tuning beginner

[kenamond]

I'm bored, so...LTFT and STFT:

The ECU has a closed-loop and open-loop mode.

The ECU has a fuel map. This tells it how much fuel to injecto for every combination of airflow and rpm. The MAF and CPS tell the ECU the airflow and rpm constantly, so it uses these to determine the base fuel to inject.

When the car is in open-loop mode (WOT), it injects fuel from the map and that's it.

When the load is lower, it is in closed-loop mode. In this mode, the ECU uses the front O2 sensor to stay right at a stoichiometric AFR (14.7). The O2 sensor will read 0.5v at stoich. The ECU looks for an alternating signal that oscillates above and below 0.5v. If the O2v stays above 0.5v, the car is running richer than stoich and the ECU notices this. If it stays below 0.5v, the car is running leaner than stoich. The ECU uses a short-term fuel trim (STFT) to make small adjustments to the fuel read from the base fuel map. If the O2v is rich, the STFT will store a number that tells the ECU to remove fuel and how much to remove. The ECU is constantly using this STFT to tweak the AFR in closed loop mode. If it gets it right, the O2v will go back to oscillating around 0.5v. If conditions are right, the ECU enters learn mode. This is where it memorizes the STFT values by storing them in a long-term fuel trim (LTFT). I've heard there are two trims from some and three trims from others. I don't know for sure. Anyway, there may be a LTFT-Lo, LTFT-Mid, and LTFT-Hi. If airflow is low, it stores the STFT value in the LTFT-Lo, etc. After it memorizes the STFT value, it resets it.

So in closed-loop mode, the ECU reads the fuel map, then adds in the STFT+LTFT corrections and injects. It monitors the O2v signal and corrects via the STFT. When it enters learn mode, it stores the STFT into the appropriate LTFT. Repeat. Typically, the LTFT values will stabilize and the STFT will hang out at the reset value (the ECU has "tuned itself". But that's only for closed-loop and learn modes. WOT/open-loop mode is blind map reading as far as fuel injection goes.

I'm not sure what triggers the ECU (1g or 2g) to enter learn mode. I'm under the impression that it's usually in learn mode during cruise/idle, but I don't know any of the specifics.

If anyone wants to add to or correct what I said, fire away. I think it's close though and should give you a good idea about trims.

 

[1992awdlaser]

I'm bored, so...LTFT and STFT:

The ECU has a closed-loop and open-loop mode.

The ECU has a fuel map. This tells it how much fuel to injecto for every combination of airflow and rpm. The MAF and CPS tell the ECU the airflow and rpm constantly, so it uses these to determine the base fuel to inject.

When the car is in open-loop mode (WOT), it injects fuel from the map and that's it.

When the load is lower, it is in closed-loop mode. In this mode, the ECU uses the front O2 sensor to stay right at a stoichiometric AFR (14.7). The O2 sensor will read 0.5v at stoich. The ECU looks for an alternating signal that oscillates above and below 0.5v. If the O2v stays above 0.5v, the car is running richer than stoich and the ECU notices this. If it stays below 0.5v, the car is running leaner than stoich. The ECU uses a short-term fuel trim (STFT) to make small adjustments to the fuel read from the base fuel map. If the O2v is rich, the STFT will store a number that tells the ECU to remove fuel and how much to remove. The ECU is constantly using this STFT to tweak the AFR in closed loop mode. If it gets it right, the O2v will go back to oscillating around 0.5v. If conditions are right, the ECU enters learn mode. This is where it memorizes the STFT values by storing them in a long-term fuel trim (LTFT). I've heard there are two trims from some and three trims from others. I don't know for sure. Anyway, there may be a LTFT-Lo, LTFT-Mid, and LTFT-Hi. If airflow is low, it stores the STFT value in the LTFT-Lo, etc. After it memorizes the STFT value, it resets it.

So in closed-loop mode, the ECU reads the fuel map, then adds in the STFT+LTFT corrections and injects. It monitors the O2v signal and corrects via the STFT. When it enters learn mode, it stores the STFT into the appropriate LTFT. Repeat. Typically, the LTFT values will stabilize and the STFT will hang out at the reset value (the ECU has "tuned itself". But that's only for closed-loop and learn modes. WOT/open-loop mode is blind map reading as far as fuel injection goes.

I'm not sure what triggers the ECU (1g or 2g) to enter learn mode. I'm under the impression that it's usually in learn mode during cruise/idle, but I don't know any of the specifics.

If anyone wants to add to or correct what I said, fire away. I think it's close though and should give you a good idea about trims.

Thats a good explanation. The only thing to add is there is infact two ltft's for a 2g and three for a 1g. Ltft lo is when the air flow is around 50 hz for both. Ltft mid is when a 1g is flowing around 150hz and 150-400hz for a 2g. The 1g also has a ltft hi for around 250-400hz.

This is out of the DSMLink wiki and has an explanation at the bottom that tells what must be true for the ecu to enter learn mode.

"Like any control system, there are two basic fuel modes in the DSM ECU: open loop and closed loop

In a nut shell, while running open loop the ECU just assumes it knows how much fuel is really going in and it assumes that accurate airflow data is being presented to it. It does not check these assumptions against anything. It just calculates how much fuel is need to achieve a certain A/F ratio based on airflow data. It looks this target A/F ratio up in an open loop fuel table indexed by RPM and engine load.

In closed loop mode, the ECU checks itself by “watching” the front O2 sensor feedback. The factory front O2 sensor switches abruptly around 14.7:1 A/F ratio. The ECU tries to adjust fuel so that the O2 sensor swings back and forth around this target. To do that, the ECU has to have some way to vary its basic fuel calculation. It uses fuel trims for this purpose. Think of these variables as dials if you want. Turn the dial one way and the ECU adds fuel, turn the dial the other way and the ECU reduces fuel.

While running closed loop mode, the ECU uses what's called a “short term fuel trim” (STFT) variable for quick changes. This is the variable is uses to force the O2 sensor to cycle. In a perfect setup, this variable would swing from positive to negative values of about equal amplitude. As it's swinging up and down, the front O2 sensor should swing up and down. This is the basic closed loop mode of operation.

The ECU will also watch what's going on with the short term fuel trim variable. If conditions are right and the short term variable has stayed positive or negative long enough, the ECU will move the long term STFT pattern into the long term variable (LTFT). This is called a learning mode. Let's say instead of swinging around 0%, STFT is swinging up and down around -5%. The ECU will eventually take note of this and move that -5% adjustment into the appropriate LTFT variable and then zero out the STFT variable. Since both are factored in, the end result will be the same but STFT should now swing around 0%.

The ECU uses at least a couple different LTFT variables depending primarily on airflow. Lower airflow values seen at idle will typically result in the LTFT Lo variable being used (and updated). Higher airflow values seen at cruise will result in LTFT Mid (2G) or LTFT Hi (1G) typically being used.

Details
There are several reasons the ECU might switch into or out of closed loop mode. In addition, even while running in closed loop mode, the ECU may or may not be running in a mode that allows learned STFT adjustments to propogate into the LTFT variables (learn mode). So there are really three different modes of operation.

Open loop (STFT and LTFT ignored)
Closed loop (STFT used but LTFT not updated)
Learn mode (STFT used and long term patterns moved into LTFT)
We'll attempt to document some of the conditions that will force the ECU into or out of these modes. First, it's probably no surprise that these conditions are different between 1G and 2G ECU codebases. So the conditions will be listed separately for each.

The ECU will force open loop mode for a number of different reasons. The following is intended to list out the more common reasons. If any one of these is true, then the ECU will run in full open loop mode. That is, STFT will not be used and, of course, learn mode will not be active either.

1G Open loop mode (any one true)
Throttle position too high (varies by RPM)
Airflow too low (units are tricky, but it's very low)
Coolant temp < 87F
The O2 sensor did not switch around 0.5v for over 20 seconds while running in closed loop mode. In that case, the ECU locks itself in open loop mode.
2G Open loop mode (any one true)
Throttle position too high (varies by RPM)
Airflow too low (units are tricky, but it's very low)
Coolant temp < 51F
Coolant temp > 228F
The O2 sensor did not switch around 0.5v for over 128 seconds while running in closed loop mode. In that case, the ECU locks itself in open loop mode.
All of the following have to be true for a full 5 seconds. If any single one is not true even for a millisecond, the timer starts over and trends in STFT will *not* be moved into LTFT.

1G Learn mode (all must be true)
Coolant temp >= 190F
Intake air temp < 123F
Baro >= 22.9 inHg
Baro &#8656; 31.6 inHg
2G Learn mode (all must be true)
Coolant temp >= 180F
Intake air temp < 133F
Baro >= 22.9 inHg
Baro &#8656; 31.6 inHg"

 

[debbie]

if your looking for new injectors I have a set of pte 680cc's here some where. they were in great shape and used for maybe 2-3 years when i took them off to upgrade to my 1000's If your interested I would let them go pretty cheap as i dont really have a use for them anyways.... pm me if you would like them. BUT, if you do I would definatly get dsm link befor you install them... I know most afc's have a hard time with anything above 550cc's. I dont know how wild your planning on going but the 680's were used with my evoIII 16g running at 23psi and maxing out around 80-85 %. was good for around 350-400hp.

how much you asking and what bhp would i expect on stock engine and turbo with these injectors

 

[debbie]

Thanks to Kenamond and 1992awdlaser for there write ups they made stuff much more clear, the strange thing is i could not explain it to someone else but i under stand it, or atleast starting to, but anyways thanks

 

[kenamond]

how much you asking and what bhp would i expect on stock engine and turbo with these injectors

If you're going with DSMlink, go with large injectors (around 1000cc/min would be nice). DSMlink can handle them and you'll have incredible growing room. If you don't plan on making a monster out of your car, something in the 800-900cc/min range would do, too.

You *cannot* upgrade your injectors without some sort of tuning tool like DSMlink, S-AFC, or AEM-EMS (there are others of course). The ECU expects stock injectors and will pulse them exactly the same in open loop mode as if they were stock (it'll look at the fuel map, determine how long to pulse the stock injectors to get the correct fuel, then pulse). If you increase injector size, the car will run rich at WOT and there's nothing you can do about it without a tuning tool. So you have to upgrade injectors and tuning tool at the same time.

Also, the stock fuel pump can't provide enough fuel flow to support more than around 15psi boost. At that point, the injectors are flowing so much fuel that the fuel pressure starts to drop which means less fuel goes through the injectors when they're pulsed open. So when the fuel pump can't keep up, you run lean at WOT and damage the motor.

And when you upgrade the fuel pump, you'll need to upgrade your fuel pressure regulator at the same time. The larger fuel pump will flow much more fuel than the stock FPR can regulate (the FPR lets fuel return from the fuel rail back to the fuel tank while keeping the pressure in the fuel rail at a certain pressure), so the fuel pressure in the fuel rail will go too high (the FPR gets "overrun" and can't return enough fuel to the tank) which will cause more fuel to flow through the injectors when they're pulsed open (higher fuel pressure forces more fuel through an open injector than lower fuel pressure).

I'd recommend reading through the stage 1 upgrade path. It explains a lot of this. There is an order to upgrading, and if you do it in the wrong order, your car will run poorly or not at all and you'll end up having to buy other parts (that should have already been installed) to get it to run correctly. But it's not really that complicated once you understand it

 

[1992awdlaser]

If you're going with DSMlink, go with large injectors (around 1000cc/min would be nice). DSMlink can handle them and you'll have incredible growing room. If you don't plan on making a monster out of your car, something in the 800-900cc/min range would do, too.

You *cannot* upgrade your injectors without some sort of tuning tool like DSMlink, S-AFC, or AEM-EMS (there are others of course). The ECU expects stock injectors and will pulse them exactly the same in open loop mode as if they were stock (it'll look at the fuel map, determine how long to pulse the stock injectors to get the correct fuel, then pulse). If you increase injector size, the car will run rich at WOT and there's nothing you can do about it without a tuning tool. So you have to upgrade injectors and tuning tool at the same time.

Also, the stock fuel pump can't provide enough fuel flow to support more than around 15psi boost. At that point, the injectors are flowing so much fuel that the fuel pressure starts to drop which means less fuel goes through the injectors when they're pulsed open. So when the fuel pump can't keep up, you run lean at WOT and damage the motor.

And when you upgrade the fuel pump, you'll need to upgrade your fuel pressure regulator at the same time. The larger fuel pump will flow much more fuel than the stock FPR can regulate (the FPR lets fuel return from the fuel rail back to the fuel tank while keeping the pressure in the fuel rail at a certain pressure), so the fuel pressure in the fuel rail will go too high (the FPR gets "overrun" and can't return enough fuel to the tank) which will cause more fuel to flow through the injectors when they're pulsed open (higher fuel pressure forces more fuel through an open injector than lower fuel pressure).

I'd recommend reading through the stage 1 upgrade path. It explains a lot of this. There is an order to upgrading, and if you do it in the wrong order, your car will run poorly or not at all and you'll end up having to buy other parts (that should have already been installed) to get it to run correctly. But it's not really that complicated once you understand it

Just want to add to this. Some people say oh I don't need an upgraded fpr my car runs fine. Well the car will be running rich and sometimes it won't make the car run worse. But, when it is doing this all the extra fuel is washing down the cylinder walls which will contaminate the piston rings which will cause blow-by which will mean you need to rebuild.

 

[debbie]

this stuff i do understand so lets see if i get this, ill need the DSM link to up the fuel and so on, bigger injectors to deliver a bigger pulse of fuel, a bigger fuel pump to provide the fuel and a regulater to control the fuel flow. (and a nice bank manager)

 

[1992awdlaser]

this stuff i do understand so lets see if i get this, ill need the DSM link to up the fuel and so on, bigger injectors to deliver a bigger pulse of fuel, a bigger fuel pump to provide the fuel and a regulater to control the excess fuel. (and nice bank manager)

Are you asking? Because if you read post number 30 that stuff was just explained.

 

[debbie]

Are you asking? Because if you read post number 30 that stuff was just explained.

no dude think i have it thank you