Megasquirt-3 Fuel system

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Intro - Single fuel pump - Low/high pressure dual fuel pump - Pump Wiring - Fuel line - Fuel filter - Fuel pressure regulator - Injector installation - Fuel Rails - Throttle Body - Injector Size Selection - Injector Impedance - Injector PWM - Injector Resistors - Peak and Hold - Wiring - Settings - Firing order - Semi-sequential - Dead Time / PWM - Small Pulsewidths -


The Megasquirt-3 forms the heart of a fuel injection system, but the selection, purchase and installation of the remainder of the system is equally important.

A typical fuel system works as follows:

A high pressure pump is connected to the fuel tank and feeds fuel to the fuel rails(s) these provide fuel directly to the top of the injectors. The fuel rail(s) are connected to an intake manifold pressure referenced pressure regulator. The regulator maintains the rail pressure a set pressure above the intake under all conditions. Excess fuel is returned to the fuel tank through the return line.

Key elements

  • Fuel pump
  • Fuel hose/pipe and fittings
  • Injectors
  • Injector mounting
  • Fuel rails
  • Pressure regulator
  • Throttle body

    Existing EFI Vehicle

    Most vehicles with EFI already fitted are readily adaptable to use Megasquirt for control. Typically all of the fuel system components will be readily suitable.

    However, if like many users you are increasing the power of your engine, you will need to consider whether your injectors are large enough and whether your fuel pump has adequate flow. In particular note that all fuel pumps can flow less fuel as the pressure increases - so if you are boosting your engine you will be needing more fuel under the conditions when your pump can supply less!

    Retro-fit EFI Vehicle

    When installing EFI on a previously carburetted vehicle or a new build you have to source all the required fuel system components. There are many choices open to the retro-fit market. Be aware that a high horsepower install will often spend more on the fuel system than the ECU.

    Single Fuel pump

    You will need a high pressure pump with enough volume at your operating pressure to feed you engine under maximum load. Typical pressures needed in the neighborhood of ~45 psi for port fuel injection, ~10-20 psi for TBI injection. A port injection pump will work with TBI, but not vice-versa.

    A standard EFI install uses a single high pressure pump connected as per the diagram above. Depending on your target power output, many OEM style pumps may be suitable. Suprisingly, some of the Bosch inline EFI pumps installed on 100hp cars are actually rated to 450hp fuel capacity. OEM style pumps are a usually a good choice as they are designed for trouble free operation for tens of thousands of miles.

    OEMs sometimes place the pump inside the fuel tank. In an EFI retrofit it is generally easier to use an external fuel pump. Ford used external fuel pumps on 1989 era 150 trucks which may be a candidate for use. These are high pressure pumps that will work in most applications. Econoline vans have these as well.

    The external pumps used in Ford F150 fuel injected trucks from the 89-93 model years are Delco EP286. At 12 volts, the operating pressure is 70-95 PSI with 36-40 gals per hour. The biggest Delco pump is the EP424, which is 75-90 PSI at 40 gals per hour. EP 268 is a GM# 25117086, EP 424 is a GM# 25176156.

    The Carter pump #P70199 (the outlet is 7/16 standard pipe thread and the inlet is 15/32 clamped hose type fitting or 3/4 standard thread. The specs are 95-PSI max, 68-93 G/Hr wide open). This is the highest flowing Carter external fuel pump in the book. It will produce up to 95 psi, and crosses over to EP7107 at Kragen for about $80 (unfortunately one end does not come off like the Carter). You might want the Ford style pump EP7109($80). You will need this if you want to be able to modify ends to be 3/8".

    Others have had luck using the external pump from various fuel injected VolksWagen models (87 VW Fox, for example). Part number is: Bosch 0 580 254 957 reportedly rated at 90 GPH@ 70PSI, you might find them for about $130 new from This pump consists of a fuel pump, filter, and an "accumulator". You can leave the accumulator in place since it does not affect the running volume or pressure, and on used pumps they are often rusted so you might not want to mess with it.

    The Bosch pumps are used on many European vehicles. You can also buy cheap clone pumps via various sellers on Ebay.

    If an OEM style pump does not offer sufficient output, there are plenty of aftermarket high volume EFI pumps on the market.
    Walbro, Aeromotive and many other vendors offer a range of pumps to suit your needs and are distributed through your local speed shop or Summit, Jegs etc.

    Low pressure / high pressure - twin pump

    For a basic retrofit, you may find that a low pressure/high pressure system is a simpler way to avoid tank modifications.

    The low pressure side can be your existing electric fuel pump. You need to add the surge/swirl tank and high pressure side. For the tank return you may already have an return or evap canister connection or could connect into the filler neck, ensuring that fuel returns to the tank and cannot leak out of the vehicle.
    Surge/swirl tank can be purchased or you can make your own. Use thick wall TIGed aluminium or brazed steel. Ensure it is totally leak free.

    Wiring the Fuel Pump

    To activate the fuel pump, MegaSquirt® provides a ground for the fuel pump relay circuit -see the main wiring diagram.
    Ordinarily, at power on, Megasquirt will run the fuel pump for 2 seconds, then when you start cranking the fuel pump is enabled again. If you stop cranking before the engine starts of you stall, the pump is turned off.

    You might want to consider a safety switch in the fuel pump circuit when installing an electric fuel pump. Holley has one (12-810, ~$20) that will ensure the fuel pump will not run unless the engine has oil pressure. It stops the pump from running if the motor stalls with the ignition on. Wiring the switch through the starter solenoid circuit energizes the pump on engine start-up. Once the engine has started, the switch continues to provide power to the pump as long as there is oil pressure to keep the switch turned on.

    Note: An inertial safety shut off switch should be installed and used to kill power to the pump upon significant impact to vehicle.

    These switches are available in junk yards from EFI Fords. The switch is on the drivers side in the trunk, near the trunk hinge, mounted so that it is between the interior bracing and the rear quarter panel (protected from being knocked around if you stuff your trunk full of stuff). It is mounted with the reset switch straight up. Note switch mounting orientation probably matters.

    It is Ford Part # F2AB-9341-AA. The wire going into it is about 14 gauge, so it should be capable of handling the full current of the fuel pump.

    The markings on the switch show that it has NO/NC (normally open/normally closed) positions so that it should be able to accommodate any possible fuel pump configuration.

    Fuel Line

    Steel tubing is recommended, but you MUST have short sections of flexible line in the feed and return lines between the engine and frame to allow for engine movement. The return line should have minimal restriction. For reference, GM systems typically have 3/8" feed lines and 5/16" return lines.

    You may be able to use your original fuel line as a return line, plumbing a new 3/8" (10mm) line for fuel supply. You can run the return line into the tank, or reroute it to a fitting or nipple you install in the fuel tank filler neck/tube assembly (in which case you may be able to use the original pick-up for your supply line). If you run a new pick-up into the tank, it will need a filter. GM sells a sock-type filter that is a good fit for 3/8" lines. It is part number 5651702 and costs about $15.

    You may have to fabricate fuel lines for your system. Tubing is available in steel, cunifer (bundy), stainless steel, and aluminum for this purpose. Do not use plain copper and is can fatigue fail with dangerous leaks resulting. The size is generally given as the outside diameter of the tubing. Unless you have a very unusual combination (or very high horsepower, well over 500+), you should be able to use 3/8" tubing for both the supply and return lines.

    Buy a good tubing bender (there are numerous styles in various price ranges) so that you don't kink or collapse the tubing while bending it. (You can also bend it over a V-belt pulley, in some cases.)

    The AN (Army-Navy) 'dash' system of hose and fitting sizing was established many years ago by the American military as a common measurement for hoses and fittings. It designates the outside diameter of the metal tube that is compatible with each size of fitting. The AN dash measure is the standard for performance hose applications. These dash sizes are expressed in 16th of an inch. For example, an -06 fitting is 6/16 of an inch or 3/8"

    Most fittings and adapters in the automotive aftermarket are based on a 37° sealing angle (SAE J514 37° - formerly known as JIC). These are also often referred to simply as AN fittings. Male and female 37° fittings will mate together for a leak-proof connection.

    Be aware that 45° fittings (commonly available in the USA) are not interchangeable with 37° fittings. In some sizes, they may thread together (-02, -03, -04, -05, -08, -10), but will not seal properly, due to the difference in sealing surface angles.

    Abrasion (the rubbing of the hose against some other component) is the number one cause of hose failure. A leaking fuel hose can start a very dangerous fire in your car, so make sure hose assemblies are routed properly to reduce the chance of any abrasion damage. Use a support every 12 to 18 inches (30 to 45 cm) to secure the hose. For chafe protection, be sure to install a grommet at any point a hose passes through a panel or bulkhead.

    Besides steel or aluminum tubing fuel line, you can also use one of the steel or nylon braided hoses from various suppliers. Generally these use the same AN 'dash' sizing system, and can use appropriate fittings to connect to 37° flare, NPT thread, or other systems.

    Note that if you are using a factory fuel rail, you may be able to find an aftermarket adapter to mate your OEM fuel fitting to an AN hose. For example, Accel offers TPI fuel rail fittings (pn 74730, ~$32) for -06 hose that will fit most General Motors TPI fuel injections systems.

    If you need a simple way to get to a barbed fitting to connect up rubber EFI hose to the General Motors 2 bbl TBI, your local auto parts house probably stocks GM fuel line repair kits in the HELP section. These consist of 9" of steel fuel line in 3/8" and 5/16" outside diameter with an O-ring and Saginaw fittings 14/16 mm, respectively, on one end and a barbed end crimped on the other. The steel lines are about $4.00 each. These pieces thread into the steel adapters on the GM Rochester TBIs. For a complete listing of various fittings with part numbers, etc. try:

    IMPORTANT: Keep the fuel lines out of passenger compartment and routed safely away from moving or hot parts to avoid damage/excessive heat. For flexible rubber hose use the SAE 30R9 EFI hose which is rated at 250 psi. EFI hose clamps are also recommended rather than gear clamps. Check with someone who knows if you are not sure about your installation. Nobody needs a 50 psi gasoline fed fire to ruin their day!

    Fuel filter

    Use a fuel injection fuel filter rated for the pressure at which your system operates. DO NOT use a universal carburetor filter - the higher pressure of fuel injection systems may cause it to burst! Position the filter downstream of the pump so that a clogged fuel filter will not over heat the fuel-cooled pump.
    However, if you fuel pickup does not include a strainer, it is wise to install a coarser filter ahead of the pump. When using original old steel fuel tanks, pieces of rust can dislodge and jam the fuel pump.

    Fuel Pressure Regulator

    The vacuum referenced fuel pressure regulator is essential. It provides constant pressure differential between fuel at injector nozzle and manifold air pressure [port EFI] or atmospheric pressure [TBI]. This makes the injected fuel quantity solely a function of the injector open time. Without the vacuum/boost reference connection you would need an excessively small pulsewidth under cruise/idle and an enlarged pulsewidth under wide open throttle or boost. Make sure the regulator is connected to a full vacuum source, not ported-vacuum. Check it has vacuum with the engine idling and the throttle shut.

    If you have an adjustable fuel pressure regulator (FPR), set the pressure with the fuel pump running, but the engine not running - that's your base fuel pressure (it is referenced to atmospheric pressure).

    The regulator is typically at the far end of the fuel rail (after the injectors) which recirculates all of the fuel, keeping it cool and free from air pockets. However, it can be installed anywhere after the fuel pump, but you may experience fuel heating and air pockets.

    If you are using an aftermarket fuel pressure regulator, it is a good idea to also install a pressure gauge, since most of these are adjustable. For TBI, use a 0-30 psi gauge. For port injection use a 0-60 psi or 0-100 psi gauge. Most of these gauges will mount directly on a fuel fitting using a 1/8" NPT thread. These are available from most aftermarket speed parts suppliers, such as Summit Racing or Jegs.

    Injector installation

    Many "high performance" vendors offer ready made EFI intake manifolds for engines that did not originally come fitted with EFI. (All typical USA V8s have a wide choice.) Or you can choose to modify your existing intake by welding, glueing or screwing in injector bungs.
    See here for dimensions of a standard injector.

    You can get information on injector bungs for port injectors by checking out for injector/manifold installation information, along with lots of other great information. The bungs are 0.530"-0.535" inside diameter [about 17/32" or 13.5 mm]. The fuel supply lines for the top of the injectors are the same size.

    CAUTION! Ensure you have a working fire extinguisher to hand before working with EFI. High pressure fuel leaks can start serious fires.

    MSD and others have an "Epoxy-In Pocket" fuel injector bung as PN 2120 (set of 8). Holley also offers them as PN 534-83 for a four pack (~$50), 534-84 for a pack of six (~$72), or 534-85 for a pack of eight (~$94).

    These bungs can be held in place with epoxy or welded and is used for fixed fuel rail systems only. These bungs are CNC-machined from aluminum for precise dimensions and have a 3/4" OD. Internally, the pockets are contoured to accept the bottom sealing O-ring of a standard injector. MSD also has "Thread-In Pockets". The aluminum pockets will screw into a 3/4"-16 hole and are supplied with a #8 O-ring to seal the pocket to the manifold. PN 2125 gets a set of 8.

    Fuel Rails

    Most injector systems will use one or more fuel rails. These serve two functions: they supply fuel to a multiple number of injectors (4 on a 4 cylinder, for example), and they physically locate the tops of the injectors. Most OEM rails can be made to work with standard engine configurations, but if you are doing a custom conversion you may have to fabricate fuel rails. Many place supply blank aluminum fuel rail extrusions in whatever length you need. One example is Ross Machine. They have two styles of fuel rail extrusion. They can also create custom fuel rails for you, with the injector holes placed to suit you.

    The aluminum extrusion comes in two sizes:

    For fabricating fuel rails, MSD has "Fuel Delivery Top Mounts", PN 2115, set of 8. These fuel delivery mounts are CNC machined from #304 stainless steel for great durability and precise dimensions. They slide over 1/2" steel tubing (MSD PN 2205) then are brazed or TIG welded in place to form a fuel rail. Fuel is routed through a 5/16" hole aligned to the mount and the injector. The PN 2105 Fuel Rail Clip is required for assembly. Their "Stainless Steel Fuel Tubing", PN 2205, comes in 2 four feet lengths of 304 stainless steel tubing, and is perfect for making custom fixed rails. The seamless tubing has a 1/2" OD and .035" wall.

    Throttle Bodies

    Your throttle body choice depends on whether you are going to use throttle body injection or port injection.

    Your throttle body needs to do 2 things:

    1. control the amount of air going into the engine, and
    2. report the throttle position to MegaSquirt® via a TPS.
    For port injection, you can convert an existing carb, to do both jobs - the carb already controls the air flow, you have to adapt a TPS sensor to it. You may choose to machine out the venturis, remove the float bowls and fuel circuits if you wish, but that isn't necessary (but may be desirable for a number of reasons, including increased power!).

    Some people use the complete individual runner (IR) throttle body and injector set-ups off late model motorcycles - they often have enough flow for automotive engines, and are frequently available cheaply on eBay.

    However, if you are planning on a throttle body injection set-up, you need a dedicated TBI unit (in order to supply the fuel to the injectors, etc.), which can be hard to find for larger engines - Holley has made a 4bbl TBI for years (in 650, 700 and 900 cfm sizes), and as the computer fails regularly on these, they are sometimes available separately on eBay. TBIs have the advantage of having the fuel pressure regulator built in.

    Note that for either port or throttle body injection you can use multiple throttle bodies to support your power levels, if your manifold configuration can be adapted for them.

    When selecting a throttle body, there are a number of considerations. You need it to flow enough to support your engine's horsepower (or more correctly, to not restrict your engines power). Generally, you want to take the throttle body from an engine that made similar horsepower to your engine.

    However, if you are uncertain of your throttle body's application, you can measure the throttle bore size. However, you can't really compare the throttle of an EFI throttle body to the throttles of a carb. This is because the throttle(s) of a EFI TB is the main restriction, but on a carb, it is the venturis that are the main restriction. So you really have to compare the EFI throttle size to the carb's venturi size. However there are also a number of other considerations, such as that you can go larger with a EFI TB than a carb without suffering so many adverse effects because a vacuum signal isn't needed for the EFI to operate. Fuel delivery is always good with EFI (well, mostly).

    However, there are some drawbacks to a too large throttle body:

    For reference, the GM tuned Port Injection engines used throttle bodies with two 48mm throttles. These support about 230 horsepower, however these throttles were not the limiting factor in the power produced by these engines.

    To calculate how much horsepower you can make from a given throttle body size, you can use the estimator below:

    Number of throttle boresThrottle Bore diameterEstimated Power
    mm HP

    Note that the above is for naturally aspirated engines having a common plenum - individual runner manifolds will need larger throttle bodies, turbocharged 'blow-through' engines can get by with somewhat smaller throttle bodies

    Injector Selection

    It is important that your injectors are correctly sized for your engine size and power requirements.
    Too small and you will run out of fuel at high power and rpms, with likely engine damage from going lean. Too large and you will encounter tuning difficulties for idle and cruise conditions.

    You can enter your engine's estimated brake horsepower (at the flywheel) and the total number of injectors in the form below and press the "Compute Flowrate" button.

    # Injectors

    Or you can use the following chart to select injectors based on the total horsepower of your engine and the total number of injectors:

    Injectors Rating Required for Specified Horsepower
    in lbs/hr and (cc/min)

    Number of Injectors









    59 (620)

    29 (305)

    15 (158)

    12 (126)

    10 (105)



    88 (924)

    44 (462)

    22 (231)

    18 (189)

    15 (158)

    11 (116)



    59 (620)

    29 (305)

    24 (252)

    20 (210)

    15 (158)



    74 (777)

    37 (389)

    29 (305)

    25 (263)

    18 (189)



    88 (924)

    44 (462)

    35 (368)

    29 (305)

    22 (231)




    51 (534)

    41 (431)

    34 (357)

    26 (273)




    59 (620)

    47 (494)

    39 (410)

    29 (305)




    66 (693)

    53 (557)

    44 (462)

    33 (347)




    74 (777)

    59 (620)

    49 (515)

    37 (389)




    81 (851)

    65 (683)

    54 (567)

    40 (420)




    88 (924)

    71 (746)

    59 (620)

    44 (462)




























































































    based on 0.50 BSFC and 85% duty cycle
    Turbo/supercharged engines should add 10% to listed minimum injector size

    Injectors are usually rated in either lbs/houror cc/min. The accepted conversion factor between these depends somewhat on fuel density, which changes with formulation (i.e., by season), but the generally used conversion for gasoline is:

    1 lb/hr ~ 10.5 cc/min

    You can use this converter:


    Another way to select injectors is to take them from an engine that makes nearly the same power as your engine will [assuming the same number of injectors].

    If your regulator is adjustable (many aftermarket ones are), you can also adjust the fuel pressure to achieve different flow rates. Changing the fuel pressure doesn't affect the flow rate as much as you might assume, since it is based on the square root of the pressure ratio. The formula is:

    new flow rate = old flow rate × √(new pressure ÷ old pressure)

    So for example, if you had 30 lb/hr injectors rated at 43.5 psi, and you went to 50 psi, you would get:

    flow rate = 30 * √(50/43.5) = 32 lb/hr

    You can use this calculator:

    Rated flow (lbs/hour)
    Rated pressure (psi)
    usually 43.5 psi for port injectors
    New pressure (psi)
    New flow (lbs/hr)

    Do not run more than 70 psi fuel pressure, or the injectors may not open/close properly.

    However, do not install injectors with a much larger flow capacity than you need. Very large injectors will create idle pulse width issues that will make tuning very difficult.

    Injector Impedance

    Injectors can typically be categorised as either high impedance (hi-z, saturated) or low impedance (low-z, peak and hold.) It is important to know which type your injectors are. Both types can be used with Megasquirt although high impedance tend to be easier to use.

    New injectors will specify which type they are or list the ohms. If you are unsure, measure them with your meter on the ohms setting.

  • High impedance injectors are typically 12-16 Ohms.
  • Low impedance injectors are often 2.5 Ohms or less.

    Do not simply connect and hope.

    High impedance injectors (12-16 Ohms)

    These injectors can be directly connected to the Megasquirt with no special precautions. No need for injector resistors and Injector PWM should be turned off.
    The V3 mainboard can theoretically drive up ten high impedance injectors on each channel. The MS3X board can theoretically drive up to four high impedance injectors per channel. For a typical maximum install a V12 would use six injectors per channel on the mainboard or a V8 would use one injector per sequential channel on the MS3X.

    Low impedance injectors (less than 3 Ohms)

    These injectors can be used, with a few connection options.

  • Injector PWM (on the two V3 mainboard outputs only)
  • Injector resistors
  • External peak-and-hold adapter

    Injector PWM

    This is available on the two bank/batch fire outputs on the V3 mainboard. When enabled the Megasquirt gives the injector full current for a short period of time to open it and then holds the injector open with a lower modulated current.
    Ensure your board and loom are wired for the injector flyback 12V return if using injector PWM.
    Do not attempt to use injector PWM on the older v2.2 mainboard.

    Injector Resistors

    This method has been used by many OEMs as a simple approach to driving low-z injectors. The installer has the option of installing a 5 to 8 ohm resistor (with a 20 to 25 watt rating) in series with each injector (in effect converting them to hi-z.)
    1980s Volvos were fitted with a resistor pack that conveniently combines a number of resistors.

    To limit the current to under 2 amps, you need:

    resistor ohms = (alternator voltage / 2.0 amps) - injector resistance

    For example:

    resistor ohms = (14.0 volts / 2.0 amps) - 1.2 ohms

    => resistor ohms = 7.0 - 1.2 = 5.8 ohms

    You can also use the calculator below. Enter your injector resistance in ohms, your hold current in amps, as well as your injector impedance in ohms, in the form below and press the "Compute resistor" button.

    Injector resistance (ohms)
    Hold current (amps)
    usually ~1 or 2 amps
    Supply voltage (volts)
    usually ~14 volts
    Required Resistors (ohms)

    The 25-watt resistors aluminium case Ohmite resistors (with 1% tolerance) from work well. Below is a picture of a 7.5 ohm resistor, Digi-Key part number 825F7R5-ND.

    Ohmite has several suitable resistors, with part numbers that start 825F (25 Watt, aluminum case with mounting ears) and end in XRY, where X and Y indicate X.Y ohms. Other manufacturers offer similar items. Ensure you buy a 1% tolerance or you may see a variation between injectors.

    With the MS3X board driving 2.5ohm low-z injectors, experimentation has shown that 3-4 ohms is a good value to use. Be aware that the resistors will got hot in operation and should be mounted to some form of heatsink. Shown here are 40W 3.3R resistors mounted to an aluminium heatsink bracket.
    resistor pack

    Contact your Megasquirt reseller for resistor pack options.

    Peak and hold

    Another option is to use an external "Peak and Hold" (P&H) injector driver box. This works similarly to the PWM option, but is available as four channels and therefore suitable for sequential installs (use more boxes for more cylinders.)
    The P&H box dissipates more heat than using PWM but is a robust way of driving low-z injectors.

    There are two main injector wiring options. The original bank/batch fire non-sequential injector outputs are available on the main connector and can operate in untimed or semi-sequential modes.
    The optional 12V flyback connection shown is only if you optional wiring has been performed on the board to bring the flyback to that pin.
    V3 injector wiring
    For sequential operation, the outputs from the MS3X connector are more suitable and support up to V8 sequential or V12 semi-sequential.
    MS3X injector wiring
    High impedance injectors (typically 8 ohms or more) can be directly driven from the MS3X outputs.
    MS3X hi-z injector wiring
    Low impedance injectors require resistors or a peak and hold box.
    MS3X low-z injector + resistor
    MS3X low-z injector + P&H


    Engine and Sequential settings
    The left hand side of the dialogue is the same as earlier Megasquirt versions [explain in more detail] The right hand side is used to configure the fuel outputs.
    Main Fuel outputs - This critical setting selects whether you are using the two bank fire outputs or the MS3X sequential outputs.
    Sequential on - Only available on MS3X outputs, choses from untimed injection, semi-sequential or fully sequential.
    Note that fully sequential fuel requires a cam sensor or a tach input from the camshaft. i.e. you cannot run sequential with only a crank mounted trigger wheel or only a simple distributor pickup.
    Angle specifies - For sequential fuel, the injection angle is tuneable. This selection chooses whether you want to tune the start, middle or end of the injection pulse. Most common is end-of-squirt.
    Injector trim - For sequential you can enable percentage based trim tables. These can be useful for balancing cylinders where injector or air flow varies. (Individual cylinder EGT will likely be required to make use of this feature.)

    Firing order
    In sequential or semi-sequential, the outputs ALWAYS fire in sequence A,B,C.... so you must apply your firing order in the loom. (Note that the "Firing Order" settings are to ensure fuel trim is applied to the correct channel, they do not change the output sequence.)

    Firing order - sequential
    e.g. on a 4 cyl engine with 1342 firing order InjA = 1, InjB = 3, InjC = 4, InjD = 2
    on a V8 with 18436572 firing order. InjA = 1, InjB = 8, InjC = 4, InjD = 3, InjE = 6, InjF = 5, InjG = 7, InjH = 2

    Firing order - semi-sequential
    (Be sure to use code 0.26 or later)
    With semi-sequential on the MS3X outputs the wiring is the same as full sequential to allow easier upgrade. The outputs are fired in pairs that are 360 degrees apart. i.e. with the V8 cycle above, in semi-sequential, outputs fire in sequence A&E, B&F, C&G, D&H, repeat

    More examples and wiring diagrams here

    Sequential and Semi-sequential using the mainboard outputs

    The two mainboard fuel channels may be used for full sequential on four stroke one and two cylinder engines if a cam sensor is used to give the full 720 degrees of position information.
    When using the two mainboard fuel channels for semi-sequential the injectors will be grouped. On a 4cyl typicals 1&4 on channel 1 and 2&3 on channel 2.

    Only certain combinations of settings are permitted with semi-sequential on the mainboard outputs. Be sure to follow this table or the code will give you a configuration error and show rpm = 65102.
    3 or more cylinders are not supported as sequential on the mainboard outputs.
    3 and 5 or more cylinders are not supported as semi-sequential on the mainboard outputs.
    CylindersModeValid option
    1Sequential1 squirt simultaneous
    1Semi-seqnot possible at the moment
    2Sequential1 squirt simultaneous
    2Semi-seqnot possible at the moment
    4Semi-seq4 squirts alternating

    Sequential and Semi-sequential using the MS3X outputs

    Semi-sequential in MS3 operates for fuel like 'wasted-COP' does for spark. Each output operates once every 360 degrees with half the fuel required for a full cycle. A cam sensor is not typically required to run semi-sequential, just a crank sensor.
    Due to the way the code operates, there are a number of restrictions in this mode and cylinder trim is not possible.
    Semi-sequential may however, but a useful 'stepping-stone' during an install as the injector wiring is identical to full sequential.
    For engines above 8 cylinders, semi-sequential operates a single injector output per injector pair. The two injectors must be wired up in pairs. i.e. V12 semi-sequential uses 6 injector channels.

    Dead Time / PWM

    The dead time (or offset) of an injector is effectively the portion of the pulsewidth when no fuel is injected, this is because the injector is an electro-mechanical valve and takes a measurable time to open and close. (Note that older documentation referred to this as just "opening time".)

    Setting the dead time correctly is an important setup step. If it is incorrectly set, you will find tuning light-load or idle very tricky and features such as air-temperature correction will malfunction. In worst cases your engine will be untuneable - especially if you have very large injectors.

    The Megasquirt-3 dead time system is found on the Fuel Settings -> Injector Dead-time / PWM menu. It allows for individual or grouped setting of these dead times and for the non-sequential mainboard injectors PWM current limiting is also available. Be sure to identify where your injectors are connected before proceeding. This will match up with the "Main fuel outputs setting" on the engine and sequential settings page.

    There are three main areas on this page

  • The MS3X injector settings
  • The non-sequential mainboard injector settings
  • The voltage correction curves. These allow fine tuning the battery voltage correction of the dead time.

    Sequential MS3X injector settings

    As previously mentioned, these injector outputs directly support high-impedance injectors. Low impedance injectors may be used through an external peak-and-hold box or resistor pack.
    The first setting Same/Individual allows either one dead-time across all injectors or if you have physically determined the precise dead time for each injector individually, you can use the Individual setting. Normally Same.
    The table below allows entry of the dead times - in milliseconds at nominal 13.2 volts.
    You can choose one of the four battery voltage correction curves. (If in doubt pick no.1 and leave the curve alone.)

    Typical MS3X settings

  • High impedance injectors - dead time = 0.8-1.0 ms
  • Low impedance injectors with resistors - dead time = 0.8-0.9 ms
  • Low impedance injectors with external peak/hold - dead time = 0.6-0.8 ms

    Non-sequential mainboard injector settings

    These injector outputs directly support high-impedance injectors or low impedance injectors with built-in PWM current limiting. The settings are:
    Dead time - dead time of bank 1
    Correction curve - which voltage correction curve to use
    PWM current limiting - enables or disables PWM current limiting for low-z injectors
    -PWM current limit - the hold duty cycle. 30% is typical
    -PWM time threshold - the peak full current time before current limiting begins. 1.0 is typical.
    -Injector PWM period - the PWM frequency. Rarely any need to change this from 66us
    Bank 2 different settings - enable this ONLY if you want to give bank 2 (inj 2) different dead time or PWM settings. Otherwise the values are shared with bank 1.

    Typical mainboard settings

  • High impedance injectors - dead time = 0.8-1.0 ms. PWM disabled
  • Low impedance injectors with PWM - dead time = 0.6-0.8 ms. PWM enabled, 30%, 1.0ms, 66us

    Special cases
    It is possible to use MS3X and mainboard injector simultaneously when using staged injection. Here you could use high-z on the MS3X and low-z on the mainboard using PWM-limiting and with different different correction curves.

    Technical description of dead time

    See also Injector Dead-time Measurement

    Injector Small Pulsewidths

    At most pulsewidths (say above 2ms) injectors behave in a linear manner, where 10% increase in effective pulsewidth gives a 10% increase in fuel flow. This linear behaviour is relied upon in the Megasquirt fuel calculations.
    However, at small pulsewidths, injectors behave non-linearly.

    small pw1

    The Injector Small Pulsewidths feature allows the user to make a correction and make the non-linear region more linear in nature. Unless you have calibration data for your injectors measured on your Megasquirt, this feature should be left turned off.
    The X-axis of the curve is the Megasquirt pulsewidth (before deadtime is applied) and the Y-axis is the required injector pulsewidth (before deadtime) that the injector needs to approximate linearity.
    Most injectors observed exhibit an 'S' shaped non-linear region. Nothing happens for the first few 0.1ms after dead time, so a curve that effectively adds on PW to the very low values will help linearise this somewhat. The non-linear region also frequently shows a 'lump' which will make linearisation tricky. See the dead time measurement page for real measured data.

    Here are some example curves based on data from real injectors. Note that no allowance is made for the 'lump' which will distort the results. The injectors were linear by 1.2ms (plus dead time) so the curve is arranged from 0-1.2ms.
    small pw2 small pw3
    Ensure that the start and end points match. i.e. here 0.000, 0.000 and 1.200, 1.200

    However, the best approach is to avoid operating injectors in this non-linear region altogether. Size injectors appropriately and use a second set of staged injectors if a large dynamic range is needed.

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