beepy

So I was tuning my SAFCII, when I starting cursing at it because no matter what I do, it will sometimes go lean and sometimes go waay rich depending on what situation my car is in.

I know why it does it: The fundamental flaw of the AFC is that it alters MAF/MAP signals (and thus fuel rate) in some function of RPM, not boost. So we do whatever we can to make the curve somewhat match the actual fuel requirements of the engine; for the majority of us, this is just making it correct for standing starts. We always run into problems, though, when we are already doing 3000 RPM and we want to step on the gas. All of a sudden our boost maxes out and we don't have enough fuel.

I really wished that the AFC trimmed fuel based on an actual MAP signal.

So then I thought, "why can't it?"

Here is the idea that came to mind: The AFC uses the tach signal as its primary input. I switched over to the "Sensor Check" and confirmed that this input is a simple voltage, about .1 volts per 100 RPM.

Is there any reason that I can't just remove the tach signal, and place a MAP signal in it's place? The Honda MAP sensors have voltage from about 0-5 volts, with ~2.* volts being the transition from vaccum into boost. So if I were to use this as the signal (possibly with a resistor or an amplifier), could I not then PROPERLY tune to fuel curve for the turbo?

Basically, I would do some measurements and figure out what the voltage curve was for the tach and for a MAP sensor, so that I could figure out a fuel correction based upon a MAP signal that I could then input into the AFC as a tach signal.

The final curve, assuming both the tach and MAP are linear, would be a base correction (-40%, etc) at voltages that equate to vacuum, and a linear progression from there to the maximum fuel requirements at full boost (or beyond).

So, does anyone see any serious errors in my thought process, or has this thing already been tried?

b16sedan

The error in your though process is the fact that the AFC does jack all for timing, which is the biggest downfall. A BTM that indescriminately pulls out timing based on boost is far from perfect, ideal, or even very good.

beepy

I asked if there was anything wrong with MY logic. The jacked timing is assumed and there are already thousands of people dealing with it with boost retards.

Joseph Davis

Interesting idea.

Keep in mind you are just flipping your ignition recurve on it's side, if you look at one of the Honduh ignition maps. Things stretch and skew along the RPM band instead of across the pressure range.

Uh, wait, are Kias MAF or SD? I've been told MAF cars won't have ignition problems with AFC fiddling, and I've been told they will. I'm really not sure... I have lots of interest but little motivation/experience with MAF.

beepy

It is a MAF, but I am still using a MSD ignition to retard timing to avoid misfiring, so I think the hack does affect timing.

I did a little more research and the Honda MAP and the standard analog tach signal are both purely linear, so this should work. I am buying a used MAP now and seeing if it works.... If I can fix my cruise-to-WOT transition problems I will worry about the timing issue later.

For Honda guys the final system would be 2 MAP sensors, one with a missing link to prevent the ECU from seeing boost, and one to trim the fuel with the AFC. With this configuration, you could use injectors that were actually properly sized for your fuel requirements instead of twice as big, so you could use less correction and have less of an issue with timing, anyway.

For my MAF, I still have to use a big correction because there is no simple way to "trick" my MAF, not that I would want to anyway.

BTW, how does Honda take care of the 5v input signal? Is it just a resistance wire or is it controlled by the ECU? I may need to build a voltage regulator.

-edit-

I think i figured out the flaw. The tach signal may read as a voltage but it is actually a square wave signal. I have to figure that out....


Modified by beepy at 8:54 PM 10/17/2004

beepy

Worked on the idea a little more. I have a basic schematic now for a voltage-to-frequency convertor that I can use to "hack the hack" into using a boost signal.

I will build a prototype using an ICL8038, various other radio shack transistors and pots for all the resistors since I suck too much at EE to calculate the resistances I need... Hopefully I will be able to build a "box" containing a 2-bar GM sensor and the linear voltage-controlled oscillator circuit that anyone will be able to DIY to make the hack just a little more useful for FI!

Of course, the timing is still an issue for at least y'all MAP guys.

On that note, I recently installed larger 30# injectors (15# stock) and used the SAFC to set a -47% correction across the board. Now I can get the car to run at 14.5:1 at all times... even under light load in boost, but I can't get it to run richer than that unless I go WOT. But at least I am no longer going lean when I decide to pass someone on the highway! Much happier with the car. The car is also much harder to start in the morning... I don't know why, but I can smell fuel so I think it goes pig rich when I first crank it.

Jefnes

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by J. Davis &raquo;</TD></TR><TR><TD CLASS="quote"> I've been told MAF cars won't have ignition problems with AFC fiddling, and I've been told they will. I'm really not sure... I have lots of interest but little motivation/experience with MAF.</TD></TR></TABLE>

(at least on Bosch ECU MAF cars)

Igntion timing IS load dependant.

How do I know? Chip tuning VR6's

Jeffrey Atwood

beepy

The preliminaries are good on this device I am building. There is about $20 worth of parts in it. I kept the price down by using a circuitboard-mounted MAP sensor instead of buying a GM 2-bar MAP. That, a few OpAmps, a voltage regulator and a linear voltage-operated oscillator circuit is all there is to it.

Right now I have it built on a breadboard, and it generates a nice 12v square wave. I still need to build a complete board and test it in the vehicle, but I am fairly confident that I will have a MAP-controlled SAFC soon!