Base FP on a 1g std is 37 psi with vacuum hose disconnected, obviously when you hook up the vacuum line up it will reduce your FP, the exact amount depends on how much vacuum your motor pulls. In a boosted car the FP raises 1 psi for every psi of boost, eg. if your running 20 psi you should see 57 psi under boost. As for the FPR it is dependent on what pump you run. Once the pump can flow a certain amount it will over run the return in the stock FPR causeing a rich condition at idle & part throttle conditions. It is common knowledge that with a 190 pump you can get away without an aftermarkt FPR (though some still recommend it) but when you step up to the 255 pump it is required. As far as I know you can't use a NT FPR as they aren't a rising rate, referenced to boost.

bootlegdsm said:

ok...in my neverending quest to "figure out" these cars, i find myself faced with yet another subject that leaves me asking wtf. ok, i have a 92 tsi, which i'm working on and in the process of studying my shop manual i've come across a piece of info i need some clearing up, if some of you old school dsm'ers can help me with. ok, the stock fuel pressure regulator on a turbo engine keeps the fuel pressure about 27 psi, now with the vacuum line unhooked, that number increases to 37. why is it, on the NT engines the fpr is about 37psi (vaccum unhooked 47-50)? this seems backwards to me....?

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The fuel pressure changes due to the force of vacuum or boost acting upon the diaphragm inside the FPR. With a base setting of 44psi (on a 2G) with the hose off. Put the hose on and the fuel pressure drops to 34-36psi. With the know scale of 2inHg=1psi, most cars operate at idle with 18-20inHg. With that in mind, 20/2=10psi pressure drop. Likewise, when under boost, the fuel pressure will rise in relationship to the boost pressure. The engine fuel demand is dictated by the amount of load the engine is under.

bootlegdsm said:

the next thing is, in updating the fuel delivery system with a larger volume fuel pump and larger injectors, do i need a higher pressure regulator? (or at what point do i need to change that?) now, at that point, could i just use a NT regulator? i may not be asking the right questions here, so if you can correct me in any point or answer why the psi's are what they are on the 2 models, please, by all means, i want to understand WHY this is, and IF i can use a NT fpr on a turbo model for the increased psi. thanks!

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IMO you need an adjustable FPR anytime you upgrade the fuel pump. Definitely others will disagree with this statement, but the problem is with the orifice of the OEM FPR. It's just too small to bypass much more fuel than stock conditions. Sure a lot of people "claim" that they don't have any issues running a 190 pump, however I'm just an anal SOB that believes that enough testing has NOT been done to confirm one way or the other. Just because something "seems" to work, doesn't mean it's correct.

What happens with a larger pump, the volume and flow increases, and the small orifice in the OEM FPR can't bypass the fuel at idle and cruise and causes the car to run rich. Under full boost conditions it's not as big of an issue because the injectors are open long enough to "bypass" the extra fuel.

I've not heard/read that a N/T FPR will work on the turbo models, if it does then all you would be doing is increasing the fuel pressure. That's not what you want to do if you're trying to do things correctly. Some cases may require a higher fuel pressure, but 99% of us don't need that. You want a FPR that is adjustable and will run base pressure and still be able to flow and bypass the added volume/flow of fuel that goes with an upgraded pump. It also needs to have a "boost reference" port that rises 1:1 as the boost pressure increases.