How to Add Additional Inputs/Outputs to an HP, Terminator X, or Dominator ECU

10 min read

How to Add Additional Inputs/Outputs to an HP, Terminator X, or Dominator ECU

10 min read

For high performance machines in the 21st Century, it’s all about control. Horsepower has never been easier to make, but to apply that new-found power means exerting control. Electronics have become the easiest way to capture data, create solutions, and maintain control over all that horsepower.

Holley has built quite a name with their line of high performance fuel injection systems and has recently expanded the product lines to include affordable ECU packages like the HP and Terminator X / Terminator X MAX line of EFI systems. While these systems use similar software to the top-of-the-line Dominator, these more affordable systems make entry into electronic fuel injection easier and more affordable.

The main advantage that the Dominator ECU enjoys over the less pricey HP and even more affordable Terminator X series is the aforementioned control. Where the Dominator differs is that it offers nearly 50 inputs and 36 output circuits – termed I/O’s. The less expensive HP and Terminator X are limited to a maximum of four inputs and four outputs. But drag racers, track day enthusiasts, and enlightened street drivers are beginning to realize that having more I/O’s offers opportunities.

Holley I/O Add-On Module

The I/O Module is compact and will fit nearly anywhere in the interior. The I/O Module kit (PN 554-165) includes a dedicated connector plus several looms of different color wire that you can use to create pathways to sensors, relays, or other devices.

Before we get into the details of this I/O box, let’s delve a little deeper into a few related items of interest. Any basic EFI ECU controls systems beyond just spark and fuel. The Holley HP and Terminator X's four inputs and outputs can be custom configured to the user’s needs. For outputs, one example might be control of a pair of electric fans where all you have to do is tell the ECU at what temperature the fans should turn on and off. Other external controls can be aimed at something fun like control over nitrous solenoids.

But once you become familiar with these systems, motorsports enthusiasts have discovered there are several other outputs that might be worthwhile additions to the race car. We’ve listed a few of these in the accompanying I/O chart so we won’t go over them here. There are enough of these useful outputs available that a Holley HP or Terminator X ECU does not have the capacity to control them all. That’s where Holley’s I/O Module comes into play.

Of course, outputs are only half the story. The other half encompasses the inputs. We can define inputs mainly as sensor information that can be added to the ECU to offer the racer or street enthusiast more information that can be used to fine tune the car for better performance. Again, we’ve come up with a list of inputs that can be used that can then be stored in the computer’s memory for use as data log reference information.

This Holley module is essentially a CAN bus device that accommodates a total of eight inputs and eight outputs that can be added to the HP or Terminator X ECU’s to expand their capability. CAN stands for Controller Area Network which is a circuitry used in computers to allow communication between devices either within the computer or with sensors outside the ECU.

While this is a new product, it does not require the latest software to allow it to operate. In fact, the module is completely compatible with all of Holley’s software right back to V2. This way, if you prefer an earlier version of the software you don’t have to upgrade just to use this new module.

Another interesting option that the I/O Module offers is that the Holley digital dash can be used to accomplish digital switching. Let’s say you’d prefer a virtual switch on the Holley LCD screen. This means after configuration, all you have to do is touch the screen to manually turn the fan on. This virtual or digital switch will then be connected to your I/O module that controls the fan.

Andy Starr 1956 Chevrolet

Among the places the Holley I/O Module could find itself useful are cars like Andy Starr’s 582ci big-block ’56 Chevy street/strip car. This Hilborn injected Rat uses a Holley HP ECU and Starr wants to expand with additional inputs that will help him improve the big-block’s e.t. and speed. Photo credit: Dave Milcarek Photography.

Andy Starr’s 582ci big-block Chevy-powered ’56 Chevy drag car uses a Holley HP-controller to run the EFI on his Hilborn-stacked Rat motor. Initially, it appeared the HP was the perfect choice and has worked flawlessly to control spark and fuel. But now with several Drag Week and other races as experience, Starr has maxed out the I/O capability of his HP ECU. This new I/O box offers a wealth of additional channels.

It has been said that there is no such thing as too much data. When it comes to controlling his ’56, Starr is discovering that the more data he can recover from each pass helps determine how to tune the car. While most guys with a low 9-second car would be running a TH400 or perhaps a ‘Glide, Starr has chosen the much more tortuous path of a G Force GF5R five-speed manual. One big challenge is making the clutch work, which is a big part of the need for additional data.

Andy Starr G-Force trans

Getting a 3,800-pound car launched with a manual transmission is not easy – nor is it easy on parts. The G-Force five speed uses a 3.17:1 First gear ratio to get the car moving and spins a 4.56:1 gear in the Moser fabricated 9-inch housing using Moser 40-spine axles.

With the existing inputs into his Holley HP maxed out with sensor data, Starr is now looking to add capacity with the I/O module. We’ve listed his current I/Os in a separate chart. His clutch release rev limiter data is actually used to limit rpm just before the clutch is released but does not require the driver to manually trigger a button like a two-step. Plus, his input rpm sensor allows him to log the difference between engine rpm and input shaft speed to determine the amount of clutch slippage on the starting line.

Starr told us his plans include adding the I/O module to free up a couple of the inputs from the HP like pan vacuum and redirect them to the I/O module. This will free up the data stream through the HP to allow a higher resolution sensor like the g-meter to be run by the HP box.

We won’t go into the details of navigating the I/O process but essentially you tell the module what sensor or output channel you intend to use and what kind of signal it will be processing. We’ve included a small chart taken directly from the Holley instruction manual that shows the different kinds of signals that can be processed. There are six different signals that can be configured to specific channels.

As an example of a simple I/O operation, we recently added a TH400 trans to a Chevelle running a 404ci LS engine. The TH400 trans requires a +12-volt signal be passed to a small straight-blade connector on the transmission whenever the throttle moves to WOT. This signal increases the line pressure in the transmission as opposed to doing so with a linkage arrangement like the Powerglide orTH350 transmissions.

We selected one of the outputs as +12v and configured the channel to trigger this signal when the TPS exceeds 70 percent throttle opening. This eliminates mechanical connections to the throttle and also reduces driver distractions. It’s a small thing but makes the driver’s job at the track that much easier.

Holley’s I/O box may appear to be not much more than another electronic accessory, but in the hands of a knowledgeable tuner, this I/O module is a powerful option on the way to making your car more consistent and quicker whether on the drag strip or the street.

Input And Output Function Examples

GM Flex Fuel sensorShift Light
Shock InputsAir Shifter Output
Front Wheel SpeedLine Lock Solenoid
Data Log Trigger12v signal to TH400 trans
Trans PressureTorque Converter Dump Valve
Trans TempBrake Lights
Pan VacuumCooling Fan Relay
Drive Shaft SpeedBoost Control Solenoid
Dome PressureWater methanol injection
Fuel LevelNitrous Solenoid
Brake PressureFuel Pump
Shock SensorActive Exhaust Valve
G-MeterElectric Water Pump Relay

Starr's '56 Chevy Current Inputs/Outputs

Driveshaft SpeedFan 1
Input Shaft SpeedFan 2
Clutch Release rev limiterWarning Output
Vacuum Pump

Each input channel on the module can be configured as one of the following types. This is consistent between all of Holley's ECUs.

Switched-high (H)12V switched input
Switched-low (G)ground switched input
5V Analog (5)Use for transducers, sensors, etc. (anything 0-5 volt input)
20V Analog (2)Use for reading a signal from 0-20 volts
Thermistor (T)2-wire temperature sensors
Frequency (F)Typically for a three-wire (Hall effect) speed sensor


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