Stroker2112

Guys, I'm sure alot of you have driven your DD car/truck right after putting on a new FAST intake...just to feel the extra 20hp or so....I'm curious to know how much it affected your A/F?...i know a new tune is wise as the extra flow has to lean out the A/F....anyone care to share some details?...

Stroker2112

speedfreak2685

iTrader: (12)

I have the same question. I had mine tuned with a ls6 and then 2 weeks later put a fast 90 on it. I drive it all the time but won't get on it all the way to red line because im scared it will lean out. Did you get on yours all the way to red line?

linegtdt52

iTrader: (12)

dude I've been rolling that bitch to 7k without a tune. She likes it too! Your O2 will increase it for how mild the adjustment would be. For all out performance, yeah get it tuned when you get a chance.

Stroker2112

I haven't put it on yet...I kinda wanted to see/hear some feedback from other members before I have to schedule a tune.

Ryan L

No. O2s are ignored at WOT.

Stroker2112

Is there anyway to tell if it's lean without a wideband sensor?.....

Ryan L

Yes, with software like HP Tuners (VCM Scanner.)

Frost

iTrader: (45)

no...

Ryan L

What about part throttle LTFT/STFTs?

Frost

iTrader: (45)

they take care of the AFR at cruise (so + or - as much as the are, it should still float around stoich) it's the WOT that will need to be checked with a WB

macca_779

STFT's are but LTFT's aren't as they apply to the whole fuel map. This is assuming one has LTFT's enabled.

joecar

LTFT's are used in closed loop (to keep AFR stoichiometric)...

when you go WOT the PCM goes into open loop (since the AFR has to move significantly away from stoichiometric).

macca_779

LTFT's are still applied to the map when in open loop though.. They just can't be created unless in closed loop.

Short Term Fuel Trim
With the information from the sensors, the PCM knows the conditions in the engine, how much air is flowing and, based on the TPS, the desired load to be placed on the engine. But how does the PCM know the amount of fuel to inject? This is the main function of the oxygen sensors. The oxygen sensors generate a small electric current, with voltage varying from around 0.1 to 0.95 volts. Less voltage indicates a lean condition and more voltage indicates a rich condition. They measure the difference between the amount of oxygen in the exhaust and the amount of oxygen in the atmosphere.

Since the PCM knows the amount of air and fuel entering the engine and, from the oxygen sensors, the amount of oxygen consumed during combustion, it can determine if the combustion process was rich or lean. This is a direct feedback system and enables the PCM to "Close the Loop"!

Because the feedback from the oxygen sensors can only be applied ?after the fact?, it causes the PCM to ?chase its tail? to achieve the correct combustion. This chase can be observed clearly in the oxygen sensors? output, which under normal driving conditions looks like a wave.

The combustion process is constantly oscillating between rich and lean. As long as this process continues to change from rich to lean in a timely manner, the PCM ?knows? that the combustion is within acceptable limits. If the oxygen sensor voltage stays high or low for too long, then the PCM adjusts the amount of fuel delivered by the injectors. This adjustment is called the Short Term Fuel Trim (STFT) correction.

Long Term Fuel Trim
The main function of the PCM is to store and maintain calibration tables. Most tables are static, they are set in the factory and do not change. The Long Term Fuel Trim (LTFT) correction table is different; it is modified by the PCM in response to the changing STFT correction.

For example, if the oxygen sensor voltage stayed low due to a lean combustion process, then the PCM would increase the STFT correction. If the correction does not bring the combustion process into acceptable limits then the PCM will increase the STFT correction even further. If that process continues and the STFT reaches its pre-determined upper limit, then the PCM records that; the particular conditions under which the engine is currently operating, require a longer-term correction to be applied. The PCM will then increase the LTFT correction and reset the STFT correction back to zero, and the whole process begins again.

What happens if the LTFT correction reaches its upper or lower limit? The PCM will set a Diagnostic Trouble Code (DTC) to alert the driver that the engine is operating outside of acceptable limits and the PCM can no longer compensate.

The LTFT corrections are stored in a table made up of 23 cells. There are 16 cells for most driving conditions, with 7 additional cells for things like WOT (full throttle) and other conditions. Each cell contains a LTFT correction for a specific engine operating condition.


The LTFT correction is used by the PCM to increase or decrease the injector base pulse width in an attempt to keep the combustion process at the desired level. You may have heard that the PCM ?learns?, or ?tunes? itself for changing conditions. The LTFT correction is the information that the PCM learns and stores. It can also be reset (by disconnecting the vehicle?s battery or by using a scan tool), causing the PCM to have to relearn the LTFT corrections. The PCM relearns each cell?s value as and when the engine operating conditions require the PCM to use those cells.

Here is how it operates; the following table is an example of a LTFT correction table.

MAP / RPM 0-899 900-1399 1400-2199 2200+
0-36 kPa (0) 1.5% (1) 1.6% (2) -0.5% (3) -3.0%
37-56 kPa (4) 2.0% (5) 1.5% (6) 3.7% (7) -1.2%
57-76 kPa ('8') 3.0% (9) 3.6% (10) -2.8% (11) -1.0%
77+ kPa (12) 2.1% (13) 1.0% (14) -3.8% (15) -2.0%


The 16 squares represent the LTFT cells, 0 through 15. The PCM knows the RPM and the MAP, therefore it can lookup the required cell for the current MAP/RPM engine condition. For example, for an engine speed of 1500 RPM and a manifold pressure (MAP) of 50 kPa, the corresponding fuel trim cell that the PCM would use is cell# (6). The value contained in cell# (6) is the percentage correction that should be applied to the PCM?s base fuel schedule. In this example, the injectors? base pulse width will be increased by 3.7%.

Stroker2112

so...if i drive the new top end w/out a revised custom tune the pcm will compensate as much as possible and/or then throw a dtc if/when it starts running too lean?....

joecar

Yes, you are correct... cell 22 is applied at WOT... (positive value is added, negative value is ignored...?)...

how is cell 22 derived (...I use OLSD so I don't remember...)...?

YellowToy/A

iTrader: (1)

An unported FAST will have less flow at low RPMs, so the LTFT and the STFT will be less. Now in the area where you now have more flow, above 4000 RPM you will have zero LTFT. I would guess the FAST will lower the A/F .5 a point or so. Not so bad if your A/F was 12.2 at WOT. If you were 13.2 at WOT and add .5 ouch. But the MAF may make up for it (not sure). Dont make changes with out a wide band. Lean is hot.