How about a Lesson on how the SAFC really tweaks our Fuel system.

[gsxtacy]

I have been searching for a GOOD description of how the SAFC affects DSM's.

Nothing. Plenty of people asking questions about set-ups and tuning issues, but absolutely nothing for the up and coming tuners to grasp THE WHY of how it works.

Did I explain my question right?

I searched DSMtalk.com also and found mixed results.

How about a Tuning Guru take the time to explain how it works?

Thanks
GSXTACY

 

[pneumo]

WHY does it work? Beause it wants to!

How does it work? The Apex Super AFC intercepts the airflow signal going to the ECU and modifies it so the ECU thinks there is a different ammount of air going into the engine. This causes the ECU to send more or less fuel into the engine.

Side effects: Since the DSM ECU also uses the airflow signal to adjust the ignition timing, adjusting the AFC in large ammounts will affect the timing in small ammounts.

 

[gsxtacy]

To me....Let me start off with that.......

To me the SAFC modifies Airflow like this:

You have 450cc injectors right? So -0- correction on your individual rpm settings is what the ECU thinks is stock, ......right?

Lets say you have 550cc injectors. 22% more volume per cycle.

Would -stock- settings ( -0- ) on the SAFC be -22% ?? or 450cc of volume?

I need something to relate to. I know the SAFC can be a good piggy back system if tuned properly.

Is it all going to come down to: Every car is different, no two cars settings can be the same?

That is what I really want to know.

 

[gsxtacy]

I guess tuning the SAFc is a Mystery. Please Someone enlighten me.

GSXTACY

 

[pneumo]

No it's not a mystery, there are hundreds of people on these boards who can tune an AFC.

550 injectors are 22% bigger than 450's, so you need to set the AFC to -18%.

I'm going to bed now, please forgive me if I don't reply immediately.

 

[kpt4321]

Time to type, I guess. I'll probably paste this over into a tech article if it turns out well.

First, a quick overview of the DSM air metering system:

The goal of the DSM air metering system, and any air metering system really, is to be able to calculate the mass flow of the air entering the motor. Now, in order to keep this short I’m going to keep the chemistry to a minimum, if you want more description in that area ask or start another thread. The basic premise is the combustion of gas and air is a chemical reaction, for which you need a certain amount of air molecules and a certain amount of gas molecules. The mass of a fluid is directly proportional to the number of molecules of said fluid, and thus by knowing the mass of something you can figure out the number of molecules.

Chemically, it turns out that in order to have a reaction that uses up all of the gas and all of the air available, you need to have about 14.7 times the mass of air, as you have the mass of gas. For example, if you have 14.7 pounds of air, you need 1 pound of gasoline to burn both of them completely.

The ECU needs to calculate the amount of fuel to shoot into the motor. In order to know how much gasoline it needs to inject, it needs to know how much air is coming in, and what the target air to fuel ratio (A/F ratio) is at that point in time. This is the job of the air metering devices, and some of the code in the ECU, which interprets the information from these devices.

The DSM air metering system consists of four parts: the karman vortex sensor in the MAF, and intake air temperature sensor, the intake air pressure sensor (barometric pressure sensor), and the ECU.

The karman vortex sensor works by detecting the frequency of small vorticies that happen when a smooth airflow is presented with an obstruction. The number of vorticies is proportional to the velocity of the airflow (I believe it follows the form frequency = kv^2). When the MAF outputs a “Hz” signal to the ECU, what you are really seeing is the frequency of these vorticies inside the upper chamber of the MAF.

Now, if the ECU knows the velocity of the airflow, and if it is programmed to know the cross-sectional area of the MAF, then it can calculate the volumetic flow of air through the MAF. Think about it, if you have air moving at 1 foot per minute, through a tube that has a cross-section of one square foot, you will have a flow of one cubic foot per minute. The flow of the motor is obviously much higher than that, but it’s the same theory.

If you remember high school chemisty, then you remember the ideal gas law. The ideal gas law states that PV=nRT, or when rearranged, that PV/RT=n. “n” is the number of molecules of the gas (which, as we said above, it proportional to mass). P is the pressure, V is the volume, T is the temperature, and R is a constant depending on the units used to describe the aforementioned items. In order to find the mass of the incoming air (n), me need to find all of the “other stuff” on the left side of the equation. We already solved for volume (in terms of volume per unit time), R is a constant stored in the ECU, and we can find temperature and pressure from the other two sensors in the MAF!

So, we do some math, and we get an air mass. Then, the ECU looks at a air/fuel ratio table (if in open-loop) or shoots for 14.7:1 (if in closed loop), and decides how much fuel it needs. Since the ECU knows how much the injectors flow, it can figure out how long it needs to open them up. For example, if it wants half the fuel that the injectors can deliver, then it will just open them up half the time.

Now, what happens when you install larger injectors? Larger injectors basically have bigger holes in the end, so if you open them for the same amount of time as the stockers, you squirt out more fuel. Of course, the ECU doesn’t know that the injectors are flowing more, so it keeps opening them for the same amount of time. Therefore, in order to get back to the A/F ratios that you want, you need to reduce the pulsewidth of the injector for each set of conditions.

The way a SAFC, or any other piggyback, does this is by reducing the amount of air mass the ECU thinks it is seeing. If the ECU thinks it is seeing less air, then it will shorten the pulsewidth. Since it’s not really seeing less air (the SAFC intercepts the airflow signal and reduces it), you get the same amount of air, plus a shorter pulsewidth, plus a larger injector, and the end result puts the A/F ratio back where it should be.

Basic SAFC operation: The SAFC intercepts the Karman frequency signal coming from the MAF. It then has a set of maps, indexed by rpm and load (TPS), which it looks to to find a correction factor. It multiples the airflow signal by that correction factor, and then sends the changed signal to the ECU. For example, if the airflow signal is 1000Hz, and the correction is -50% (maximum reduction) it will end up sending a 500Hz signal to the ECU.

One of the main things I think people don’t understand about the SAFC, is the way the rpm and load based interpolation works. While I won’t go off on a tangent on load here, the basic theory is that TPS is a representation of the amount of load on the motor (although not a very good one on a turbo car). The two different maps allow you to have different corrections under different load conditions, so that, for example, you can have the car richen up more as load increases in order to attempt to avoid knock. The use of the RPM points is obvious, as they just allow you to change the correction based on engine speed.

The question that comes up is, what happens if you’re between points? In reality, it’s pretty simple. The SAFC just builds a curve (although it is built out of straight lines between the points, as with most engine management). If the correction is +10% at 2000 rpm and +20% at 3000 rpm (not likely, but it makes for an easy example) then the ECU will basically “connect the dots” and when you are between those two points it just looks where that line points to on the correction map.

Examples: If you are at 2500 rpm, since you are halfway between the points, you will get +15% correction (15 is halfway between 10 and 20). If you are at 2250 rpm, you will get 12.5% correction, because that is one-quarter of the way from 10 to 20.

The SAFC does the exact same thing with the throttle points, between the high and low maps. If you are halfway between the two throttle points, then the SAFC will find the correction that is halfway between the corrections on the high and low maps. When you are below the lo throttle point, it just uses the low throttle settings, and when you are above the hi throttle point, the SAFC just uses the high throttle settings.

One of the main side effects of changing the airflow with a SAFC, is the profound effect it has on timing. The timing map in the ECU is set up as such: the ECU looks at the engine speed and the airflow (actually the airflow per rev, but we wont go there right now), and then finds the point on the timing map. The timing map is just set up like a spreadsheet, with the columns representing either engine speed or airflow, and the rows representing the other. The tendency of the timing map is that lower airflow (less load on the motor) gets more timing advance. This is for a couple reasons, but generally lower load means less heat and less cylinder pressure, which means you can use more timing advance to get the mixture to combust at the correct point. The effect this has is that if you reduce the amount of airflow that the ECU sees, it will move down on the timing map, and you will get more advance.

You need to be very careful with this. Not only do you get more advance at WOT, but you also get more advance at cruise and part throttle. In most cases, the WOT knock that can be caused by too much advance can be tuned out, but the knock caused at part throttle and when the turbo is spooling can’t always be.

 

[kpt4321]

Oops, I missed this:

Originally posted by gsxtacy
Lets say you have 550cc injectors. 22% more volume per cycle.

Would -stock- settings ( -0- ) on the SAFC be -22% ?? or 450cc of volume?

No, you don't just use the injector size difference as the correction. Think of it this way:

If you have injectors that are twice as big as stock (900 cc/min), that means they are 100% larger. However, if you used 100% as the correction (not possible) then you wouldn't get any airflow to the ECU at all!!

If the injectors are twice as big, you want to cut the airflow signal in half, right? Tha means -50% correction. So, where did that number come from?

correction = 1 - stock injector flow / new injector flow

For 900's, we get: 1 - 450/900, or 1 - 1/2, which is 1/2. 1/2 is 50%, so you use -50% airflow correction!

Make sense?

 

[bluelightning92]

This is like a frickin dictionary and my brain is soaking it all up!!
very nice post - it taught me a ton. mad props yo

 

[kpt4321]

Thank you.

This is also here in the tech article section, so that people can always find it:

http://www.dsmtuners.com/forums/showthread.php?s=&threadid=105569

 

[awdEAGLEtsi]

kpt i thought this sounded familiar, you already wrote this before...it was already in the tech section. i don't think gsxtacy looked to hard. this is the same article i got that knock question from, from the other post. but still great info....people need to read those posts first they are all like this one...VERY VERY informative!!

 

[GeneralChaos]

Originally posted by kpt4321
Oops, I missed this:



No, you don't just use the injector size difference as the correction. Think of it this way:

If you have injectors that are twice as big as stock (900 cc/min), that means they are 100% larger. However, if you used 100% as the correction (not possible) then you wouldn't get any airflow to the ECU at all!!

If the injectors are twice as big, you want to cut the airflow signal in half, right? Tha means -50% correction. So, where did that number come from?

correction = 1 - stock injector flow / new injector flow

For 900's, we get: 1 - 450/900, or 1 - 1/2, which is 1/2. 1/2 is 50%, so you use -50% airflow correction!

Make sense?

tell me if im doing this right.

so I do 1 - 450/550 (.81) and get .19 so....-19% airflow correction

and say 1- 450/660 (.68) = .32 so -32% airflow correction?

seems pretty simple now if im doing this right, just took me a minute to get the rust off my brain, been a while since I was in math class

let me know if this is right though, I dont want to get into tuning with a AFC and mess it all up. Think I got it right though.

 

[kpt4321]

Yessir, you are right on.

Keep in mind that those correction values are ONLY A BASELINE and should only be in the SAFC long enough for you to go for a drive and tune.

 

[GeneralChaos]

yea I imagine I will have to change the % up or down a few after a drive to get things as close to perfect as I can.

best way on determining that is a logger right?

 

[silverkorn]

Originally posted by GeneralChaos
yea I imagine I will have to change the % up or down a few after a drive to get things as close to perfect as I can.

best way on determining that is a logger right?

wrong.

the best way is to take it to a shop to have then tune on a dyno which can hold loads and rpms on your car with a wide band. but a logger with a UEGO will get you close, its just a matter of how many times you need to go out and log to get close to your requested AFR reading

and great write up kpt, couldnt of said it any better myself

 

[kpt4321]

Actually no. A wideband is NOT the best way to tune the low throttle, closed loop section of the SAFC.

When the car is in closed loop, the fuel trims have a certain correction range. The only time that the A/F ratio (as read by a wideband) will show the A/F mixture being incorect, is if these are out of their range.

For example, if the fuel trims are 135%, you probably would want to correct them on the SAFC. However, a wideband would say that everything is fine. A datalogger is the best tool for tuning in closed loop, because you have active feedback on exactly what the ECU is doing.

At WOT, a wideband is a GREAT tool to have, I can't argue with you there. Even still, you need to keep an eye on knock
.

 

[ProjectTwin]

File -> Save As -> Web Archive

Great thread!