Driving Injectors

Dynamic EFI

Bringing TBI and Multi Port Fuel Injection to a New Level.

Driving Injectors

This is an area that garners a lot of questions and confusion. Part of the confusion is the difference between the injector driver, and the injector firing rate. First we talk about the driving of the actual injector, regardless of how often the injectors are fired. Let's start with the meat of the matter. What injectors can be driven from TBI injector drivers.

The Physical Drivers:

The stock TBI ECM will drive:

2 TBI injectors, 1 on each driver
4 port saturated injectors, 2 on each driver (in parallel)
4 port peak-and-hold injectors, 2 on each driver (in parallel)
6 port saturated injectors, 3 on each driver (in parallel)
8 port saturated injectors, 4 on each driver* (only if the injector resistance is greater then 14 ohms) (in parallel)

* saturated port injectors range from 12 to 17 ohms. FMS/Bosch are typically in the 12 ohm range. AC Delco/Delphi are typically in the 16 ohm range.
*A simple modification to the TBI ECM will allow any 8 port saturated injectors, 4 on each driver (in parallel). This mod is required for saturated injectors in the 12-14 ohm range.

With the 4-injector upgrade (a Dynamic EFI exclusive) the TBI ECM will drive:

4 TBI injectors, 2 on each driver (in parallel)
6 port saturated injectors, 3 on each driver (in parallel)
6 port peak-and-hold injectors, 3 on each driver (in parallel). Not exactly correct as-is, this can be changed to suit if a user is running a 6 cylinder with PnH port injectors.
8 port saturated injectors, 4 on each driver (in parallel)
8 port peak-and-hold injectors, 4 on each driver (in parallel)

There are other combinations that will work. However, they are in the "this is not the best the way to do it" book.

The Flip Side of Injector Driving:

The other side of injectors is how often they fire. The normal injector firing is synchronized to the spark plug firings. This for both port and TBI injector firing schemes. The difference is in how often the injectors are fired.

A TBI system typically fires the injectors every other spark plug firing. That is, 2 injectors, each firing alternately on each spark plug firing.

A port system typically fires the injectors once every engine revolution. That is, no matter the number of cylinders, every time the engine does one full rotation the injectors fire. Which is twice per intake cycle per cylinder.

Firing Rate:

The stock TBI ECM firing rate is:

With a TBI system, 2 or 4-injector driver, the injector(s) will fire in alternating fashion.
With port on a 4-cylinder, the injectors will fire in alternating bank fire (stock TBI ECM firing order)
With port on a 6-cylinder, the injectors will fire in a strange 3 for 4 order. It works, but not as it should (zucchini).
With port on a 8-cylinder, the injectors will fire twice per engine revolution. This is OK for low speed engines (below 4400 or so RPM). The issue is that the injector duty cycle becomes an issue. Too much time is wasted opening and closing the injector.

With the Dynamic EFI port conversion, everything changes. An inexpensive modification to the TBI ECM along with the EBL Flash allows for proper 6 & 8 cylinder port injection firing rate. Recall that TBI & port 4-cylinder engines the firing rate is OK with the stock ECM. For a 6 or 8 cylinder engine there is an available Port Mod board.

With the Port Mod the TBI ECM firing rate is:

Port on a 6-cylinder, the injectors will fire once per engine revolution.
Port on a 8-cylinder, the injectors will fire once per engine revolution.

And the reason for different drivers, Injector Impedance:

The impedance of the injector coil defines which drivers work for which injectors. There are two basic types: peak-and-hold (PnH), and saturated.

The PnH is also known as a low impedance injector. They are in the 1.2 to 2.4 ohm range.

The saturated is also known as a high impedance injector. They are in the 12 to 16 ohm range.

When you use a DVM to measure an injector it is really the resistance that is being read. The impedance includes the inductance, which if you don't understand don't worry about it. The resistance is enough information to decide which driver is required.

A TBI injector is 1.2 ohms, which is a PnH type. It is driven to peak at 4 amps, then folds back to hold at 1 amp.
A port PnH injector is 2.4 ohms. It is driven to 2 amps peak, then folds back to hold at 0.5 amp. Which is why two in parallel can be driven from one TBI driver.

A port saturated injector ranges from 12 to 16 ohms. The injector is turned on and held on throughout the pulse. Now, if you recall from the earlier discussion, a stock TBI driver has trouble with four saturated port injectors once the resistance drops below 14 ohms. The reason is that the resistance of the four injectors is low enough to cause the PnH TBI driver to peak and fold back to hold. The problem is that the injectors won't stay open at the low hold current.

With four 16 ohm injectors the total resistance is 4.0 ohms. Divide the vehicle voltage (14 volts) by 4 for 3.5 amps. Not enough to trip the 4 A peak on the driver.
With four 14 ohm injectors the total resistance is 3.5 ohms. Divide the vehicle voltage (14 volts) by 3.5 for 4.0 amps. Which is borderline on tripping the 4 A peak on the driver.
With four 12 ohm injectors the total resistance is 3.0 ohms. Divide the vehicle voltage (14 volts) by 3 for 4.7 amps. It now trips the driver into hold mode. In this case it is a simple matter to disable the PnH function of the driver. It just turns it into a saturated driver which works for four port saturated injectors on each driver.