FRP Fuel Pressure Regulator vacuum line. why?
 

I have searched and i know that the line on top of my SARD FPR plugs into the manifold. I dont understand why. I love understanding the science behind it all. I searched and everyone acknowledges the need for it, but i dont know fuel systems well enough to understand the manifold pressure line.

you set your static pressure (40psi) with the car off.
once you start the car, the vaccum will lower your fuel pressure a bit.
once you hit boost, the regulator will raise the fuel pressure at a 1:1 ratio.

ex. 40 psi static pressure, at 15 psi boost, your fuel pressure will be 40+15=55psi.

nahmean?

thanks! I was plugging it in, and i had no understanding why.

yes, it adjusts fuel pressure based on manifold pressure. Reasoning is you always want your active injector pressure to be the same. Under boost, the fuel pressure has to overcome the manifold pressure to get the same "delta" nozzle pressure. Same goes under vacuum, it needs less pressure.

That line is VERY important. Make sure your using quality hose and zipties. If it comes off, your fuel pressure will drop and cause a lean condition.

thewird

https://www.rx7club.com/attachment.p...1&d=1263876457

I have collected a large PDF library of technical documentation from various OEM's which explains the "science" of fuel injection and engine control. PM me if you are interested, I can email you some stuff.

Bringing this one back. Does anyone know if the 1:1 ratio applies to vacuum state? I have my base pressure set at 44psi. At idle, my vacuum is about 11" but I'm only showing a decrease of psi to 41. I thought it should be much lower than that? I'm running an aeromotive FPR.

Yep, vacuum acts on the diaphragm in the same way boost does. But, remember that roughly 2" vacuum = 1 psi of pressure (or in this case, negative pressure)

If your FPR isn't dropping lower, it could be that the line to it isn't big enough, the gauge isn't fully accurate, or it's getting a weaker vac. signal.

Got SUPER lucky yesterday as this line popped off without me realizing it and was boosting a bit.. YIKES!

Also need to burp your liquid filled gauges as it can affect the reading.

Well the gauge I have is a Nordskog digital gauge so I can moniter the pressures in my car. The nipple that came with the FPR is the same size as all the other nipples. This is the regulator I have.

http://www.summitracing.com/parts/AEI-13109/

:dunno:

have you noticed a change in idle AFR at idle? and is your vacuum stable at 11" or does it fluctuate a bit?

I've had lots of fluctuations recently with my AFR with my engine on the test stand. I think it's all voltage related as when the alternator start charging, the engine doesn't run that well at idle and shakes a lot. When the engine is warm, and I do fresh start (no throttle movement), the engine idles perfect (no shaking) until I rev it a bit to turn the alternator on. Then it starts shaking when it gets back down to idle. It's a strange issue I've been dealing with for a while. I think it's the low amperage of the alternator that's giving me those problems (especially when I turn the fans on). I'm currently running a 80 AMP 626 alternator till my new Fd alternator shows up. The vacuum is stable but the pressure doesn't seem to really lower more than 41 psi. I've tried other nipples on my TB. I'll see what happens since I''m currently putting the engine back in the car so I can do a base tune. On decel my engine pulls about 22" as I have my old boost/vacuum gauge hooked up. I keep thinking there may be a problem with my FPR. I'm gonna borrow a friends vacuum pump and hook directly to the nipple to see how low I can make the pressure go.