LS tuning fundamentals

• LS Gen III/IV; 1997-2013: The Sniper EFI system requires a high-pressure fuel pump capable of 60 psi, with a built-in regulator factory-set at 58.5 psi, and at least 3/8-inch feed and 5/16-inch return lines.
• LS Gen III/IV; 1997-2013: The wideband oxygen sensor should be mounted where it samples a good average of one bank's cylinders, slightly after they merge; with long-tube headers that means roughly 1-10 inches after the collector, with at least 18 inches of exhaust pipe downstream of the sensor.
• LS Gen III/IV; 1997-2013: If the vehicle has catalytic converters, the oxygen sensor must sit between the engine and the converters, and it must never be installed at the bottom of the exhaust tube so condensation cannot enter the sensor.
• LS Gen III/IV; 1997-2013: The Sniper's hot idle target is the RPM it targets once coolant temperature is above 160 degrees Fahrenheit.
• LS Gen III/IV; 1997-2013: Converting to EFI will not fix mechanical problems: the engine must have no vacuum leaks, correctly set ignition timing, and sound mechanical condition, because EFI cannot cure bad valves, damaged head gaskets, worn piston rings, or cracked leaking vacuum lines.
• LS Gen III/IV; 1997-2013: Closed Loop status on a Holley ECU means it is adding or subtracting fuel to hold the target air/fuel ratio; the system is intended to run in closed loop almost all the time.
• LS Gen III/IV; 1997-2013: CL Comp is the moment-by-moment percentage of fuel the Holley ECU is adding or removing to hit the target: a minus sign means fuel is being removed, no sign means fuel is being added, and it reads 0 percent in open loop.
• LS Gen III/IV; 1997-2013: The Holley Target AFR changes with engine speed and load, and closed loop compensation continuously adjusts fuel so the wideband reading (AFR Left) stays near the Target AFR.
• LS Gen III/IV; 1997-2013: Holley self-tuning (Learn) only runs when engine temperature exceeds 160 degrees Fahrenheit, the engine is in closed loop, and Learn is enabled; the status reads Learn when active and NoLearn when it is not.
• LS Gen III/IV; 1997-2013: Idle is set on a fully warm engine: in neutral, adjust the idle screw until IAC Position reads between 2 and 10 percent, and do not attempt this below 160 degrees Fahrenheit.
• LS Gen III/IV; 1997-2013: The Sniper's fuel prime shot fires 2.5 seconds after key-on; turning the key quickly without waiting skips the prime and can make the engine take longer to start.
• LS Gen III/IV; 1997-2013: A practical self-tuning drive cycle: warm the engine past 160 degrees, drive varied speeds, loads, and gears with A/C on and off; when CL Comp sits near zero in an area, the self-tuning is considered complete there.
• LS Gen III/IV; 1997-2013: Key-on/engine-off sensor baselines before first start: TPS should read 0 and sweep to 85-100 percent at wide open throttle, MAP should read 95-102 (as low as 75 at high elevation), and battery should be at least 12.0 volts.
• LS Gen III/IV; 1997-2013: For the first start, use non-timing-control ignition, then set static timing to 15 degrees with a timing light on the balancer before handing timing control to the ECU.
• E38; LS Gen III/IV; 1997-2013: Closed loop is the built-in feedback control: the ECU reads the exhaust oxygen sensors, estimates the actual air-fuel ratio, and adds or removes fuel to bring the mixture back to its target.
• E38; LS Gen III/IV; 1997-2013: Narrowband oxygen sensors are only accurate at the 14.7:1 stoichiometric ratio for pump gasoline; richer or leaner than that they are effectively useless for tuning.
• E38; LS Gen III/IV; 1997-2013: A narrowband sensor cannot measure how rich the engine is at wide-open throttle under power enrichment, where the target is much richer than stoichiometric, so it cannot be used to fully tune the engine.
• E38; LS Gen III/IV; 1997-2013: To properly calibrate an ECU across its operating range, a wideband air-fuel ratio sensor is required.
• E38; LS Gen III/IV; 1997-2013: Short term fuel trim is an instantaneous correction based on an error that exists right now: if the engine is running too rich against the stoichiometric target, STFT removes fuel to drive it back.
• E38; LS Gen III/IV; 1997-2013: If a short term trim stays negative for a significant period, the ECM decides there is an error in the calibration at that point and applies a long term fuel trim that absorbs what the short terms were doing, letting STFT return toward zero.
• E38; LS Gen III/IV; 1997-2013: Closed loop control deliberately cycles the air-fuel ratio slightly rich and lean across the stoichiometric point, which is what makes catalytic converters work efficiently.
• E38; LS Gen III/IV; 1997-2013: A good tuning target is closed-loop trims as close to zero as possible, ideally within plus or minus 3 percent, with up to plus or minus 5 percent tolerated before calibration work is warranted.
• E38; LS Gen III/IV; 1997-2013: GM ECMs rely primarily on the mass airflow sensor, which directly measures the mass of air entering the engine so the ECU can pick the injector pulse width for a target air-fuel ratio.
• E38; LS Gen III/IV; 1997-2013: The MAF sensor has some latency under transient conditions such as sharp throttle openings, which is why GM added a speed density subsystem to complement it.
• E38; LS Gen III/IV; 1997-2013: Speed density tuning models airflow from a three-dimensional volumetric efficiency table indexed by manifold absolute pressure and RPM rather than measuring it directly.
• E38, E67, E40, P59; LS Gen III/IV; 1997-2013: Early Gen III LS1 ECMs used a conventional built-in VE table, while later GM ECM generations moved to a virtual volumetric efficiency (VVE) system.
• E38; LS Gen III/IV; 1997-2013: If you want to tune using only short term fuel trims, you must disable long term fuel trims first, otherwise the combined data will be inaccurate.
• LS1; 2000-2013: EFI tuning software such as EFILive is a tool rather than a tune in itself: it reads, modifies, reflashes, and datalogs the factory ECU's operating parameters.
• LS1; 2000-2013: A quality wideband oxygen sensor is required so you can read what the engine is doing in every driving scenario, and the tuning software can interface with many wideband brands.
• LS1; 2000-2013: The VE table represents the engine's actual pumping efficiency at each point in its operating range: an engine is basically an air pump that is more or less efficient at different operating parameters.

MAF vs speed density

• HP Tuners: The GM MAF sensor outputs a frequency signal proportional to the airflow through the meter; the MAF Airflow vs. Output Frequency table looks up airflow (g/sec) for a given frequency to compute airmass for fueling.
• HP Tuners: Speed density is normally the PCM's backup mode if the MAF fails, but some vehicles are not fitted with a MAF at all and use speed density mode all the time.
• HP Tuners: The VE Correction Factor is simply the ratio of the MAF airmass to the VE airmass; it offsets VE-calculated airmass during transients so steady-state/unsteady-state transitions stay smooth.
• HP Tuners: The VCM constantly cross-checks MAF operation against a speed/density-calculated airflow and sets P0101 (Maximum Delta Air Flow) if the difference exceeds the threshold at that speed/density airflow.
• HP Tuners: 'MAF Sensor Fail High' is the frequency threshold at which the VCM sets the High MAF Frequency DTC, P0103.
• HP Tuners: When changing the MAP sensor, set the MAP Sensor Calibration Linear B value (if specified) to 1/3 of the Linear slope calibration; this applies to 2- or 3-bar code.
• HP Tuners; E38: GM ECUs incorporate both a mass airflow system that directly measures mass airflow and a speed density system (based on manifold absolute pressure) that calculates mass airflow — the two inputs should normally calculate out to approximately the same number.
• HP Tuners; E38: In GM terminology the speed-density side of the ECU's airflow model is known as virtual VE, or virtual volumetric efficiency.
• HP Tuners; E38: On a sharp throttle transient there is often a lag in the MAF signal response, so the ECU can instantaneously revert to the speed density lookup table to handle the transient event — the SD subsystem exists even in MAF-based operation.
• HP Tuners; E38: At higher RPM the GM PCM focuses solely on the mass airflow sensor input; below the dynamic airflow thresholds it can use predicted airmass calculations based on both the MAF meter and the speed density (virtual VE) tables.
• HP Tuners; E38: The MAF Calibration table (VCM Editor > Engine > Airflow > General) is a two-dimensional table of airflow values versus the frequency output of the MAF sensor, with the numbers expressed in grams per second; it has an exponential-shaped curve.
• HP Tuners; E38: Later GM ECUs such as the E38 do not show a conventional VE table under the Speed Density tab — instead there are VE coefficient tables, and GM calculates the current volumetric efficiency on the fly from the coefficients and current operating conditions.
• HP Tuners; E38: In VCM Editor version 3, the Edit > Virtual Volumetric Efficiency menu directly generates a viewable, graphical volumetric efficiency table from the E38's coefficient parameters, so the tuner can edit VE in a conventional table form.
• HP Tuners; E38: After editing the virtual VE table, clicking the Calculate Coefficients tab makes the VCM Editor generate new coefficients that are put straight into the VE Coefficients tables — completing the round trip from table edit back to ECU coefficients.
• HP Tuners; E38: The technically correct MAF scaling workflow: first disable the speed density system, then scale the MAF sensor calibration to suit the engine changes, tuning only one subsystem at a time.
• HP Tuners; E38: The second stage of the workflow: flash the MAF-scaled calibration, then run the ECU solely in speed density mode (ignoring the MAF input) and calibrate the virtual VE table with the same measured-vs-commanded-lambda error process.
• HP Tuners; E38: Once both MAF scaling and virtual VE are corrected, the ECU is flashed back into mixed mode using both the MAF sensor and the speed density/virtual VE system — the way GM intended the ECU to operate and the combination likely to give the best results.
• HP Tuners: HP Tuners software patches can rewrite factory ECU code to remove the mass airflow sensor and revert to speed density tuning, and to run positive boost on a naturally aspirated engine while keeping fuel and ignition control correct.
• HP Tuners: To build a VE-tuning histogram in VCM Scanner, use the two speed-density table parameters as axes: manifold absolute pressure on the vertical (y) axis and engine RPM on the horizontal axis, with AFR error as the logged data populating the cells.
• HP Tuners: For the VE histogram to be useful, its breakpoints must match the breakpoints of the speed density table — easily done by copying the header data out of the speed density table in VCM Editor and pasting it into the histogram setup.
• HP Tuners; E38: The Airflow Dynamic tab's High RPM Disable and High RPM Re-enable parameters define the RPM above which the speed density system is completely ignored and only the MAF input is used; the E38 defaults are 4,000 rpm (disable) and 3,900 rpm (re-enable).
• Holley EFI: The Holley EFI V4 'Estimated VE' data-monitor parameter estimates volumetric efficiency taking into account what the closed loop fuel trimming is performing, and can be used to more quickly tune the base VE table when manually tuning.
• Holley EFI: The Holley EFI V4 'Base Fuel VE' data-monitor parameter indicates the base fuel value (in VE) from the base fuel map with all modifiers excluded — the raw VE-table fueling before corrections apply.

Checking the tune

• HP Tuners; GM V8: A GM V8 is well tuned when its long-term fuel trims sit within ±4%.
• HP Tuners; GM V8: A GM V8 is well tuned when its short-term fuel trims sit within ±4%.
• HP Tuners; GM V8: If STFT is more than 3% away from zero, the LTFT value is still learning and the tune is not yet stable.
• HP Tuners; GM V8: The VE table correction for a cell is derived from the formula: (+-LTFT%) + (+-STFT%) = Final % Learned.
• HP Tuners; GM V8: Worked example: LTFT +7% and STFT -2% at 1,600 RPM / 30 kPa MAP gives a total +5% correction; apply about half (2.5%) to the VE cell to avoid overshoot.
• HP Tuners; GM V8: After reflashing the VCM with a new VE table, use the VCM Controls function to reset the fuel trims before recording new data, giving a clean slate.
• HP Tuners; GM V8: Fuel trim cells 20, 21 and 22 are not permanent corrections (used for DFCO, power enrichment fuel, charcoal canister purge); do not use histogram corrections logged for those cells.
• HP Tuners; GM V8: The VCM uses 16 fuel trim cells, numbered 0 through 15, to cover the VE table.
• HP Tuners; GM V8: In the HP Tuners sample layout, the LTFT RPM boundaries are 1,300 RPM, 2,500 RPM and 3,500 RPM.
• HP Tuners; GM V8: In the HP Tuners sample layout, the LTFT MAP boundaries are 32 kPa, 56 kPa and 80 kPa.
• HP Tuners; GM V8: LTFT corrections are stored in the VCM's RAM and remain there until power is removed from the VCM or a reset is forced via the scanner.
• HP Tuners; GM V8: The VCM learns fuel trims in closed-loop fuel mode; when logging, stay in closed loop, make slow throttle changes, and hit as many histogram table locations as possible.
• HP Tuners; GM V8: Do not let the engine enter power enrichment mode while logging VE corrections, because PE turns off closed-loop fuel.
• HP Tuners; GM V8: STFT corrections are instantaneous, do not use cells, and are not permanent; they act as a fine tune for the LTFT cell adjustments.
• HP Tuners; GM V8: Expect a 2-4% change in fuel-trim learning from day to day due to weather, fuel and engine dynamics; do not chase LTFT numbers down to zero permanently.
• HP Tuners; GM V8: The STFT-only tuning method requires the VE table to be no more than 10% off; if it is further off, start with the LTFT method first.
• HP Tuners; GM V8: With the STFT-only method (LTFT disabled), the VE table cell corrections are simply the +-% STFT error from the histogram.
• HP Tuners; GM V8: After finishing the STFT-only tuning method, re-enable the LTFT.
• HP Tuners; GM V8: Keep the VE table smooth: modify surrounding cells when making changes, because transitions over cliffs, ridges or spikes in the table cause poor results.
• HP Tuners; GM V8: During adaptive idle, the goal of the long-term idle trims (LTITs) is to make the short-term idle trim (STIT) equal zero.
• HP Tuners; GM V8: Validate idle tuning by logging the LTITs and the STIT with the scanner and confirming the values, once stabilized, are within the adaptive idle airflow limits.
• HP Tuners; GM V8: LTIT values are slow moving; use the scanner to make sure the LTITs have reached steady state in the changed region before making any tuning judgments.
• HP Tuners; GM V8: The current STIT is always reset to zero when not at idle; when idle conditions return, the STIT begins to learn again from its remembered position.
• HP Tuners; GM V8: Startup airflow consists of two components added together, Friction Airflow and Startup Airflow; it adds to the base airflow and decays away to zero as the engine begins running.
• HP Tuners; GM V8: Off Idle Startup Scalar multiplies the current startup airflow on an off-idle transition: 0.5 halves it, 1.0 has no effect, and 0.0 decays it to zero instantly.
• HP Tuners; GM V8: Increasing the Airflow Final Minimum values can result in hanging RPM.
• HP Tuners; GM V8: Throttle follower decay values are subtracted from the current IAC airflow value every 12.5 ms, so higher numbers mean faster decay.
• HP Tuners; GM V8: The Fan 1 idle airflow value is extra airflow added to the base running airflow to compensate for the extra electrical load when a single cooling fan is active.
• As a general guide, the sum of the short-term and long-term fuel trims at any point should sit between plus and minus 5%; within 5% the ECU is doing its job well.
• When choosing within the acceptable trim range, slightly negative fuel trims (removing a little fuel) are preferred over slightly positive trims.
• Keep a safety margin of approximately two to three degrees from the onset of knock when optimizing ignition timing.
• Holley Terminator X: Holley self-learning requires coolant temperature above 160 deg F, closed-loop operation, and Learn enabled; in the Learning Status, NoLearn means not learning and Learn means active self-tuning.
• Holley Terminator X: Holley Terminator X typical Target AFR values are Idle 13.5-15.0, Cruise 13.5-15.5, and naturally aspirated WOT 12.5-13.2; larger cams may need richer idle/cruise targets.
• Holley Terminator X: On a Holley Terminator X, Closed Loop Compensation reads 0% in open loop; a negative value means the ECU is removing fuel and a positive value means it is adding fuel.