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The important thing often overlooked is that the nozzle outlet pressure of the injector MUST REMAIN CONSTANT against the forces of increasing/decreasing pressure in the manifold. Imagine a door you are trying to keep shut against the wind, but this door has no latch, so it’s up to you to keep enough pressure applied to keep it shut as the wind against it increases or decreases. In order to keep the door in the same position, you would have to apply equal force on your side against the wind speed (force) trying to open it.

Now, what if you wanted to allow some air through the door, but you never wanted it to rush in faster or slower. The principle would still be the same as above with one exception: you would have to apply slightly more opposing force to the door at all times to keep the rush of air constant coming in as the wind changed.

Now, an example of what goes on in your manifold: Let’s use an easy one. Suppose you have 10 psi boost inside your manifold. The injector (Ford) is rated at 39 psi operating pressure. What fuel pressure do you need to overcome the boost pressure in the manifold?

fuel rail pressure - manifold pressure = nozzle outlet pressure (or delta pressure).

In order to keep the delta (Ford’s rated operating pressure) constant (39 psi), the fuel pressure in the rail would HAVE TO BE 49 psi at 10 psi boost, because 49 - 10 = 39.

Now, a not so easy one. If the idle pressure was 30 to 32 psi (typical on most Ford systems), how could you get 39 psi at the nozzle outlet in the manifold? Since the manifold now has vacuum and not boost, how can you produce 39 psi at the nozzle with only 30 psi in the rail?

You can do it because remember the most important thing: it’s all about nozzle outlet pressure (delta). Because we read vacuum in inches, it’s confusing (who’s idea was that anyway?). Write this down: Every 1 psi of atmospheric pressure = 2.036 inches of mercury (Hg). So, if you are sitting there idling at 18 inches (Hg), how many psi would that be?

Inches of Hg / 2.036 = psi or 18 inches Hg / 2.036 = -8.84 psi (ahh, only this is NEGATIVE boost)

In order to keep the delta (Ford’s rated operating pressure) constant (39 psi), the fuel pressure in the rail would HAVE TO BE 30 psi at 18 inches vacuum, because 30.12 - -8.84 = 39

(Note, gauges vary a little - if you see ~15 to 16 inches at idle, then you should see ~ 31 to 32 psi in the rail).

Now you know what your fuel pressure should be at all times under all conditions: vacuum or boost! If you have 15 psi boost, then you better have 54 psi in the rail, right? What if the vacuum was 10 inches? Then you better have 34 psi in the rail (10 / 2.036 = -4.91, or 34.09 - -4.91 = 39).

Another way to look at it is vacuum or boost + 39 = desired fuel rail pressure (-4.91 psi + 39 psi = 34.09 psi).

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