stock S5 TII FPR question
 

It's my understanding that the stock series 5 TII FPR:
-cannot be removed
-is rising rate (1 psi fuel pressure per 1 psi boost pressure, right?)

So, if I didn't want to deal with the rising rate portion, could I just cap off the FPR instead of running a vacuum line from it to the UIM? Would that keep it at a static 37psi fuel pressure (that is the base FP, right?)

Would this have any positive or negative side effects that you guys can think of? Would base pressure be insufficient for high power levels?

Thanks
Victor

Why would you want to take away the rising rate function? I am not positive if that is how it works but it makes sense to me.

Are you running like no boost?
You're going to run lean if it doesn't know what boost your engine it at.


-Ted

Injectors are controlled by the E6K. If the FPR keeps fuel at a steady 37psi or whatever, then it would function the same as an aftermarket FPR set to 37 psi, right? I don't care if it rises as I boost...

Am I making myself more clear? People tell me the rising rate just makes tuning more difficult. So I was thinking, Hey, let's just get rid of it.

There is one problem...

As boost rises, the pressure differential at the fuel injector goes down...unless it's compensated.  This is the reasoning behind the 1:1 rate for most boost-dependent FPR's.

At 15psi of boost, if your base fuel pressure is 35psi at 0 manifold and it doesn't change, the effective fuel pressure at the fuel injector tip is only 20psi!!!

Do you see why the system is set-up that way?


-Ted

That makes sense.

What do the guys with static adjustable FPRs do? Set it at ~60psi and let it be?

I just figured (and have been told by some others) that a RR FPR is basically just another complication.

What's really the purpose of an adjustable static FPR, then?

Not recommended on boosted engines.


It is, but it's not going to complicate fuel tuning that much.


NA engines?


-Ted

Re: stock S5 TII FPR question
 

Yes, the fuel pressure does increase linearly with manifold pressure, which is why the stock FPR is called a linear-rate FPR. It is not a rising-rate FPR. All stock FPR's, whether turbo or NA work this way. The pressure differential between the fuel pressure and manifold pressure is kept constant so that the injector flow rate stays constant. This means fuel pressure is lower than the static fuel pressure when the manifold pressure is below zero (vacuum). The only difference between turbos and NA's is that on NA's the fuel pressure stops at the static pressure when the manifold pressure reaches atmospheric (WOT) but on a turbo it keeps rising as manifold pressure goes beyond atmospheric and into boost.
That would be very foolish. If the fuel pressure stays at the static rate, the engine will run very rich at low load and idle because fuel pressure is much higher than normal and run very lean at high load because fuel pressure is much lower than normal. Engine failure is highly probable. Do not remove the FPR's vacuum line!

Alright, alright, I get the idea, guys :)

My logic was that you would tune for a static pressure, ie pull fuel at low load, add fuel at high load. Apparently I am misunderstanding something.

But whatever works, works.

Thanks, guys.

Injector duty cycle is already tiny at idle and large at high load. You would need to make low load duty cycle even smaller and high load duty cycle larger. Both will cause you problems.

Basically there is no point in doing this; it just makes no sense. Let the FPR do its job properly.

Good point.

Why, then, does everyone recommend aftermarket regulators for higher-power cars?

Larger capacity upgrade fuel pumps will "outflow" the stock FPR, and you will get significantly higher rail pressures.

You also have the option to adjust your base fuel pressure to whatever you want, and there is some gains to running higher rail pressures for better atomization and spray pattern.

You need to also keep the max fuel rail pressure down to about 70psi or so, as excessively high fuel rail pressures can cause headaches with fuel injectors not being able to open correctly.


-Ted

Just to expand a bit on what Ted said, you have to remember the FPR is positioned after the injectors, and it increases pressures by restricting the fuel flow through the return line. So they don't need to be upgraded because they don't let the engine to get enough fuel (like a pump), they need to be upgraded when they can't flow enough to keep the pressure of an upgraded pump down.