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Username Post: c90 datalog shows air/fuel ratio 18.0
Danny Cabral 
Member
Posts: 3447
Danny Cabral
Loc: Connecticut
Reg: 11-03-04
06-14-12 01:25 AM - Post#2236964    
    In response to Danny Cabral

Here's the entire C950 Fuel Map tuning procedure (using the Data Logger),
collectively in one post, for others who may be searching for it:

  • Danny Cabral Said:
Especially for tuning purposes, I would:
A) Set the 'Max TPS for Closed Loop Operation' at two numbers higher than your 'WOT TPS Position'. You don't want any chance of the TPS value moving slightly over the wide open throttle value, and entering open loop (due to thermal expansion, flexing, etc).
B) Set ALL the 'O2 Compensation Limits (%)' to 25. This will allow the ECU to do the most it can, and give you a greater range of tuning calculation. When you start tuning the Fuel Map, that preset 15% will limit the ECU's compensation range, and you'll be wondering how much more the ECU would've corrected the Fuel Map (necessitating the need for more data logs).

This will set the ECU in constant closed loop mode, allowing you to data log (O2 Mod), and tune the Fuel Map based on the ECU's corrections (Closed Loop Compensation). In other words, let the ECU tell you which direction to tune, and how much. Of course, this requires a good Target Air/Fuel Ratio Map.

Use the data log's O2 Mod, MAP & RPM to tune the Fuel Map by adding (rich) or subtracting (lean) the O2 Mod's percentage from each cell. Use a calculator, and don't tune the same cell twice. Locate the Fuel Map cell you want to tune by intersecting the horizontal MAP value, and the vertical RPM value's axis point. You're finding the MAP & RPM axis values from the data log, and applying them to the Fuel Map. You'll need to do this many times by scrolling along the data log, and changing Fuel Map cell numbers.

Afterwards, the Fuel Graph needs to be smoothed out. Be conservative because adjusting the Fuel Graph actually changes the Fuel Map tuning (cell numbers). It should look like the example on page 35 of the C950 manual (and even that one could be a bit smoother). Study the shape (curve) of the Fuel Graph...it's that way for a good reason because it matches the torque curve of the engine; not the horsepower curve. (Make sure you also understand the 'graphical representation' of the Fuel Map/graph on page 31.) Read this link for more information on feeding the engine's torque curve: https://www.chevytalk.org/fusionbb/showtopic.php?ti...

You can do all of this on your home PC at you leisure. If you need further instruction on the mathematical calculations (O2 Mod % from the data log), and how to apply them to the Fuel Map, see below.


  • Danny Cabral Said:
Record a short steady state data log.
Locate the cell by intersecting the MAP & RPM values.
At that point, look at the "O2 Mod" percentage.

100% is the baseline, the ECU wasn't adding or subtracting fuel.
Anything over 100%, the ECU was adding that fuel (too lean).
Anything under 100%, the ECU was subtracting that fuel (too rich).

Using cell value 60 for example, with an O2 Mod of 115% (too lean);
60 (Fuel Map cell value) + 15% (O2 Mod) = 69 new Fuel Map cell value.
If the O2 Mod is under 100%, you would simply subtract the percentage.

After tuning the specific cells, enter the Fuel Graph, and blend the
surrounding areas to those tuned cells. Smooth over the entire graph.
Do this until the data logs indicate a consistent 95%-100% O2 Mod.
Of course, ensure your Target Air/Fuel Ratio Map is programmed to your liking.


  • Danny Cabral Said:
After tuning the Fuel Map, the O2 Compensation Limits % should be decreased in the high load/full throttle areas. So if you have a wideband O2 sensor error/failure, the engine can still run fairly well (without the ECU modifying 25% of the fueling). Change to Open Loop mode until you can replace the O2 sensor. If you start tuning without the full 25% O2 Compensation, it will require you to perform more data logging because a new Fuel Map will need more than a "minimum" amount of tuning. Keep in mind, I'm talking about temporary settings for initial tuning purposes (with a new C950/wideband system).

There is an exception, when you can decrease the O2 Compensation Limits, from the start: If you used the C950 'Map Builder' Excel program (C950CALC in C:drive software) accurately, then your base map should be fairly close, and won't require an excessive amount of O2 Compensation when tuning. Use the C950 'Map Builder' program first for WOT, blend the Fuel Graph next, and then begin tuning (you'll be smoothing the Fuel Graph again after each data log/tuning session).


  • Danny Cabral Said:
I'd like to add some necessary information to my aforementioned 'data log tuning' procedure.
When applying the data log changes to the Fuel Map, only make changes from the 'steady-state' periods of the data log. In other words, when reviewing the data log, focus on the areas where you were not accelerating. During acceleration, the base map is modified by adding the 'transient fuel' (acceleration enrichment) parameters. If you tune the Fuel Map during these transient fueling periods, the base Fuel Map will end up being overcompensated. (Acceleration enrichment can't be tuned until the base Fuel Map is steady-state tuned.)

The key here is to look at the green RPM line on the data log. For initial Fuel Map tuning purposes, ignore the diagonally upward periods - transient fueling. As a matter of fact, I usually only select the green RPM line for simplicity. Read the O2 Mod's % value during flat horizontal (steady-state) periods of the data log. If in doubt, verify that the TPS and/or MAP values are not increasing. Continue to tune those Fuel Map cells by intersecting the horizontal MAP & vertical RPM axis points (MAP & RPM points acquired from data log). At 4000+ RPM, acceleration enrichment is virtually nonexistent, so the task becomes easier. At 4000 RPM & higher, just read and tune directly off of every point on the data log.

Knowing how to drive for data logging purposes is especially useful, and will ease your tuning task.
First, I like to record short data logs. This will lessen the chance of tuning the same Fuel Map cell twice, and the data log will look less daunting. Second, since the objective is to steady-state tune, avoid a lot of acceleration (or do it as quickly as possible). I like to start by recording several seconds of idling (neutral), in gear and with the A/C on too. Then in 1st gear, hold the RPM steady for several seconds according to each RPM point on your Fuel Map. Repeat this process for each number of gears/speeds your transmission has (overdrive will be recorded on the highway only). In other words, you'll be doing this many times for each gear and RPM point. Smooth the Fuel Graph after each tuning session.

A good example of a second data log would be repeating this entire procedure going uphill or at some points, gently applying the brakes to reach higher MAP load points. (Better yet, if you can get your vehicle stuck in the mud or snow, you can use this to substitute a 'rolling road'...don't laugh, it works.)
Lastly, for initial tuning purposes, I would disable the 'decel fuel cutoff' function by enter 9000 RPMs, otherwise it will correctly indicate a maximum lean 18:1 air/fuel ratio, preventing you from tuning some deceleration points of the Fuel Map. Ideally, the entire fuel map should be tuned to 95-100% Closed Loop Compensation.

May God's Grace Bless You

'78 BRONCO: 508" stroker, TFS heads, Dominator MPFI & DIS, A/C, Lentech Strip Terminator AOD, 3:1 Atlas II, modified Dana 44/60-lockers-4.10s, hydroboost/4-disc brakes, ram-assist/heim joint steering, 4" lift, 35" tires

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