Hey everybody, like the title says. I removed my rat's nest, and was wondering where is a good place to get vacuum for the FPR?

 

I'm not sure if it's the same on an NA, but on a TII we use a nipple right where the subzero used to be, under the UIM.

 

Can anybody confirm or deny this is a logical location for an S4 N/A?

 

What is the FPR, and what kind of vacuum signal does it need?

 

FPR= Fuel Pressure Regulator

 

What kind of vacuum signal does it want? Manifold, constant, or something otherwise modified?

 

Manifold, I would imagine.

 

It would certainly make sense since fuel regulator would probably want a vacuum signal in order to regulate fuel delivery, a metric based on fuel need, which is in turn dictated by throttle position (and RPM). Sounds like any vacuum port coming from the manifold engine-side of the throttle plate itself would be a viable candidate.

 

I have a few spots on the intake manifold I can use then.

 

Anything between the throttle and the keg would have the same vacuum signal.

 

This is the location I chose, on the LIM passenger side:

 

It's not quite that complicated.
Fuel pressure determines injector spray pattern. When the intake manifold is under vacuum due to closed, or partially closed, throttle plates, the injectors are also under vacuum, and when the injector opens, the vacuum attempts to suck the fuel into the engine.
Similarly, under boost, the manifold pressure attempts to push the fuel back into the rail.
So, to keep the pressure differential roughly a constant ~40-45 psi, the fuel pressure is referenced to the intake pressure, dropping the rail pressure approximately 1 psi for every 2 inches of vacuum, and increasing rail pressure at 1:1 for every psi of boost.

 

Good to know the specific whys and hows. Your application specific explanation of what's going on sounds a bit more complicated than I described (I certainly didn't allow for any consideration for forced induction, spray pattern, etc.). Some day I'll figure out all this newfangled electrickery...

Regardless, the end result is the same. Anything between the throttle plate and the intake port will get the job done. It's all the same signal; whatever is most convenient.