ECU Compatibility
Determining the correct amount of injector fuel flow is only part of injection selection process. You also have to match the injector's operating parameters to your Electronic Control Unit. Before we delve into these operating systems, you need to understand how an injector operates.
When an injector is energized by the ECU, its internal parts physically move to let the pressurized fuel flow through. The amount of fuel delivered is controlled by how long the injector is energized by the ECU and its driver circuit. This time is usually specified in milliseconds (ms). An injector is "pulsed" open for short periods of time at idle, and held open longer as rpm and the engine's airflow increases, which requires a corresponding increase in fuel flow.
In the case of MSD's "top feed" Competition Fuel Injectors, an internal ball is electronically lifted off its seat to allow the high pressure fuel to pass and exit the six metering holes. MSD injectors produce a narrow spray angle of 10° - 15°, while other injectors' spray angle can be as wide as 30°. This narrow spray cone delivers a fully atomized fuel charge that is suspended in the intake charge, preventing wetting of the intake and cylinder walls.
There are two main styles of driver circuits so there are two main styles of injectors: Saturated Circuit and Peak and Hold. It is important to note that even though the two injector styles look similar and will physically fit in either system, they are not compatible and one style cannot be swapped for the other! Interchanging the different style injectors can result in slower delivery rate, overheating of the injector or driver and even engine damage.
Saturated Circuit Drivers and Injectors
Most domestic OE production EFI systems use an ECU with 12 volt Saturated Circuit drivers. These systems are very inexpensive, simple, and reliable. This type of driver works by supplying 12 volts to the injectors and the ECU turns it on and off to establish a fuel injector pulse. In general, if an injector has a high resistance specification (12-16 ohms) the ECU uses a 12 volt saturated circuit driver to control it. This means that the current flow in the driver and injector circuit stays low keeping the components cool for long life.
The downside to a Saturated Circuit driver is that it has a slower response time (opening and closing time) than a peak and hold type. This slower time can somewhat decrease the usable operating range of the injector energized by this driver. An injector operating on a saturated circuit driver typically has a reaction time of 2 milliseconds while a peak and hold driver typically responds in 1.5 ms.
MSD offers three higher flow injectors that are designed primarily for quick response time with a 12 volt saturated circuit driver. The PN 2018 Injector is rated at 38 lb/hr with 12 ohms and the PN 2013 is a 50 lb/hr, 12 ohm injector. Our new Jammer injector is rated at 60 lbs.-hr (PN 2030) and can be used in either Saturated or Peak and Hold systems.
NOTE: You can measure an injector's resistance with a Digital Volt-Ohm Meter (DVOM) by connecting it across the injector's electrical plug contacts.
NOTE: Ohm's Law can be applied to calculate the current in the injector and driver circuit when using a high resistance injector like the PN 2018 (38 lb/hr static flow, 12 ohm). Remember, Ohms's Law is I = E/R, where I = the circuit's current in amps, E = battery voltage in volts available to the injector, and R = injector resistance in ohms. So, for our 12 ohm injector being supplied 12 volts, I = 12v /12 ohms, which is 1 amp of current in the circuit to operate the injector.
Peak and Hold Drivers and Injectors
These type of injectors and drivers may also be called current sensing or current limiting. Peak and Hold injectors are primarily used in aftermarket high performance systems. Because they are more expensive and complex than saturated circuit drivers, they are not generally used with domestic production ECUs.
Most high flow injectors are low resistance (2-5 ohms) and use a peak and hold driver to activate them. With this type of driver, 12 volts is still delivered to the injector, but due to its low resistance, the current in the driver circuit is high. How high? Using Ohms's Law we can calculate the current rating (12v/2 ohms = 6 amps). This is substantial current flow and a Saturated Injector cannot handle it.
The Peak current quickly opens the injector while the lower Hold current rating is used to keep it open for the duration of the ECU command. Because these injectors have larger physical parts and often work against high fuel pressure, they require an extra “kick” from the higher current to keep the opening and closing time of the injector stable at the higher fuel flow rate.
Peak and Hold drivers come in two values; 4 amp peak/1 amp hold, and 2 amp peak/0.5 amp hold.. Even though 6 amps may be available to operate the injector, the maximum it is allowed to reach is 2 or 4 amps, depending on the driver's current limit. Note that the MSD PN 2014 (72 lb/hr, 2 ohm) and PN 2015 (96 lb/hr, 2 ohm) injectors require a 4/1 amp driver.
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