Fuel, E85 & boost tuning

• Terminator X, HP, Dominator; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Aeromotive's bypass regulator provides a vacuum/boost reference port for fuel pressure compensation at a 1:1 rate, a feature aimed primarily at blow-through centrifugal supercharger applications.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: AEM's universal adjustable regulator has a 1:1 vacuum reference, meaning each pound of boost adds one pound of fuel pressure, and its adjustment range runs from 20 psi up to whatever the fuel pump can deliver.
• E38, P59; LQ4, LQ9, LM7; 1999-2007: ZZ Performance's boost-referenced fuel pressure kit for return-style GM systems lowers rail pressure to 38-42 psi at idle under full vacuum and rises 1:1 under boost, but the change requires PCM programming to match.
• E38, E67, Terminator X, Dominator; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: The flex fuel sensor is designed for installation in the fuel return line; its straight-through 3/8-inch (9.5 mm) tube adds minimal restriction, which suits high-horsepower fuel systems, and it resists the corrosive properties of high-ethanol blends.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: In observed real-world consumption, pure E85 raises fuel use by roughly 25-30% over gasoline (E50 only about 5-10%), and pump-labeled E85 can legally contain anywhere from 51% to 83% ethanol.
  Note: Stoichiometric math implies about 52% more fuel; real-world consumption usually rises less than that.
• P01, P59; LM7, LQ4, LQ9, LS1, LS2; 1999-2007: HP Tuners' 2-bar/3-bar MAP support (the 2.5-bar custom OS) adds a Boost VE table spanning 105 kPa to 255 kPa with better than 5 kPa resolution in the boost region, retains stock VE resolution in non-boost areas, adds a MAP-referenced Boost Enrich table in 5 kPa steps, and runs in speed-density mode only.
  Note: A 2-bar sensor reads to about 14.7 psi of boost; the OS boost tables read to 255 kPa when paired with a 3-bar sensor.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: A boost-referenced fuel pressure regulator uses a vacuum/boost port with a 1:1 ratio: every pound of manifold pressure raises rail pressure by one psi, so injector differential pressure stays constant. For example, 10 psi of boost on a 45 psi base takes the rail to 55 psi.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Without a boost-referenced regulator, manifold pressure pushes back on the discharge side of the injector and reduces effective fuel delivery. On a 45 psi system with 10 psi of boost, the injector only discharges at 35 psi, making the engine run lean; 40 psi of boost would leave just 5 psi of discharge pressure.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: A boost-referenced regulator also compensates in reverse under engine vacuum: it lowers rail pressure proportionally so that intake vacuum does not pull extra fuel through the injector, keeping the differential ratio consistent at all times.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: The net result of a properly boost-referenced fuel pressure regulator is a constant injector flow rate regardless of intake manifold pressure.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Fuel injectors are rated at a specific differential fuel pressure, most commonly 43.5 psi (3 bar) and 58 psi (4 bar); effective flow rate at any other pressure scales with the square root of the pressure differential.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: When sizing injectors, confirm the selected flow rate at the fuel pressure the system will actually run, and account for a boost-referenced regulator raising base pressure proportionally with boost.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: The industry-standard maximum operating duty cycle for port fuel injectors in continuous high-performance use is 80%; at 100% the injector becomes an uncontrolled open orifice and the ECU loses fine fuel control.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Injector minimum flow per cylinder is computed as (Gross Design Power x BSFC / cylinders) / 0.80, which enforces the 80% duty cycle ceiling directly in the sizing math.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Fuel systems on forced-induction builds must be sized for gross power including the supercharger's parasitic draw; a ProCharger F1-X consumes roughly 160-200 HP at full boost, so a 1,200 BHP target needs injectors sized for 1,360-1,400 BHP of fuel demand.
• E38, E67, P59; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Advanced Fuel Dynamics' E85 system uses the GM-style inline sensor to monitor ethanol content and automatically steps injector output up as ethanol rises; the sensor re-reads content every 1/10 of a second, so blends adjust on the fly without a retune. Pump E85 can range from 51% to 83% ethanol.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: E85 has a blended stoichiometric air/fuel ratio of approximately 9.7:1 to 9.8:1 versus about 14.7:1 for gasoline; ethanol's much lower energy density by mass is what forces considerably higher fuel mass flow for the same power.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: By brake-specific fuel consumption, E85 (BSFC 0.80 lb/hp-hr) demands roughly 45% more fuel mass than gasoline (BSFC 0.55) to make the same power.
  Note: Real-world fuel use on E85 often runs closer to 30% more, while the stoichiometric ratio alone (14.7 ÷ 9.65) implies about 52% more.
• P01, P59; LS1; 1998-2002: Converting a fourth-gen F-body LS1 to E85 required swapping to high-flow E85-compatible fuel pumps and larger injectors, because ethanol's substantially lower lubricity quickly wears out a standard pump not designed for it; OBD-II-era fuel systems otherwise already tolerate the ~10% ethanol in pump gas.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: A proper E85 fuel system needs 30-40% more flow capacity than the gasoline equivalent; injectors should be sized to stay at or below 90% duty cycle and the pump should carry 10-20% headroom. Materials to avoid include PVC, nitrile, natural rubber, urethane, non-anodized aluminum, zinc, brass, and untreated ferrous metals, and pre-2001 OE fuel systems were not engineered for any ethanol concentration.
• E38, E67, P01, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: For E85 service, injectors with stainless steel internals (such as Bosch EV14) resist the corrosive environment, Viton O-rings are the preferred seal material, and a serviceable inlet filter on each injector serves as the last line of defense against fuel-system residue clogging.
• LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: On an E85 carbureted setup, use an AN-10 line from tank to regulator and AN-8 from regulator to carb, all braided stainless or hardline with no rubber, a pump capable of 250 gph, carb fuel pressure at 6 to 6.5 psi, and steel-micron filters instead of paper.
• E38, E67, P01, P59; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: As boost rises, the ignition curve benefits from a small retard function because the denser, more turbulent boosted mixture burns faster; this same concept also permits keeping more advance at low-speed cruise while pulling advance back at high boost.
• E38, E67, P01, P59; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: A boost-pressure-actuated ignition retard is a compromise: it progressively retards timing as boost climbs even when no detonation is present, so mid-range timing ends up sub-optimal in order to be correct at maximum boost, costing noticeable mid-range torque.
• E38, E67, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: A flex fuel sensor sits inline in the fuel line before the fuel rail and measures the ethanol-to-gasoline ratio so the ECM can adjust ignition timing and injected fuel quantity for the actual blend in the tank.
• E38, E67, P59, Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: The GM/Continental flex fuel sensor uses a three-wire connector: ground, battery-positive power, and a signal wire carrying ethanol percentage as a frequency signal. The ECM supplies an internal 5V pull-up on the signal circuit, which the sensor pulses to ground; normal operation is 50-150 Hz, while 180-190 Hz flags contaminated fuel.
• Terminator X, HP, Dominator; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Aftermarket plug-and-play flex fuel harnesses for Holley EFI insert into the 4-pin power tap on the main harness; the signal lead pins into an open frequency-type input, and the required pull-up resistor comes pre-wired. These harnesses fit Terminator X, HP, and Dominator systems with GM/Continental sensors.
• Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Terminator X ECUs expose four input types (12V, ground, 5V, and frequency); the frequency input is the one used to read a flex fuel sensor.
• Terminator X; LM7, LQ4, LQ9, LS1, LS3, LS6; 1999-2013: Terminator X ECUs carry an onboard 1-bar MAP sensor suited to naturally aspirated or nitrous combinations; for boost, the internal sensor is disabled and an external sensor is used: GM part 12592525 plus Holley adapter 558-416 for 2-bar and up.

Air-fuel ratio targets

• HP Tuners; E38: In closed loop, the GM ECU constantly targets stoichiometric, which is 14.7:1 AFR; the MAF reading and injector size let it calculate the pulse width to achieve that target automatically.
• HP Tuners; E38: If the closed-loop fueling is not quite where it needs to be, the short term and long term fuel trims take care of the discrepancy — trims are the ECU's correction mechanism around the 14.7:1 target, not a tuning target themselves.
• HP Tuners; E38: Under wide open throttle (or past a throttle setting), the GM ECU switches into open loop mode, which in GM lingo is referred to as power enrichment — fueling is then commanded from the PE tables instead of the 14.7:1 closed-loop target.
• HP Tuners; E38: In the HP Tuners VCM Editor Fuel tab, the Power Enrich tab holds the power enrichment equivalence ratio tables — the single 2D table vs RPM where the tuner defines what air/fuel ratio the engine runs at WOT; there is no load axis in this table.
• HP Tuners; E38: GM equivalence ratio is simply the inverse of lambda: EQ = 1 divided by lambda. Raising the EQ number commands a richer mixture; lowering it commands a leaner mixture.
• HP Tuners; E38: A PE equivalence ratio of 1.225 converts to a lambda target of about 0.82 (1/1.225), and multiplying by the 14.7:1 stoichiometric ratio gives a commanded AFR of 12:1.
• HP Tuners; E38: To do the tuning job properly a wideband air/fuel ratio meter is required so the tuner can actually see what the AFR is doing during WOT operation in power enrichment mode; factory narrowband sensors are only good at cruise or idle.
• Holley EFI: In the Terminator X handheld, the BASIC FUEL menu 'Allows changes to the Target Air/Fuel ratio at idle, cruise, and wide open throttle' — the single entry point for the three target-AFR settings.
• Holley EFI: The Terminator X 'Target AFR at WOT' setting is typically 12.5 to 13.2 AFR on naturally aspirated engines; running richer may reduce power, and running leaner may reduce power or cause potential engine damage.
• Holley EFI: The Terminator X 'Target AFR Boost Offset' slider inputs the desired target AFR offset per 7 lbs of boost, so raising the offset richens the target under boost and lowering it leans it.
• Holley EFI: The Terminator X 'Acceleration Enrichment' setting changes the accelerator-pump function of the fuel injection: raising the number increases the amount of fuel added when the pedal is pushed, lowering it decreases the added fuel.
• Holley EFI: The Holley EFI V4 'Base Target AFR' data-monitor parameter indicates the base target air/fuel from the target A/F table with all modifiers excluded, so it shows the raw table target before closed-loop, enrichment, or other modifiers apply.
• General: Lambda describes how much richer or leaner the air-fuel ratio is compared to stoichiometric, expressed as a percentage: 1.0 is stoichiometric, numbers above 1 are lean (excess oxygen), numbers below 1 are rich (excess fuel).
• General: Lambda is actual AFR divided by stoichiometric AFR: a measured 12.5:1 AFR on pump gasoline (14.7:1 stoich) equals 0.85 lambda, because 12.5/14.7 = 0.85.
• General: AFR is lambda multiplied by the stoichiometric ratio: a lambda value of 1.05 on pump gasoline converts to 1.05 x 14.7 = 15.4, i.e. 15.4:1 AFR.
• General: Lambda reads as a direct rich/lean percentage: 0.90 lambda is 10% richer than stoichiometric and 1.03 lambda is 3% leaner than stoichiometric (likewise 0.85 is 15% richer), letting a tuner make fast percentage fuel-table corrections without a calculator.
• General: Lambda targets stay constant across fuels: a turbocharged engine switching from pump fuel to E85 can safely target 0.80 lambda at 1 bar of boost on either fuel, while the equivalent AFR target changes completely.
• General: At 0.80 lambda the commanded AFR differs by fuel: on pump fuel (14.7:1 stoich) it is 0.80 x 14.7 = 11.8:1, while on E85 (9.8:1 stoich) it is 0.80 x 9.8 = 7.8:1 — the same lambda, very different AFR numbers.
• General: On E85, many tuners still talk in gasoline-scale AFR numbers (12.5:1 and so on) even though E85's own stoich is about 9.8:1. It works as shorthand, but it confuses people. Lambda avoids the problem: 1.0 is stoich on any fuel.
• General: There is no single air/fuel ratio that must be applied to every engine and every application — the correct target is a moving target that depends on RPM, load, and the engine combination.
• General: In one dyno test on a naturally aspirated 3.5L V6, maximum torque (337 Nm) came at a lambda target of 0.93 at WOT and 2,000 rpm.
• General: That test showed a torque plateau: from about 0.86-0.87 through about 0.98 lambda torque barely changed, so the AFR target over that wide range has no dramatic effect on power; torque drops off at the very rich and very lean ends.
• General: A naturally aspirated engine normally makes maximum power at approximately 0.85 to 0.92 lambda; dyno testing should establish the best AFR for a particular engine rather than assuming a fixed number.
• General: A forced induction engine generally makes best power at approximately 0.82 to 0.85 lambda depending on the application, though a richer mixture may be needed for safety since the extra fuel controls combustion temperature.
• General: As boost increases, combustion temperature climbs, so tuners tend to target a richer air/fuel ratio with more boost to control temperature and prevent engine damage.
• General: A modified engine with a more aggressive cam profile may idle best slightly richer, around 0.90 to 0.95 lambda, because low-RPM efficiency suffers with big duration/overlap; raising lambda toward 1.0 leans it, lowering richens it.
• General: Don't tune richer than 0.90 lambda at idle: very rich mixtures foul spark plugs (hard starting), wash oil off the bore walls (excessive wear), and dilute the sump oil (long-term engine wear).
• General: A standard engine idles happily at lambda 1.00, and typical road-car cruise tuning targets lambda 1.0 since cruise is a light-load, fuel-economy operating area.
• General: An engine with a very aggressive camshaft and lots of overlap may cruise more smoothly slightly richer, but typically should not need to go richer than about 0.95 lambda at cruise.
• General: Maximum fuel economy at cruise is normally achieved at approximately 1.05 lambda; going leaner reduces torque, eventually forcing a larger throttle opening that cancels the fuel saving.
• General: Adding fuel (richening the mixture) reduces combustion temperature, which is why richer WOT mixtures are used both to make maximum power and to control chamber temperature for engine reliability.
• General: With increased camshaft overlap, unburned fuel ends up in the exhaust system, producing an inaccurate AFR reading that doesn't truly represent the actual air/fuel ratio of the combustion charge — so wideband readings at idle on big cams must be interpreted cautiously.