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Discussion Starter · #1 ·
Should I boost refrence the regulator or not? Im pretty sure I have more than enough injector to get the job done at base fuel pressure and still stay under 80% duty cycle so I dont see a need. Im new to EFI so Im really not sure if I should or not.
 

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Discussion Starter · #3 ·
OK and why is that? Not doubting your word just like to know reasoning behind it. My thought process is the computer would raise the duty cycle accordingly with the A/F ratio under boost.
 

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The injector flow rating is at a certain pressure drop across the injector. If you raise the pressure in the intake manifold without raising fuel pressure, you drop the pressure across the injector and make it harder for it to spray fuel, effectively lowering the injector flow rate.
 

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Well the short of it is you want high pressure when holding the injector open long so that the fuel sprays very atomized and finds its way into the chamber as even and fluid as it can. I know people who just run super high base pressure but that makes idling difficult. But yeah, you want to lower fuel pressure at low load or vacuum to make it easy to control the injectors - if you ran high base pressure your pulsewidth on the injector would be super duper short, which isn sometimes too short that the physical latency in the valve in the injector is incapable of moving that fast for such a short burst.

So you run low base pressure to idle, and you need high fuel requirement at high load, so raising the pressure with boost 1:1 means for every psi of boost your fuel pressure raises and the velocity of air past the injector is compensated with higher velocity fuel with better atomization into the mixture.

Also another thing to think about is let's say you have a 20 psi fixed pressure fuel system - what happens at 35 psi of manifold pressure? Your air pressure will be greater than your fuel pressure! It will make it less likely that the fuel will spray in the pattern/atomization/velocity that you intend!

And finally, if looking at a fuel map, your fuel values make more sense. If you run a low pressure fuel system your numbers at low load and high load will be VERY far apart. This can hurt your resolution when it comes to tuning. Having pressure increase will have your fuel pressure increase and allow for an intuitive fuel table.
 

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What all these guys above said X2.
Same reason you need more intake valve spring pressure when running in boost vs. NA. The boost pressure is trying to open that valve too. When you have pressure sitting on the outlet side of the injector it will not flow the same (pattern-wise) nor the same amount.
 

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The easiest way of looking at it IMO is like this:

Right now you're breathing in an atmosphere pressurized to 14.7 psi, right? That means that before you even turn your engine on your fuel rails have 14.7psi of fuel pressure and your engine has 14.7psi of air pressure. Now it's not hard in this situation to see that barely any fuel is going to flow because both sides are at the same pressure (before and after the injector).

When they say an injector is flowed at 43.5psi they are talking about the differential across the injector. Let's say you're running 30psig of boost and 30psig of fuel rail pressure. No fuel would flow because just like the example above there really is no pressure forcing fuel to go through the injector! If you're running 30psig of boost pressure you need 73.5psig to get the same flowrate that the injector was rated at @ 43.5psi.
 
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