Although the primary benefit of EFI systems like the Holley HP and the Terminator X is their ability to dole out fuel in precisely measured increments, these ECUs are also capable of monitoring and managing a number of other aspects of an engine thanks to the inputs and outputs that are integrated into their design. And as Ryan Witte of Holley Performance points out, you don’t need to have an established game plan when adding an EFI system in order to leverage these abilities down the road.

“With these EFI systems, you’re giving the fuel system the ability to communicate with a lot of other elements of the vehicle, and that ability means that you can configure the ECU to do – or not do – a range of different things based on the data that’s being shared. And that programmability can add an entirely new layer of control over how various systems behave in different situations and conditions. It allows you to move beyond managing an engine to managing the entire vehicle. That might not be something you need to do today, but having the functionality built into the system means you can add things later on as needed.”

Zooming further out on this concept, inputs and outputs give the user the ability to not only monitor and manage elements outside of the EFI system itself, but also trigger events based on the data received. Fundamentally speaking, it’s procedural programming – if “X” happens, then do “Y.”

“Think of it as a series of logical questions and answers, if you will,” says Witte. “You can have something turn on or off based on one data point, or several different criteria if you want. With these Holley EFI systems, you can program as few as one input or as many as five inputs to trigger an output.”

At this point, you might be saying to yourself, ‘That all sounds neat, but what would I use this for in my vehicle?’ Fear not, we’re going to look at some of the most common uses for inputs and outputs as well as a few particularly creative ways of leveraging this capability. Witte will also give us the lowdown on configuration, and what you can do if you start to run out of available IO connections with your system.

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Getting Automated

“An input’s primary role is to monitor, it doesn’t do the controlling,” Witte says. “For the user, the data points you’re probably going to be interested in here are things like pressure, temperature, shock travel – stuff like that.” Accelerometers are also another common use for inputs.

“These provide a G-force reading,” he tells us. “That can be really useful in a variety of different racing disciplines, but in drag racing, you live or die by it. If you increase the acceleration reading, the car is going to be quicker. And that comes in handy when you’re dialing in a setup or a tune. The data can tell you whether or not the changes that you’re making are moving things in the right direction.”

With Holley systems, you’ll find a few different types of inputs like thermistor, 0-5 volt, and 0-20 volt, along with frequency or digital speed inputs, the latter of which are designed for digital voltage input from a speed/rotation measuring device like a Hall Effect sensor. Things are a little simpler on the other side of the coin, though – an output either controls the ground or sends power. Outputs can be used for more simple automation like shift lights or to automatically turn on a secondary fuel pump under certain conditions, and they can also be utilized to create more elaborate processes.

“So here you can either see on/off, or you can pulse it with pulse width modulation, or PWM, which means the output is being turned on and off at programmable rate,” Witte says. “You’d typically use pulse width modulation for something like nitrous solenoids, which are sometimes pulsed.”

PWM can also come in handy for things like variable speed cooling fans, as custom programming can allow you to define a specific rate of speed that the fan will operate at for different engine temperatures, or under certain conditions, like when an air conditioning system is also running. “And you can custom program that in our software to do whatever curve you want, and add criteria like engine speed, coolant temperature, vehicle speed, and so on.” Combining the functionality of inputs and outputs allows you to create more advanced procedural programming that can keep various systems in check and tangibly improve performance in turn.

“You can get into things like wheelie control,” he says. “Using laser height sensors, and combined with the shock travel data, you can use the advanced tables to pull timing, boost, or something else to regulate power output to keep the front end of the car down as needed.”

Once you get the hang of things, the rabbit holes you can go down with creative IO usage are vast.

“I’ve done things like passed an input to PWM output, and then used that PWM output to replace a different input,” he says. “That was done to basically ‘hijack’ a signal so we could make it do something different. A while back, a drag racer that we were working with asked if we could use a G-meter as part of their traction control strategy instead of a driveshaft curve. In his application, he knew that if the G reading went above a certain level early in the run, the tires were going to spin. Driveshaft speed doesn’t give you the same information – it’s more variable. This approach made more sense for what he was trying to do, which was pull timing if the G reading exceeded a certain threshold. Clean passes were more important than trying set a world record for him. So, in order to make that work, we came up with a method that allowed us to use the G-meter input and setup an output based on it. And because of how our software is designed, I could choose that output for the traction control.”

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Sorting It Out

Although the specifics of setting up inputs and outputs are largely dependent on the sensors involved and what you want to do with them, you’ll be running some wires from the ECU to those sensors in order to establish that connection regardless. Witte notes that using appropriate wire gauges and verifying that your connections are secure will help minimize the potential for headaches later on. But ultimately there’s very little guesswork involved, even for those who may be new to the wiring game. “In our software, we have what’s called a pin map, which tells you what each terminal on the ECU connector is configured to do. We’ll tell you that pin number 15 can accept 5-volt and thermistors, for instance. When you set up your input, you’ll see a label on there that says what that input requires. You can drag it onto a given pin if it’s compatible, but the software will stop you if it isn’t.”

Once everything’s hooked up, you can configure the new connection using the IO Individual Configuration File, or ICF, in the software.

“This is a section of the tune that’s dedicated to user-configurable inputs and outputs,” he explains. “You go into that section of your tune, you name this new input and tell it what type it is terms of 5-volt, thermistor, a frequency, etc. Then you hit the configure button and tell it what to do. Is it a temperature sensor, or a pressure sensor? If it’s pressure, are we measuring PSI or kPa? The drop-downs in the software make the process pretty self-explanatory, you just have to make sure that you’re giving it accurate information.”

The process for output connections follows a similar pattern, as the pin map feature illustrates in no uncertain terms where different types of components should be connected. But there is one important difference. “Because you’re often controlling something with a lot of amperage on the output side, it’s very important to relay things that pull any type of significant current,” Witte says. “The ECU can deliver about 2 amps on an output, so anything more substantial than a small solenoid should be on a relay.”

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Going Above And Beyond

While it varies by vehicle and use-case, Witte offers a good rule of thumb for the minimum number of inputs and outputs that folks should keep an eye out for when shopping ECUs. “I think that six to eight inputs and outputs is really the minimum these days. When we came out with the HP ECU thirteen years ago or so, we did four and four. At that time, that was a pretty good amount. But with the growing popularity of more advanced engine features like VVT and variable runner timing in the years since, the situation has evolved because those features have eaten up some of those inputs. At this point I think you really should have four spare inputs to do what you want, and six is preferrable.” Witte also says that four outputs is a good baseline, while six will provide some additional flexibility.

Those who are running out of IO connections on their existing Holley EFI ECUs can bring more onboard with a CAN IO Expansion Module, which adds eight inputs and outputs to the system. “This plugs into the CAN bus, and there’s a good mix of input and outputs to ensure that you’ll be able to do what you want without having to really worry about how you’re plotting everything out.” This expansion module can also be utilized with a new EFI installation, of course, but Witte suggests running the numbers before going that route.

“The key here is to plan for the future,” he says. “Don’t buy the bare minimum ECU if you think that IO is going to be a significant part of your build. If you’re trying to choose between an HP and Dominator, and you’re planning on adding a bunch of sensors, I’d recommend just getting the Dominator. The difference in cost between that system and an HP with an expansion module isn’t as significant as you might think.”

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