A lot of times, even though we may not admit it when other people are watching, we read the instructions. They are great for taking something that you may not be an expert on, and telling you how to make that something work properly. Just because we can read instructions, doesn’t mean that we actually understand what is going on with the part we just installed.

To that end, Ben Strader of EFI University wants to make sure that everyone actually understands how and why things work, not just parroting what the instruction manual – or worse, the internet – says. To do that, he has released the first in a series of tech videos, bluntly titled “How Not To Be An Idiot.” In the first installment, he covers mechanical fuel pressure regulators.

“Most of the time when you ask someone what the job of a fuel pressure regulator in an EFI application is, the most common answer is that it’s there to make sure you have constant fuel pressure,” says Strader. “That’s actually wrong. It’s there to make sure you don’t have constant fuel pressure, because if you did, you’d never have the proper fueling for you engine.”

Now before you walk away mid-article, thinking we’re all idiots, Strader starts off by explaining the job of the regulator in a high-level view. “The regulator is fed by the fuel pump, and then the regulator allows fuel to bypass and return to the fuel tank in order to maintain a specific pressure through the system, to the fuel injectors,” he says. The key phrase to note there is “a specific pressure,” which is distinctly different from “a constant pressure.”

Strader points out, that the key to the regulator’s job is the port which is connected to your intake manifold. That port mechanically references the intake manifold’s actual pressure – be it boost of vacuum. “Imagine what would happen If you had 50 psi of boost coming into the cylinder with 50 psi of fuel pressure. No fuel would come out when you open the injector,” Strader explains.

“By connecting the regulator to the intake manifold, when boost goes up, fuel pressure goes up, and we get the same pressure differential across the injectors as they fire. The same thing is true under vacuum. If the manifold is under vacuum, I now have a vacuum cleaner actively sucking fuel out of the injector, and they will be flowing more than they would at strictly atmospheric pressure. That’s not good either.”

So now, you are probably starting to understand the difference between a specific fuel pressure and a constant fuel pressure. “The reality is, as you drive your car, your manifold pressure is constantly changing,” Strader continues. “With that constantly changing, and the tip of the injector inside of the intake manifold, and subject to that fluctuating pressure, the last thing I would want is the injector receiving a constant pressure on the supply side. If that was the case, the injectors would always be flowing something different when they were open.”

That vacuum port on your fuel pressure regulator is the key to the magic that happens inside of the machined-aluminum housing, which allows the proper amount of pressure to be supplied to the injectors, whether you are in vacuum or boost.