Theory: An Injector Model


Understanding your fuel injectors is one of the most important things you can do to ensure that fueling is appropriate for your engine.  First, some vocabulary:

  • ECM, ECU, Engine computer : used interchangeably to mean the computer operating the fuel injectors and running the engine
  • AFR, Air – Fuel Ratio : the ratio between how much air and how much fuel an engine is receiving or how “lean” or “rich” it is running
  • Solenoid : a solenoid is an electromagnetic electromechanical device.  It operates by using electricity moving through a coil to generate a magnetic field which moves a plunger. (more)
  • Injector : a special type of solenoid that allows fuel to flow through it when energized (more)
  • Pulsewidth : the length of time the engine computer applies electricity to the injector, or how long the injector is commanded to be open
  • Flow Rate : The amount of fuel an injector flows once open.  These values are typically given in units of cc/min or lbs/hr at a specified fuel pressure. (injector flow rate varies with the square root of fuel pressure.)
  • Latency : the length of time after the injector is turned on before it achieves its linear flow rate.

Everything you ever wanted to know about injectors but never knew to ask

Injectors are pretty simple devices: turn on the electricity, wait till the fuel starts flowing.  Right?

Not quite…

Injectors are mechanical devices – once electricity is applied, the injector needs to move from its resting position in which no fuel flows to its open position where fuel is flowing at its published flow rate.  The problem is that this transition from “closed” to “open” is far from instant – some larger injectors can take several milliseconds to open fully.  During this time, injectors do not flow at their linear flow rate.  How long injectors take to open varies from injector to injector largely due to mechanical reasons.  Fuel pressure can also affect injector latency because of the force applied by fuel on injector internals.  And most importantly, the amount of electricity you supply to the injector controls how much magnetic force coils inside the injector can create.  Bottom line: when your battery voltage decreases (such as when cranking) your injectors take longer to open and fuel injector latency increases.

Tuning for Injector latency

Most engine computers have some kind of table to compensate for injector latency.  They can be called many things – “Injector Battery tables” or “Injector battery offset” or “battery tables” but they frequently look very similar: a table of how long to open the injector before it achieves linear flow (“latency”) versus measured battery voltage.  The idea here is that the ECM opens the injectors for a period of time (from the battery tables) to compensate for variations in injector opening time versus battery voltage.  If you change injectors, you probably need to update your battery tables, too.  If you vary fuel pressure, you may want to try changing the battery tables as well as other tables to account for changes in latency.

A basic method for tuning injector latency requires a wideband and a multimeter (or better yet, datalogging battery voltage from the ECU).  Follow this procedure:

  1. Start by hooking up you multimeter or starting datalogging battery voltage.  If you are using a multimeter, use a voltage source close to the ECM if possible.
  2. Fire up the car and hold it at a few thousand RPM.  Observe battery voltage – it should be fairly high. (13.8 – 14.5 volts, depending on the vehicle)
  3. Gradually, let the car return to idle while keeping an eye on battery voltage.  Many vehicles will run anywhere from 0.75 to 0.1 volts lower at idle compared to cruising RPMs.
  4. Problems with battery tables can contribute to hunting or unstable idle.  Once the car is idling, do everything you can to put an electrical load on the car – turn on headlights, turn on the stereo, turn on the fan for the climate control inside the car.  As you do so, keep and eye on battery voltage and observed air fuel ratio.
  5. If you see the car run progressively leaner when you turn on electrical accessories and voltage drops, start increasing injector latency at the battery voltage you observe until you minimize changes in air fuel ratio when changing electrical load.  This will result in a curve with a steeper slope.
  6. If you see the car run progressively richer when you turn on electrical accessories and voltage drops, start decreasing injector latency at the battery voltage you observe until you minimize changes in air fuel ratio when changing electrical load.  This will result in a curve with a flatter slope.
  7. If you feel really adventurous, you can disconnect the large cable between the alternator and the + side of the battery (or sometimes a wiring distribution block) while the car is running.  When you do this, the battery will stop charging.  Voltage you observe at the ECU will decrease as the car consumes the battery’s charge.  You can generally tune a much wider range of the battery table by doing this but it is much more of a pain to do and will eventually drain your battery to the point the car will not run.
  8. Note: these injector battery tuning methods assume the car is reasonably well tuned close to idle and will idle at a reasonably steady AFR.  Doesn’t need to be perfect, but you may do more harm than good messing with injector battery tables when the tune is jacked.

Another sign that your battery tables may be off is when the car runs poorly at small throttle angles compared to large throttle angles.  Sometimes changing latency is a quick way to fix a car running too rich / too lean that runs well close to wide open throttle.  Latency changes will have a large effect at low pulsewidths (i.e. closed throttle) but will have comparatively little effect at high pulsewidths (i.e. open throttle.)

You shouldn’t be afraid to adjust injector latency as part of tuning but always remember that it is a BROAD SWEEPING CHANGE THAT WILL AFFECT HOW THE ENGINE RUNS EVERYWHERE.  If you have a problem in a specific load condition, chances are your problem is elsewhere.  When you start seeing PATTERNS of problems (i.e. closed throttle too lean, close to idle where battery voltage too lean, hard starts/cranking when battery voltage lowest, etc.) then it is worth looking into whether a latency adjustment can solve your tuning issue.

You can always sanity check your injector battery tables visually.  Injector latency always increases as battery voltage drops.  If you look at a 2D graph of battery voltage versus latency, it should always be relatively smooth.  As voltage increases, injector latency should level out and change much more slowly than at lower voltages.  This is not a Ford thing or a Honda thing – this is a universal thing that all cars that use fuel injectors will follow.

Tuning for Injector Flow

We haven’t said that much about injector flow up to this point, but it is equally important to having your engine run correctly.  Injector flow is the “obvious” thing that most people change when installing different injectors.  Most older systems account for injector flow with a “fuel constant” (it is called many different things in different systems such as… ) – when you change the size of injectors, you multiply the fuel constant by the difference in flow between your old injectors and your new injectors.  For example:

  1. Fuel constant = 16.4
  2. You have 24lb/hr stock injectors
  3. You install 32lb/hr stock injectors
  4. 24 (old) / 32 (new) = .75
  5. New fuel constant = old fuel constant * change in injector size = 16.4 * .75 = 12.3

Keep in mind, this is just a guideline to get you close.  You can use the injector size / injector constant to make sweeping, global changes to fueling if your tune is off everywhere.  You *should* be able to get a tune very close to where it was before an injector change by changing nothing more than battery tables and an injector size / injector constant.

Some systems (Ford, GM LSx, newer Dodge / DCX Hemi, others) use a dynamic flow model of injector behavior rather than a single “injector constant.” These systems try to more precisely account for the flow of injectors by modeling how injector flow changes as a function of how long they are open.  Most ~87-2009(ish) Ford uses the concept of injector slopes.  There is a “low slope” and an “high slope”, along with a threshold to change from one to the other and often a minimum pulsewidth.  The injector slopes can be thought of as TWO injector flow constants and the ECM changes from one to the other as the injector opens.  When changing injectors on Fords or other manufacturers that use dynamic flow models, a good starting point is to scale both slopes (or all members of a dynamic flow table) uniformly by the predicted difference in injector flow rate.  An even better approach is to copy values from another OEM calibration that uses the injectors you have installed.  Some injector suppliers (but not many – Injector Dynamics is the one that comes to mind) do dynamic flow testing and can supply you with data precise enough to plug in.