shiftnmadkwik

several questions, first of all what is the stock fuel pressure on an s4?
i searched but could not find a convinving answer

also does anyone here use nitrous on a rising rate regulator?

or is it better off just having a self contained fuel delivery system for the nitrous when a rrfpr is in use? perhaps a 2 gallon fuel cell with its own low pressure pump and then i could run 100 octane in just that tank .

i would imagine sizing the fuel jets to match the nitrous at static fuel pressure would be the way to go, but as pressure increases will there be way too much fuel? or would the margin of safety produced by this rich condition be worth it.

i just dont want the system to totally bog once the fuel pressure starts going up.

any thoughts?

shiftnmadkwik

no ideas?

scathcart

39-42 psi base.

What good would it do you? With an N/A, you have no boost to affect the "rising rate", so the RRFPR would function just like a regular FPR, aftermarket or stock.

Again, why use a RRFPR? Even with a turbo, why?
Why use a low-pressure pump for feeding the fuel for your nitrous system? Aiming for poor fuel atomization through your fogger?

Not worth it.
Again, why are you going to be using a rising rate FPR? Your sig says you have an N/A, so using a RRFPR makes no sense.

How about you explain what you're trying to do, and then we can go from there. There are ways that you can get around a RRFPR with wet nitrous, but I'm not going to get into it without at least hearing why you are so set on using a RRFPR.

shiftnmadkwik

sorry for being vague,

the car is getting a supercharger, with a standard holley flange.
to compensate for boost in the mean time, it is going to have a boost referenced rrfpr. until i decide on standalone or not

since the blower has a standard holley flange, i am going to sandwich a plate system between the blower and the uim.

most plate systems are designed for low pressure fuel systems, and therefore a seperate pump would work fine, if that is what is needed.


my question remains that if the base fuel pressure is 39-42 and at max boost 7-8 psi for now, if it is a linear rate rrfpr the ending fuel pressure will be in the neighborhood of 50psi, or 25% more fuel?

if my fuel jet calculation is made upon the ~50 psi number, will it run way too lean off of boost? or if i calculate it at 40 psi, will it bog once the pressure starts rising
maybe a number somewhere in the middle would be more appropriate. or perhaps the real answer is hooking up a wideband and watching the numbers,

next time i will be more specific

scathcart

Makes sense now. FMU's are pretty much solely used now for supercharging. Are you planning on using a carb or retaining fuel injection? If you're retaining fuel injection, a S-AFC is dirt cheap at $75 or so used and would offer much better performance without the hassles of an RRFPR.

You can run your high pressure fuel into a plate simply by using the appropriate jet.

Now, with a standard fuel pressure regulator, under boost, you have to keep something in mind... the pressurized manifold will attempt to be pushing the fuel back into the fuel rail. This is why fuel pressure regulators increase their fuel pressure with regards to boost and vacuum: to keep a the fuel pressure differential between the rail and the manifolds a constant. Without a increase in boost, at 42 psi of boost, no fuel would be sprayed in.
Since the boost pushes the fuel injector fuel back, it also pushes the nitrous system fuelling back into the system as long as the nitrous system is mounted AFTER the compressor.

SO.... you can tune your nitrous system at 40 psi fuel pressure with zero boost which will have the same jet tuning as 50 psi fuel pressure with 10 psi of boost.

What I normally do when I am tuning these nitrous systems is: Get my baseline fuel pressure via a fuel pressure gauge. Tune the car for performance with the nitrous system off, aiming for my target AFR's whether the car is N/A, Turbo'd, or S/C. Then I calculate my fuel jet size at my baseline fuel pressure, and install the jet, and make some WOT runs with the nitrous on to establish my new WOT w/nitrous AFR's. I then pull out fuel jet sizing to get as close without going over to my target AFR's from before; on an N/A, I make the car run richer under nitrous injection, and on a boosted car, I will lean out the car since nitrous injection is a detonation suppressant.

shiftnmadkwik

gotcha, as long as i use a rrfpr that is referenced 1:1, which im not sure is the case. i will have to determine what ratio rrfpr is being used.

the problem is that the nitrous will be pre compressor, where the injectors are post compressor. therefore pressure at the fuel solenoid will be rising
so the question remains will it run pig rich as that rate goes up.

thanks for your suggestions.
"on a boosted car, I will lean out the car since nitrous injection is a detonation suppressant"
didnt even come to my mind but its a good point

scathcart

A fuel pressure regulator with a 1:1 pressure increase when referencing boost is not a rising rate fuel pressure regulator; its just a fuel pressure regulator. ALL fuel pressure regulators increase fuel pressure at a 1:1 ratio, even the stock N/A fpr. A rising rate FPR is simply one where the rate at which the pressure increase when referencing boost can be increased past 1:1... say increasing the rail pressure by 2 psi for every single psi of boost.

Running a standard FPR and tuning with a injector controller will be your best bet. The problem with RRFPR's is that they are not able to tune for different operating conditions or rpm loads... so if you tune your RRFPR to stay rich enough to be safe at all rpms, you will be running power-robbingly rich under other rpms.

The cost of an aftermarket RRFPR is typically equal to that of a used S-AFC, so there really is no point to using such a crude form of tuning as the RRFPR.

Also, don't mount the nitrous pre-compressor. Its just as easy to mount it after, its safer, and it takes out the worry of tuning with respect to a rising fuel pressure.