Which Holley ECU Is The Best For You?


Which Holley ECU Is The Best For You?


Hot rodders and gear heads are the true definition of control freaks. In the early years of performance and racing, the focus was entirely on making horsepower. But now making power is much easier. Now the goal is as much about controlling the engine both from how much fuel and spark each cylinder needs along with accurately monitoring every nuance. This can’t be done by analog means with manual gauges and maybe a movie camera like our grandfathers did it. That’s just too cumbersome. Today it takes computer power.

We’ll take a look at the three levels of Holley engine computers and how these systems build on a basic high performance foundation to offer three optional tiers to choose from. We’ll approach this like building a house. First we have to have a simple yet solid foundation that supports the addition of multiple floors, if you will, of engine control. We will begin with the Terminator X, add features and control with the HP, and finally hit the penthouse suite with Holley Dominator ECU.

To set the stage, the Dominator was actually the first ECU that Holley created that offered incredible power and performance capabilities. Once that flourished, the engineers found they could offer simpler systems for applications that did not require nearly as much complexity and these systems could be offered at a reduced price. But the point that must be emphasized is that even the least expensive Terminator X employs a feature-rich software and identical computing power to the Dominator. Even the basic electrical harnesses are the same between all three ECU’s...with some differences, of course.

Terminator X and Terminator X Max

Holley ECU Terminator X

The Terminator X is the step above the Sniper but offers the same sophisticated software program as used by the top of the line Dominator ECU. The Terminator X and Terminator X Max are an excellent choice for a basic street oriented EFI application.

The simplest of the Terminator X systems is designed to offer EFI control over various engine platforms such as old school engines like the small- and big-block Chevy, small-block Ford, as well as the more modern GM LS, Chrysler Gen III Hemi, and the Ford Mod motors. The Terminator X is designed to control both fuel and spark as well as offering control for optional systems like nitrous.

In keeping with its affordable approach, The Terminator series is designed to manage only high impedance injectors. This helps keep the price of the system affordable by not requiring more robust, high amperage injector drivers. This less complex design also limits the total number of what are called Inputs and Outputs (I/O). Inputs refer to the number of channels of additional data the system can track such as fuel pressure, transmission temperature, or other useful data. Outputs would be such items as electric fans, nitrous solenoids, a shift light, or any number of other systems. The importance of these I/O’s will become apparent when we get to the HP and Dominator series where racers can use these for more sophisticated control strategies.

The Terminator X is the base unit for this series and is augmented with the Terminator X Max. The Max versions offer additional control over electronic transmissions as well as electronic drive-by-wire (DBW) throttle bodies. A specific application would be a Terminator X Max designed specifically for a Gen IV 6.0L LS engine with a 58x crank signal, DBW, and a 4L60E electronic transmission.

Holley ECU Terminator X 2

The latest addition to the Terminator X Max series of EFI kits will allow you to retro-fit tunable EFI to a GM LT Gen V engine like the LT1, L83 or L86 engines. This package uses a separate driver box to accommodate the larger injector drives necessary for the high pressure in-cylinder injectors.

The Terminator X series was intended as an entry level EFI system that does not require a laptop to install or tune. Instead, the Terminator X comes with a separate handheld display that makes the installation and tuning very easy with a few screen touches. This makes it easier for enthusiasts who may be intimidated by complex laptop software maneuvering required by more sophisticated systems like the HP or Dominator. However, the Terminator X also does offer the ability to go into more detailed software changes with a laptop if desired.

One interesting note about the Terminator X that is different from most other entry level ECU’s is that it contains its own internal manifold absolute pressure (MAP) sensor to reduce the confusion that can often occur with using a separate, external MAP sensor. The Terminator X sensor is a one bar device intended for normally aspirated engines. It can be quickly adapted to supercharger or turbocharger applications by using the supplied harness connection to allow the use of an external MAP sensor. The internal sensor just makes the setup procedure easier. The more sophisticated HP and Dominator ECU’s require an external MAP sensor that must be specifically identified in order to work properly with the software.

All this should reveal that the Terminator X and Max are much more than bare bones EFI systems. The Terminator X also offers nitrous control that includes progressive control over the solenoids. But if turbocharging is more your fare, there’s also electronic control for boost available that can be managed using time, rpm, gear, or speed parameters. Terminator X does not offer internal data logging but data logging is retrievable on a separate SD card located in the hand-held. Plus, as we will see with both HP and Dominator, the Terminator X offers advanced individual configuration file (ICF) work.

Holley ECU Terminator 3

The Terminator X and X Max systems all use a hand-held device that provides a simple-to-use portal for input into the ECU, making a laptop unnecessary. Of course, a laptop can be used for more finite adjustments, if you choose to do so.

In addition to these amenities, Terminator allows the optional use of a knock sensor circuit to protect valuable engine parts like pistons and head gaskets from failure due to the harmful effects of detonation. This can be found in the tool bar under the ICF logo which stands for Individual Configuration File.

Holley has also recently released a separate Terminator X Max kit specifically designed to control the Gen V direct-injected engines such as the L-83 5.3L and L-86 6.0L truck engines and of course the LT1. This system requires heavy-duty electronic drivers to trigger the high amperage loads of the high pressure fuel injectors so the Terminator X Max box is supplemented with a separate box housing the required injector drivers.

This Gen V Max system offers tuning tables for variable valve timing (VVT) that is present on all Gen V engines. Control tables are already loaded in the software to make it easier to set up the engine quickly and start making horsepower.

Holley ECU Terminator X 5

Electronically-controlled transmissions like this 4L60E can be controlled with either a Terminator X Max ECU or with a Dominator ECU. The HP does not offer transmission control.


Holley ECU HP EFI 1

The HP is an affordable version of the Dominator ECU with similar software opportunities but still limited to the same 4 input and 4 output options of the Terminator X series. It does, however, offer the ability to run 8 low impedance injectors.

For those looking to step up to more sophisticated electronic control, the HP is the next level in the Holley family of ECU’s. The HP offers more options but must do so through the use of a laptop as it does not require a handheld device although one can use a hand-held monitor with either HP or Dominator but they will need to be purchased separately.

Starting with a nod to durability, both the HP and Dominator ECU’s are fully potted, which means the circuit boards are encased in an epoxy that improves the ECU’s ability withstand vibration. Beyond that, the most significant upgrade that the HP offers is the ability to control 8 low- or 16 high-impedance fuel injectors where the Terminator is limited to only 8 high impedance injectors. This offers many more options for the enthusiast looking to flow greater volumes of fuel.

Low impedance injectors require higher capacity drivers to manage the electrical current necessary to run these injectors. In addition, the HP also offers 2GB of internal data logging capability which is much more user friendly.

Building on the Terminator series, the HP also offers the ability to manage 8 distributor-less outputs capable of supervising factory style “smart” ignition coils or Holley DIS coils. This is a great feature when eliminating a distributor on an older engine like a small- or big-block Chevy in order to achieve improved timing accuracy.

Holley ECU HP 2

One optional feature that can provide a direct visual connection to either HP or Dominator is Holley’s 12.3-inch wide Pro Dash that offers outstanding graphics to enhance the whole ECU operation.

Additional features include both nitrous and turbocharger control with far more sophistication. The HP can manage as many as four stages of nitrous with progressive solenoid control. Building on the sophisticated turbocharger boost control similar to the Terminator X, the HP also contains software to run a Holley water-methanol injection system.

It’s worth noting the HP is not designed to offer electronic transmission control or drive-by-wire throttle control. To gain that you will have to step up to the Dominator. The HP also limits the number of I/O’s to four for each side, the same as the Terminator X and Max boxes but offers more programming functionality. To expand that capability without investing in the Dominator, Holley offers a separate, add-on I/O unit that increases to 8 inputs and 8 outputs.

When working with outputs, you may see a reference to PWM control, which is defined as pulse-width modulation. A classic example of PWM control would be voltage modulation of an electric fan. If the fan is commanded to run at 50 percent duty cycle, full electrical power is only applied over half of the total time the fan is running. This is a little bit like pedaling a bicycle where you maintain a given speed but only pedal for half the time and rest the remaining time.

Holley Dominator EFI

Holley ECU Dominator 1

The Dominator offers immense control, and is able to run up to a 12-cylinder engine with the ability to run 12 peak and hold injectors. Or, you can run multiple combinations of 24 low and high impedance injectors along with an enormous selection of custom configurable inputs and outputs.

Okay now we get to the big boy in this trio of ECU’s. Every control system that we have previously discussed with Terminator X, Terminator X Max, or the HP is available with the Dominator – plus much more. The Dominator’s potential is really aimed at more sophisticated users usually in formal competition where much of this increased control can be exploited. Of course, the Dominator’s can also be successfully used on the street as well.

The Dominator can also be configured to operate some electronically controlled automatic transmissions, a feature not available on the HP. So if that is part of the overall plan, then the Dominator fits nicely.

Because control is the name of the game, the Dominator increases overall potential by managing as many as 16 sequentially-driven individual injectors with 8:2 control leaving four more injector channels for uses such as charge pipe injectors. The standard pattern would be 8 amps to initially open the injector and 2 amps to keep it open. This means the Dominator has the ability to control16 individual injector drivers (which requires the use of the injector driver box) instead of 8 with the HP. The system can also be configured to control as many as 24 individual injectors, offering an immense amount of options for staged use when using a power adder like nitrous, a supercharger, or turbocharger.

Holley ECU Dominator 2

This image indicates the Input/Output location on the Holley software toolbar (arrow). Clicking on this icon will open up a workplace from which you can create new inputs or output configurations.

Another useful addition to the Dominator’s capability is a dramatic increase in the Dominator’s I/O capability. While the Terminator and HP are limited to 4 inputs and 4 outputs, Dominator offers an amazing potential with as many as 47 inputs and 36 configurable outputs. That’s an enormous amount of control – even for a committed data hoarder.

There’s also an advanced individual configuration file (ICF) capability that offers virtually an unlimited ability to tune based on fuel, spark timing, boost, nitrous, and more. Individual cylinder timing is fully adjustable and the Dominator also provides the potential to integrate power modulation based on nitrous use or boost when used in conjunction with an add-on traction control device.

We’ve condensed a ton of information about these three Holley ECU’s leaving much more detail to wade through in order to fully understand all of these features and which would be best for your application. But within these three ECU’s there is one that will be a perfect match for your next build.

Holley ECU Nitrous graph

Nitrous has become such a common power adder that even the basic Terminator X system offers the ability to do finite nitrous control that is much more than just an on/off switch. Essentially you can choose the on/off RPM, specify a nitrous delay from the onset point, and establish a progressive curve for nitrous all within the Terminator X software.

Holley ECU boost graph

All three ECUs in this story offer electronic boost control, with a more basic system found in the Terminator X and more sophisticated options open when you step up to the HP and Dominator ECUs. This graph illustrates a simple linear boost control limited to 10 PSI over 15 seconds.

ECU Input/Output Options

Terminator X44
External Driver Box88

Inputs consist of six speed of either either Hall effet or magnetic plus 30 0-5 volts.

** Outputs consist of 20 high-side 12v PWN or switched 12v

Input and Output Options

GM Flex Fuel SensorShift light
Shock inputsAir shifter output
Front wheel speedLine lock solid
Data log trigger12v signal to TH400 transmission
Transmission pressureTorque converter dump valve
Transmission temperatureBrake lights
Pan vacuumTorque converter lockup
Driveshaft speedSecondary fuel pump
Dome pressureElectric fans
Fuel levelAir conditioning cut-out
Brake pressureWarning light
Ride height

Diagnostic LED lights on Holley Terminator X

Engine Run
ECU Power
Wideband Status
TPC Calibration
Crank Signal
Fuel Pump
VVT Status (when applicable)


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