STFT & LTFT question

[jmakado]

i'll keep it simple so i don't confuse the situation. if more info is needed just lemme know and i'll post it.
the setup:97 gsx, walbro190, 560's, afc II
the problem: i have my STFT pretty good when i'm cruising. my LTFT are flaky however. they are at -8 when i am at any throttle lower than 10%???

they are at 0% at EXACTLY 10% or above....what the hell? also, i can't get my idle (750rpm) LTFT or STFT to change? help please!!!

 

[CanadianTSi]

Take out the 560's and put your stock Injectors back in Untill you get a bigger turbo

 

[Mitchel63]

actually he has a 16g and stock injectors have a tough time making 320hp

 

[UK-DSMer]

I wouldn’t worry too much about LTFT unless its way off. You have 560 cc injectors so basically you probably have the low settings somewhere in the -22 to - 26% range to lean the injectors back out around the stock level. With this taken in to account the ECU should be able to make adjustments based on the o2 signal.

On the 2g from what I understand you want the LTFT to be as close to 0 as possible, this then causes the ECU to do very little long term adjustment. If you have your low table settings to far off and the ECU can not compensate you will get at change engine light, fuel control related.

When you tune the high table, take in to account the turbo spool point and the fact that this is now increasing fuel pressure 1psi boost vs. 1psi fuel, which intern is increasing your theoretical injector size.

 

[lowridin2g]

i suck at tuning my low throttle settings. the way i tune my low throttle is i just free rev to 1,000 and hold it and adjust till my stft is at zero. i do that all the way up to 5,000 then i just use the same settings i did for 5,000 for 5600, 6000, 6600, 7000. i have 650cc injectors with 255 pump and aeromotive afpr set at 43 psi base fuel pressure. my low settings are at around -17 and -16 across the board.

tuning my high settings on the other hand im pretty damn good at. my 02 voltages look like, .92 from 4,000 - 4600, then .94 all the way up. maybe .96 by redline depending where i get gas. and im getting 20 degrees timing advance by redline also. by redline my high settings are set to -43 on the safc.

 

[Blk_99gst]

1g's, have a low, mid and high fuel trims. Low for idle, mid for mid, and high fuel trim for more high way crusing/above a certain % throttle.

2g's only have stft and ltft. But, since they don't have individual low,mid,high, the ltft will change once it crosses some threshold of airflow or throttle percentage. So you will see one value of ltft at idle, but then driving on the highway you will see another.

 

[jmakado]

i appriciate all the responses, but i think some of you guys are missing my point. i have no problem tuning the HI settings. i also have my STFT and LTFT pretty damn close. my question is this: does it matter that my LTFT are at -8% below 10% throttle and why is it like that. i think that blk_99gst is correct about the different map.....but shouldn't the afc change all the maps???

 

[jmakado]

Take out the 560's and put your stock Injectors back in Untill you get a bigger turbo

uuuuuhhhhhh no, wrong answer.....

 

[98spydert]

i appriciate all the responses, but i think some of you guys are missing my point. i have no problem tuning the HI settings. i also have my STFT and LTFT pretty damn close. my question is this: does it matter that my LTFT are at -8% below 10% throttle and why is it like that. i think that blk_99gst is correct about the different map.....but shouldn't the afc change all the maps???

The SAFC is going to make the corrections you tell it to. It doesn't know the specific map you're in or the correction the ECU is making. The problem is the SAFC doesn't adjust fuel based on airflow like the MAFT does so if you're right in between maps(going from low to mid) and you've made adjustments for one, the other map may need different adjustments to keep the trims happy. You can't adjust based on airflow though so you're stuck. 10% throttle at 3000rpm and 30% throttle at 3000 rpm are getting the same adjustments from the SAFC even though you're probably in two different airflow maps and they require slightly different settings.
Have you set your low settings based on idle or while cruising?

 

[Blk_99gst]

my question is this: does it matter that my LTFT are at -8% below 10% throttle and why is it like that. i think that blk_99gst is correct about the different map.....but shouldn't the afc change all the maps???

If you are at -8% ltft at idle, then the ecu is having to remove 8% of fuel at idle, so you are a little too rich at that rpm point. That rpm point on the safc is probably 1000rpm for you, so I would lean that out maybe 5% and see how that affects the ltft at idle. While you are crusing (50mph, 10 or 20 percent throttle) your ltft will read something different, and you adjust that one using the other safc rpm ranges, 2000 through 4000. For all rpm ranges above 4000, they should be set to whatever value the 4000rpm is.

 

[jmakado]

i set them first idleing and then fine tuned them a little by driving at *X* rpm and making small adjustments. i have them to 0% all the time unless the throttle is below 10% and then they are at -8%. which brings me to idle. i will try going a little lower witht the 1000rpm point to get the STFT to 0% at idle......it's really just me being way too picky. the hi settings were easy on a dyno, but all this fuel trim lo-setting BS is making me wish i had dsmlink more every day....anyone want an AFC-II with less than 500 miles in the car????

 

[ahains]

When you tune the high table, take in to account the turbo spool point and the fact that this is now increasing fuel pressure 1psi boost vs. 1psi fuel, which intern is increasing your theoretical injector size.

Minor correction - the amount of flow through an injector is determined by the size of the opening, and the *pressure differential*. The "effective" pressure can be determined by subtracting the manifold pressure from the fuel line pressure.
When you add 1psi of boost, you *must* increase the fuel pressure by 1psi to compensate or else you will be changing the A/F ratio. So this does not increase your effective injector size at all.

HTH,
Adrian

 

[Blk_99gst]

i set them first idleing and then fine tuned them a little by driving at *X* rpm and making small adjustments. i have them to 0% all the time unless the throttle is below 10% and then they are at -8%. which brings me to idle. i will try going a little lower witht the 1000rpm point to get the STFT to 0% at idle......it's really just me being way too picky. the hi settings were easy on a dyno, but all this fuel trim lo-setting BS is making me wish i had dsmlink more every day....anyone want an AFC-II with less than 500 miles in the car????

Short term fuel trims can move around so much that it's hard to tune for them. Just make sure the ltft are moving in the correct direction (ie closer to zero). Also make sure you car is warmed up before doing any fuel trim tuning. If you drive on the highway at 60mph, your ltft will be the "high" ltft value. Then if you instantly let off the gas, the ltft will switch to the "low"/idle ltft value. This "low"/idle ltft value will also exist at idle...makes sense that it should!

And you're right, you don't have to be that picky with the fuel trims. I say as long as you are plus or minus 5 percent, that's fine. They don't need to be dead on zero. And they will change with weather condictions, different gas, and other factors.

anyone want an AFC-II with less than 500 miles in the car????

There's a local guy looking for a safc; if you're interested in selling it pm me.

 

[jmakado]

thanks for all the help guys!! as far as selling it......i will be but probably not for a couple months when i can pony up for DSMLINK. i'll pm you when i do though and see if your friend still is looking for one. thanks again!