanding is .
MORE AIR MEANS more FUEL ...
So how can possitive side be more AIR less fuel ?
for a car to run faster means you presure it with more AIR therefore burn more FUEL , correct ?

So in my head i always think possitve side INCREASE the number alot once the Vtec engagement point and slowly decrease a bit . Before vtec engange point everything put under the negative side of the graph . ( The setting should be the same right for both Wide and Narro Throttle ) this is like programing in if and else condition .

True.. more air means more fuel.. but the correction is done on the pressure, not the actual amount of air coming in. The pressure read by the MAP is in a negative value (you can see this in VAFC in monitor mode)

That's why I'm saying that those numbers at the graph +1% or -1% isn't directly related to fuelling... or simply adding air.. It's modifying the MAP signal taken from the sensor and returning it to the ECU and it's modifying the AIR pressure, which (indirectly tweaks the voltage of the MAP sensor returned to the ECU)...but notice that VAFC says that it's modifying the AIR, not the voltage..

And the MAP sensor reads manifold vaccuum.. not air flow per se... so that's why right now I *think* that the negative air correction results in the ECU thinking that there's more air(flow)...

Its like IF Throttle < 50%
Run Narrow throttle setting
Else if throttle >= 80
Run Wide throttle setting

So it should be right to set both setting the same ?

Yup.... but tweaking it usually results in better part throttle response though... and better fuel economy..

What do you mean by positive air correction ? issit just a term ? And what you mean by he positive numbers decrease fuelling ? Does it mean positive side means more AIR LESS FUEL ?

Positive air correction is the term that VAFC uses for adding (positive) correction to the absolute pressure manifold signal.. while negative air correction is the opposite.

It's been a while since I played with VAFC.. but the principles of VAFC is correcting the manifold pressure signal (which is read from the MAP)

So, from what I remember.. the positive air pressure % results in a smaller vaccumm pressure number being returned to the ECU, which is interpreted by the ECU as less air(flow)... and well.. you know the rest.

As I already said.. get an A/F meter.. and test.. I sold off mine long ago and the last time I tuned a VAFC was like maybe 6 months ago.. even then I used an A/F meter.

Anyway.. you should be able to know by the stock settings, if you already have your VAFC tuned before.. Is the upper rev ranges in the positive section, or the negative section?.. what fuel pressure are you running? with that settings?

That in itself should give you some clues whether positive air correction or negative air correction gives you fuel enrichment.

 

[shiroitenshi]

back to topic...does the VAFC has a ON/OFF button whereby we can switch if off then the setting goes back to whatever we have in the ECU?...without having to unplug the VAFC itself?.....ada ka?...

haha... you didn't understand the earlier post kah?

If you set all back to 0%.... means you're running the stock ECU settings lah.. :P

One thing you can do for first timer (like I did) is set part throttle in [setting] --> Narrow Thr. to all zero.. then set the T/H Point to Lo = 60%.. means for normal driving where my foot isn't pressing the pedal past 60%... I'm running on the stock ECU. And above that.. you can see the air correction number starting to increase.. :P

Then when you know more.. can tweak it further later for better fuel economy and lower end response.

Don't worrylah.. udeng.. if you car stock.. all the reason you can play with VAFC without worry of detonating your engine.. just don't go overboard with more than 2-3% of air correction until you know exactly what it's doing, and again.. Make sure you know how to listen for knocking.. :P