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VAFC users, how far past -40% can you go at idle

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VAFC users, how far past -40% can you go at idle

Old 06-15-2004, 03:46 PM
  #1  
0.5 BAR
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Default VAFC users, how far past -40% can you go at idle

Hey guys, I use a vafc and 450's and currently have it correcting the fuel the same values as mentioned in the faq. At idle and up to around 3,000 rpm it runs too rich, so I was wondering if any of you have taken it past -40% at idle and if so how much further -41%...-%42? Turboef9, when you put those values in the faq, were they intended for someone boosting 10 psi? Since im running only half a bar, I feel like I should go past -34% after 6k rpm, to maybe -32% at redline. Then for idle since im running rich there, go past -40% to maybe -42%. What do you guys think about that? I will be getting a wideband and some dyno time soon, just wanted to see if people have tried the settings I said above for 7.5 psi. I realize 10 psi gets me further away from timing advance in the table but I don't have a mbc yet.
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Old 06-15-2004, 04:17 PM
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Default Re:VAFC users, how far past -40% can you go at idle

I wouldn't go too far.

Stock flow difference without latency compensation is 46%. But, due to the latency of the larger injectors a 40% flow differential is used to supply adequate fuel.
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Old 06-15-2004, 04:24 PM
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Default Re:VAFC users, how far past -40% can you go at idle

Ok cool. In regards to the ending correction values past 5k though, did you intend it for 10 psi in the faq so that -34% took you back to 1psi values?
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Old 06-15-2004, 05:03 PM
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Default Re:VAFC users, how far past -40% can you go at idle

Well, see the common misconception with the VAFC hack in regards to "fuel for x PSI" is that it's all dicatated by your injector size.

When you scale a VAFC you're not scaling for "how much boost you want to run", you're scaling for "how big of injectors" you have. However, this scaling, however, directly effects how much boost you can run.

Let me throw some figures at you...

Assuming stock fuel maps tuned for 240cc injectors NA, we want to run 10psi. What size injectors will we need?

Lets try 310ccs..

(240 / 310) - 1 = 22% flow difference..

(We're not going to worry about latency compensation for sake of example)

3.1v is where you CEL will kick on for the MAP code, and put your ECU into limp mode. So, we continue..

10psi is roughly 4.9v.

4.9 - (4.9 * .22) = 3.8v.

What does this mean? At 10psi with a -22% fuel differential, you'll hit a CEL at about 6 psi.

Lets use the same math for 390cc..

(240 / 390) - 1 = 38% differential..

4.9 - (4.9 * .3 = 3.08v.. this looks like it's cutting it a bit close. Elevation will play a part in whether you'd be able to use 390cc injectors to hit 10psi.

Anyway.. as you can see, 450s will fit fine for 10psi.. depending on your settings.

You never want to go below your flow differential.
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Old 06-15-2004, 06:24 PM
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Default Re:VAFC users, how far past -40% can you go at idle

Jason Parker=engine management Jedi Master

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Old 06-15-2004, 09:49 PM
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Default Re:VAFC users, how far past -40% can you go at idle

Ok, I understand your figures. Im just saying that instead of trying to run 10 pounds, I only run 7.5 which is about 4.5 volts, and in that case I can use a -32% correction after 6k rpm to get me closer to the end of the table instead of -34% if I was at 4.9 volts. I am wondering if anyone else running half a bar uses this setting, and how does the wideband show?

With your calculations you use fuel flow differances in ratio with map voltage as if each column on the table had equal fuel pulse increases, whereas on stock tunes it can jump from .53 ms to .67 ms differances between columns. So 40% of the 3.1 volts isn't exactly 40% of the max fuel pulse value, but its close enough. So when your figuring out what the max boost you can run on certain injector sizing is with those equations you used, its a good figure but not a precise calculation right?

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Old 06-15-2004, 10:02 PM
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Default Re:VAFC users, how far past -40% can you go at idle

i used -47 for idle and up to -50 hi corrections on obd2. all setups are different for different cars. best thing to do is to wideband and egt tune.
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Old 06-16-2004, 01:42 AM
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Default Re:VAFC users, how far past -40% can you go at idle

Originally Posted by Tell Me Turbo
With your calculations you use fuel flow differances in ratio with map voltage as if each column on the table had equal fuel pulse increases, whereas on stock tunes it can jump from .53 ms to .67 ms differances between columns.

...

So when your figuring out what the max boost you can run on certain injector sizing is with those equations you used, its a good figure but not a precise calculation right?
But we're not dealing with fuel pulses, we're dealing a a linear MAP sensor voltage.. which is plotted by the ECU in a linear manner. Therefore, percentage cuts will be accurate for column placement, but, yes, you're right about column increase vs pulse duration is not linear. But, if you also notice, MAP heading increase across the maps are not linear either. They're averages of the column boundries. Without going into a long draw out explaination of column boundries vs RAW MAP values.. suffice it to say, percentage cuts are an acceptable way to tune your car (as a VAFC does) on stock maps.

Secondly, about your calculations based on your target boost, you can't tune that way. You'll run entirely too rich.

You _must_ tune for your injector size. All of your tuning will fall into place for target boost level. If you do not tune for your injector size, you will run lean because, yes, you _will_ be at the higher portions of the fuel map with the larger injectors pulsing at durations that do not equate to thier fuel flow differential.

Hrm, in writing this, I think I'll elaborate on why flow differential percents are accurate. It's pretty simple to understand, but normally an overlooked fact..

Think about it like this.. If say you had a pulse duration that flows the correct amount of fuel for that amount of air flow at a 2ms duration with 240cc injectors.

To get that _exact_ same amount of fuel, for the SAME air flow with larger injectors, say, 450cc, you would have to scale that by the flow differential percent, which is 46% (Note, this does not include latency compensation).

So real world numbers..

2 - (2 * .46) = 1.04ms It would take a pulse duration of 1.04ms with 450cc injectors, to flow the same amount of fuel as 2ms would with a 240cc injector. See where this is going?

It doesn't matter that the fuel maps are no linear. It's the correct use of the flow differential percent, for that cells pulse duration that counts.
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Old 06-16-2004, 02:38 PM
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Default Re:VAFC users, how far past -40% can you go at idle

Thanks for explaining why you can't choose your ending value on the table without running rich/lean. Also that your injector size chooses it for you with the fact you have about 5% of play going up 1% per 1000 rpm before getting rich. Most of the stuff you were saying about the fuel flow percent attached to map voltage, I think we were misinterpreting ourselves on because I understand what you just wrote, but it was that subject of the injector dictating the best ending table values for you that I needed clarification on. Thanks again.
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