SAFC setting Q, how low can you go?

[sids]

ok here's my NE points on my SAFC

1000
1500
2000
2500
3000
4500
5000
7000


I want to tune the lo throttle 1000 - 3000 RPM range for fuel economy. I use this the most because most of my driving is cruising in traffic.

currently here's my guesstimate settings

1000 -4%
1500 -5%
2000 -6%
2500 -6%
3000 -5%
4500 up to 7000 is zero out

so how low can i go in the 1000 to 3000 rpm range without any problems? I don;t thing knock will be problem because there's hardly any boost in that range, i just want as lean as i can get

oh BTW stock turbo, fuel pump and injectors

 

[steve]

Keep in mind that while the ECU is in closed loop mode any changes you make with the SAFC will be counteracted by the ECU increasing the fuel trim.

Steve

 

[kpt4321]

Steve is right on. The fuel trims will bring the AFR back up, and they have a range of around 20-30% of full correction.

Once you get it lean enough that the fuel trims are maxxed and they can't add any more fuel, the car will start to lean out. Timing will also be jacked up, and if you lean it out too much the car will start to run like crap.

You really don't want to tune like that.

 

[1995badgsx]

keep in mind that injectors size and fuel system have everything to do with this, if you will post that, then everyone could give you a better start out setting

and are you hitting fuel cutt or does it smoke alot under hard acceleration????

 

[CanadianTSi]

Think about it...If you have the stock Injectors and pump why do you need to change it?? the ECU knows exactly how to do it.

 

[steve]

keep in mind that injectors size and fuel system have everything to do with this, if you will post that, then everyone could give you a better start out setting

read the last line of the first post

 

[sids]

ok let me rephrase my objective. I want to tune my lo rpm range and lean it out all for the sake of getting better mileage. here's my understanding.

DSms run on the rich side no matter what rpm, even if its in closed loop mode, the ecu will still give more fuel than neccessary, its not going for 14.7:1 AF, probably something like 12:1

now like what kpt said got me thinking:

1. low throttle and the ecu is in closed loop mode, taking readings from the o2 sensor (which is basically a rich/lean indicator as i understand it) and squirts fuel according to what it sees.

2. making the SAFc correction factor will make the engine see less air, so then it will add more fuel in order to richen things up, bad for fuel economy, OR

3. use SAFC to richen things up making the ecu see more air than is really there causing it to squirt less fuel because it thinks its too rich.

so is it option 2 or 3? or is there a better way of doing it?

 

[steve]

I understood what you were asking.

Let me give you a simpler answer. While the ECU is in closed loop you can't do what your thinking unless you find a way to lie about the output of the O2 sensor. The switching point of the O2 sensor is what decides the target AF ratio in closed loop. With a good O2 sensor that switching point is intended to be 14.7:1.

You might want to read the 1G DSM tech manual to better understand what's going on.
http://www.twingles.com/manualcd/dsmgift/

Steve

 

[kpt4321]

ok let me rephrase my objective. I want to tune my lo rpm range and lean it out all for the sake of getting better mileage. here's my understanding.

So, the same thing you said in the first post.

DSms run on the rich side no matter what rpm, even if its in closed loop mode, the ecu will still give more fuel than neccessary, its not going for 14.7:1 AF, probably something like 12:1

No. They run at 14.7:1 in closed loop, if the O2 sensor is working and the fuel trims are in range.

1. low throttle and the ecu is in closed loop mode, taking readings from the o2 sensor (which is basically a rich/lean indicator as i understand it) and squirts fuel according to what it sees.

Yes.

2. making the SAFc correction factor will make the engine see less air, so then it will add more fuel in order to richen things up, bad for fuel economy, OR

If the ECU (not the engine) is seeing less air, why in the world would it inject MORE fuel?

Less air = less fuel.

3. use SAFC to richen things up making the ecu see more air than is really there causing it to squirt less fuel because it thinks its too rich.

Again, what? If the ECU sees more air, it gives you mroe fuel.

so is it option 2 or 3? or is there a better way of doing it?

Option 4, which is YOU STILL CAN'T FREAKING DO IT.

Read the two posts Steve and I made about fuel trims, again. And then maybe again.

 

[sids]

ok bear with me dsm wisemen, what kyle said got me thinking

SAFc modifies AF signal:
assumming zeroed out settings, ecu sees 14 air (simplifying tihngs here a bit but makes for better understanding) and so it squirts 1 fuel, giving you a 14:1 ratio which the o2 will see as stoich.

SAFc correction factor of say -10% making the ecu see 13 air, so it squirts out 0.92 fuel, still maintaining 14:1 ratio and o2 sensor still reads as stoich

increase the correction factor to -20% and ecu sees 12 air and squirts out 0.85 fuel, still 14:1 ratio and still stoich

so is this how the ecu works?

 

[steve]

Nope.

Closed loop when you apply AFC correction so that the ECU sees less air and computes less fuel needed the O2 still reads actual combustion of real air and computed fuel so it reports lean causing the ECU to start adding more fuel until it negates your correction or goes as far as it can and maxes the trims out.

The error in your examples is that the O2 doesn't still read stoich, it reads lean because you really have 14 parts of air entering but less than 1 part fuel because of your fooling the ECU.

So why do people use an SAFC?

To adjust the AF ratio in open loop. On a stock car this allow you to lean out the AF at WOT.
To adjust for larger injectors. Open or closed loop you need to compensate for the fact the ECU thinks is has smaller injectors than it does, so you lie to the ECU telling it that less air
is entering the system to balance things out. In addition you usually lean the high throttle settings for the same reason as above.

I'm starting to wonder if your paying attention

Steve

 

[sids]

that's why I spend most of my time in the non performance mods section :laugh:

anyway if the ecu compensates and sefl regulates in close loop, why then is there a page on RRE on how to tune your lo-throttle? and if its purpose is to fool the use when using larger injectors (which i get when in open loop), then during crusing around town in lo throttle, no adjustment needs to made because the ECU will automatically reduce injector pulse width based of o2 sensor readings

then why does everyone here seem to have a lo throttle map?

I do like playing with it though by maxing the correction factor -40% and making the engine stumble at idle and increasing it a few notches up

 

[CanadianTSi]

This also got we thinking.

If there is no way to change your A/F Ratio with stock Injectors then how come it works with bigger Injectros?? (talking about low throttle of course)

 

[steve]

This also got we thinking.

If there is no way to change your A/F Ratio with stock Injectors then how come it works with bigger Injectros?? (talking about low throttle of course)

The royal we?

Keep thinking. What I've said so far is that in closed loop the ECU uses the feedback from the O2 sensor to try and maintain 14.7:1.

With larger injectors installed the ECU will always be providing more fuel than is needed. The O2 feedback will say to lean things out and fuel trims will drop and depending on how much larger they are you will max the trims and not be able to reach the target AF.

When you use the SAFC with larger injectors your low settings are negative to fool the ECU into thinking less air is entering the system so that it will call for less fuel, since the injectors are bigger you wind up with the same amount of fuel as you would with the stock injectors and zero correction. The O2 feedback is right on the money at 14.7:1 and your fuel trims stay centered. Your not changing the target AF, just making it easier or possible for the ECU to get there.

High settings are used to adjust open loop where you can adjust AF.

This has all been slimplified to make it easier to understand. An astute reader might ask why the O2 sensor cycles if the ECU holds 14.7:1

Steve

 

[sids]

gee I suppose then this lo throttle setting for road race is bogus since it recommends a plus 50% for stock turbo and fuel system (which is what i have right now, along with the other mods)

2G Turbo#### Stock#

T-25 turbo, RSR Cat Back, RSR DP-no cat, Filter, 14psi#
Stock Injectors, Side and Lower honeycombs out, Adjusting screw backed out flush#

Low Setting:
1K +50#### 2K +18## 3K +10## 4K +8#### 5K +6#### 5.5K + 6#### 6K +6##### 7K +6#

High Setting:
1K +0##### 2K -1###### 3K -1##### 4K 0####### 5K 0###### 5.5K +3#### 6K +4###### 7K +5#

TH-Point
Lo# 20%########### Hi## 60%#

Now I'm not syaing that because its from RRE that its the bible or anything but then why would it be there in the first place?

 

[steve]

Because they hacked the MAF. "Side and Lower honeycombs out, Adjusting screw backed out flush" that causes more air to enter the system than would normally and would cause the car to run very lean without correction.

Steve

 

[UK-DSMer]

gee I suppose then this lo throttle setting for road race is bogus since it recommends a plus 50% for stock turbo and fuel system (which is what i have right now, along with the other mods)

T-25 turbo, RSR Cat Back, RSR DP-no cat, Filter, 14psi#
Stock Injectors, Side and Lower honeycombs out, Adjusting screw backed out flush#

Read RRE's site again, carefully, thats not what you have at all.