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I need a couple things about the Vafc straightened for me.

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I need a couple things about the Vafc straightened for me.

Old 04-28-2004, 12:26 AM
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Default I need a couple things about the Vafc straightened for me.

Ok, so I have some Q's about the ever popular vafc hack.

I understand how to set the vafc, and why -40% to -35% fuel settings are used for all narrow/wide + low/high cam.

To keep things organized, if you know the answers please respond to part A and B seperately.

A: What I am having trouble grasping is the actual shift thats occuring on the spark and fuel tables. Ok so the vafc controls fuel amount by altering the map signal and then placing you at that value on the table... but since the stock honda ecu's shut down after seeing 1 psi of boost voltage, exactly what is the vafc doing to the map voltage when manifold is at say 5 psi of boost?

B: So how do you figure out which table column you are at in terms of inHg/psi with the -40% setting? The -40% is just relative to fuel amount over stock 240's and latency, so how do you relate it to the inHg/psi voltage the vafc is sending to the ecu?

Thanks
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Old 04-28-2004, 03:05 AM
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Default Re:I need a couple things about the Vafc straightened for me.

I am going to answer the questions with what I now understand for those of you that might have wondered the same.

A: The vafc automatically scales boost readings into vacuum readings for you without yourself doing anything. That is why you are only then subtracting an additional amount of columns to get to a smaller pulse width to make sure the larger 450 injectors don't run too rich. I had never read that the vafc takes boost readings and flops it over to the same place on the vacuum side of the table automatically. I thought you had to first get yourself over to vacuum values again, and then move more leftward for the injector compensation. Knowing that the vafc automatically makes a 11 psi voltage equal to a 1 inHg one for you answers this question.

B: The -40% and -35% numbers ARE directly related to the inHg readings. The change in volts that the vafc knows to move you based on the value you gave it would move you to a leftward column in our example (-40%), and thats the corresponding vacuum value. If you are at 1.3 lbs right before the stock tables stop and you put a +10% fuel curve, you will not get 10% more fuel because there is no column to move to in the table with a higher fuel pulse. Or if you were at 2 psi at 1,100 rpm with a -60% vafc fuel pulse reduction, you would throw a check engine light too because there is no column leftward that has a 60% lower fuel pulse.


Also you will throw a cell if one of your fuel correction values puts you past 26 inHg voltage reading on the left side of the table looking for a lower pulse value. This is what was confusing me as well, made me think the table could loop somehow through crazy formulas in the vafc when really it can only flip between boost and vacuum sides and add or subtract fuel by moving columns (voltage) UNTILL you ask for a fuel pulse that is not stored already in the rpm row you are looking at. (aka beyond the stock voltage range).

If any of what I said is incorrect please post about it.
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Old 04-28-2004, 08:09 AM
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Default Re:I need a couple things about the Vafc straightened for me.

another thing that you may want to note is that when the ecu is using the fuel tables for vacuum while you are in boost then it is also using the ignition tables from the vauum areas. This can be a problem unless you are running fairly high boost like 10psi. The less boost you run the higher into vacuum you will go. As you probably know the ignition timing is advanced more in higher vacuum which means you will be under boost and running extremely high ignition timing. This is why the more boost you run the lower the amount of vacuum the map will see and the ecu will look up tables with lower ignition timing beceause the tables will be getting closer and closer to 0psi as you raise boost. I hope that made sense..
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Old 04-28-2004, 12:58 PM
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Default Re:I need a couple things about the Vafc straightened for me.

Yep, good post.
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Old 04-28-2004, 02:25 PM
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Default Re:I need a couple things about the Vafc straightened for me.

thanx
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Old 04-28-2004, 03:25 PM
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Default Re:I need a couple things about the Vafc straightened for me.

Originally Posted by Tell Me Turbo
If any of what I said is incorrect please post about it.
OK, I will.

In response to B... what?!

There is no "stored" information about fuel pulses in the VAFC or SAFC. That is why they are generic, and can be used on Hondas, Nissans, DSMs.. whatever has a linear analog voltage scale for the MAP/MAF sensors.

I'll explain.. Your stock fuel table is 0 to ~3.2v. Anything higher will throw a CEL because it's "out of bounds" of the fuel and ignition tables. So how do you boost?

Your AFC intercepts the MAP/MAF voltage reading that is being sent to the ECU.. SCALES it based on the percent you entered, then sends it to the ECU.

For example, You're at WOT, 10psi, 4.9v (Your MAP sensor will never hit 5.0v due to the design of the diaphram) ..scale percent is -36% (Negative.. because you're scaling back, and 36% is how much you're taking off).

4.9v - (4.9v * .36) = 3.14v

That 3.14v is now sent to your ECU which brings you back to the last column of the fuel and ignition maps.. where your 3.1v would land you WOT NA..

To summarize, there is no direct corrolation of the scaling percent to the pressure readings (inHg/psi). The scaling percent is for the MAP sensors voltage. However, the voltage dictates what the ECU thinks the manifold pressure is.


[EDIT]
I just reread the last portion about throwing a CEL because of there not being enough "columns" left or right. That's incorrect also.

The reason you throw a CEL when you scale to far back is because the ECU knows that vacuum value doesn't exist. The reason for a CEL up top.. because on an NA motor (which is what the MAP sensor routine was designed for) a pressure value greater than 1.3psi (~3.1v) will not exist. These are just internal checks on voltage.

You can manipulate the voltage all you want, as long as it's within the internal bounds checking. This includes running positive manifold pressure. The ECU cares not about "columns" or "rows" as the fuel and ignition table cell value lookup routines have a "catch all". Which means if you are at a higher pressure value than the table reads, it'll use the last available value. This is because the ECU places all of its trust in the MAP sensor, and the voltage bounds checking.
[/EDIT]
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Old 04-29-2004, 03:32 AM
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Default Re:I need a couple things about the Vafc straightened for me.

Turboef,

Thanks for correcting me on the fact the vafc scales voltage directly, and not by first knowing what fuel pulse you want to be at. Also you are saying the vafc does not automatically put boost voltages to stock ones, that that is infact the -40% and -36% percent voltage inputs we are telling it to move that keeps us from throwing a cell, and not just correcting fuel alone. Why then in your faq does it relate the -40% and 35% numbers to fuel flow directly, as if the ecu knows fuel tables? I gues I am understanding it wrong.

Other then those though, I was thinking I was writing what you posted.

For instance, you wrote:
"The reason you throw a CEL when you scale to far back is because the ECU knows that vacuum value doesn't exist. The reason for a CEL up top.. because on an NA motor (which is what the MAP sensor routine was designed for) a pressure value greater than 1.3psi (~3.1v) will not exist. These are just internal checks on voltage. You can manipulate the voltage all you want, as long as it's within the internal bounds checking. This includes running positive manifold pressure. The ECU cares not about "columns" or "rows" as the fuel and ignition table cell value lookup routines have a "catch all". Which means if you are at a higher pressure value than the table reads, it'll use the last available value. This is because the ECU places all of its trust in the MAP sensor, and the voltage bounds checking."

I meant the same thing. I thought people would know that if I was saying you cannot add/subtract fuel because there is no column to the left or right, they knew that meant no voltage value that the ecu wants to read between 0 and 3.2 like you posted; since a table column (vacuum)is voltage and nothing else to the ecu process.

I said:

"The change in volts that the vafc knows to move you based on the value you gave it would move you to a leftward column in our example (-40%), and thats the corresponding vacuum value. If you are at 1.3 lbs right before the stock tables stop and you put a +10% fuel curve, you will not get 10% more fuel because there is no column to move to in the table with a higher fuel pulse."

Again, that meaning no voltage to increase to that the ecu will work with since that voltage has no corresponding table value. I was using way too much lamens terms and sentences in my post for everyone to understand. BTW, I corrected my original post so no one would read it and get confused.


Thanks for your post.


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Old 04-29-2004, 10:08 AM
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Default Re:I need a couple things about the Vafc straightened for me.

Originally Posted by Tell Me Turbo
Why then in your faq does it relate the -40% and 35% numbers to fuel flow directly, as if the ecu knows fuel tables?
Because it's easier to explain that way. It doesn't help the cause to confuse people in the FAQ. AFCs directly control voltage of the MAP sensor, which directly defines which "column" to use.

But, all of the fuel pulses are static in ECU. Changing column is what changes the fuel. Thus controlling fuel flow, not pulse.
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Old 04-29-2004, 03:23 PM
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Default Re:I need a couple things about the Vafc straightened for me.

I thought the point of the faq WAS to confuse people. ha

But if the vafc has no generic stored information about fuel flows, then how does the faq help people to create the percentage scales out of the differances in injectors size and latency? Is it just manipulation that your injector differance percents between 240's and 450's match the percents of voltage that need to be input to not throw cells? How does it work that the percents you're using to cut voltage also match the percents you were using in the faq for injector flow differances only since the vafc does not utilize fuel flow information?

Yea the pulse doesn't change but the column changes to a different pulse so the pulse is changing. lol, Were on the same page with that.
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Old 04-29-2004, 04:48 PM
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Default Re:I need a couple things about the Vafc straightened for me.

Originally Posted by Tell Me Turbo
I thought the point of the faq WAS to confuse people. ha
Heheh.. I've been aquised of being "too geeky" sometimes, so I tried to make it simple.

But if the vafc has no generic stored information about fuel flows, then how does the faq help people to create the percentage scales out of the differances in injectors size and latency?
Because the increase percentages push it to a column with a long fuel pulse, in turn opening the injector longer, and "compensating" for the lack of inertia in the moving mass of the larger injector at low rpms. As RPM increasew

Is it just manipulation that your injector differance percents between 240's and 450's match the percents of voltage that need to be input to not throw cells?
In relation to what? The reason you scale the flow difference between the two injectors is not for CEL reasons, it's for pulse duration reasons. Like we've stated, the pulse durations for the map are pretty linear. So when you scale back the flow difference, you're putting yourself at a part of the map that, essentially, equates to the same fuel flow that you would have if the larger injectors were native to that fuel map. It just so happends that you're cutting hte MAP voltage to do this, with in turn decreases resolution of the fuel map allowing for higher MAP pressures, I.E, boost.


How does it work that the percents you're using to cut voltage also match the percents you were using in the faq for injector flow differances only since the vafc does not utilize fuel flow information?
Because of the fuel tables "linear" nature. Stock tuned maps are essentially linear, so you can move around them (adjust MAP voltage) to find the pulse you need. IT just so happends that it takes about 5% increase (moves a column over) to get the longer pulse durations, to compensate for the latency and flow the same amound of fuel that a 240 would at a higher vac column. This is only needed for lower RPMs as the interia increases as RPM increase.

Yea the pulse doesn't change but the column changes to a different pulse so the pulse is changing. lol, Were on the same page with that.
Yep.
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