ghoonk

Sorry for the cross-post:

Okay guys, need some advice regarding risking rate fuel pressure regulators, e.g. HKS fuel pressure regulator, SARD fuel pressure regulator.

When installing an adjustable fuel pressure regulator (FPR), many people make the mistake of setting the fuel pressure with the vacuum line on the FPR still connected. From what I now understand, the correct way is to set it at a fuel pressure high enough to get the engine running, then disconnect the vacuum hose on the FPR, THEN set the fuel pressure to the 'correct'/desired level before reconnecting the vacuum hose. This will typically cause the fuel pressure to then drop.

For the purpose of this question, let's say we set the fuel pressure to 2.8 bar (40.6 psi)with the vacuum line disconnected.

Now, according to a number of online references from FPR manufacturers, this means that the fuel pressure at WOT is 2.8 bar, and the idling fuel pressure should be lower. In many cases, this would be irrelevant since we should only be concerned with fuel pressure under throttle.

Now, with the understanding that

a. fuel pressure should be set in accordance with the target peak boost level (i.e. to provide the right level of fuel pressure at max boost), and

b. this seems to be consistent with the description of the purpose of the differential pressure warning lamp in the Defi Link Control Unit II,

what is the 'correct' fuel pressure to set (with the vacuum hose off) if I am running 1.5/1.6 (21.7-23.2 psi) bars of peak boost, assuming that I am using stock fuel injectors and a Walbro fuel pump?

dubbleugly01

iTrader: (24)

anywhere around 40-45#'s will be fine. It's hard to tell if you understand that the purpose of the vacuum line connected to the fuel pressure regulator will "correct" the fuel pressure to always be your set pressure (say 42#'s) above the manifold pressure. For instance, if the intake manifold is under 7#'s of vacuum, your fuel pressure will be 42-7= 35#'s. If you're at 20#'s of boost, your fuel pressure will be 42+20= 62#'s. Taking it to an extreme... if you tried to run 42#'s of boost without the vacuum line connected to the regulator, your injectors wouldn't spray! But with the vacuum line connected, your fuel pressure raises to 42+42=84#'s so that they still work consistantly.

The more fuel pressure you run, the more you increase your injectors capacity, but decrease your fuel pump capacity. The less pressure you run, you cut your injector capacity but increase your fuel pump capacity.

Hope this helps.

[EVO]Cookie

iTrader: (3)

According to the Mitchell service manual idle pressure is around 33psi. For stock injectors. There are a lot of different adjustable FPR the EVO uses a rising rate 1:1 The pressure is only limited to the fuel systems flow power (pumps,lines,fuel rails ect..) A 95psi injector has higher demands on the fuel system vs a 45psi injector. Non rising rate FPR's need higher fuel pressure over all and they can be hard to tune for idle because of the flat line pressure. Trying to tune the injector latency's for idle is trying to say the least. Rising rate are easy to tune set the base pressure and go.



Rising rate FPR will rase the pressure as the demands increase.


This depends on the FPR type rising rate or non rising rate.

If you have a rising rate FPR with stock injectors you need stock fuel pressure. If you raise the pressure to high it will start idling rough. You can see this for yourself if you adjust the pressure while the car is running to much or to little and it will stall. I wouldn't run more than 43psi at idle with that setup and more like 35 to 38psi.

PVD04

iTrader: (23)

1. Pull the vacuum line
2. Set pressure to 43.5 psi
3. Reattach vacuum line
4. Don't touch it any more and begin tuning.

ghoonk

Thanks for the replies and advice, everyone. This has really got me thinking some.

@PVD04 and [Evo]Cookie, why 43.5 psi?

According to a number of references, increasing fuel pressure increases the amount of fuel that the injectors can fire. There are two lines of rationalization that follow:

1. assuming that the capacity of the fuel rail to be a constant, the amount of fuel that the injectors can fire does not change.

2. however, by increasing the number of times that the injectors can fire in a second (is this injector pulse width? I'm not a tuner, but I'm willing to listen and learn), and having fuel pressure and fuel rail capacity held constant, it would also imply that I can fire a greater volume of fuel since fuel is being constantly fed at a high pressure, i.e. when the injectors fire and drains the fuel rail of fuel, the FPR set at a fairly high pressure would continue to force fuel into the rail, preventing a drop in fuel pressure, hence ensuring that the fuel rail is always at max capacity and pressure.

cpoevo

iTrader: (1)

this is from the Mitsu factory Manual:

9. Disconnect the vacuum hose from the fuel pressure
regulator and measure fuel pressure with the hose end
closed with your finger.
Standard value: 289 − 309 kPa (42 − 45 psi) at curb idle


here is the link to AMS website for installing and setting their FPR kit:
http://www.amsperformance.com/instru...structions.pdf

[EVO]Cookie

iTrader: (3)

This can be very hard to explain at times. Lets start with the base pressure of 43 with rising rate FPR. Idling it's at 43psi when you go WOT depending on your fuel systems power it could raise to 60psi+ hence, rising rate.

Now physics of fluid and viscosity and flow. Are you sure you want to get into this you could fill page after page on this it would be easier if you just Googled this on your own to get a better understanding of it.

[EVO]Cookie

iTrader: (3)

I can give you a calculator for IPW injector duty cycles also some good general tuning info there to.
http://www.stealth316.com/2-calc-idc.htm

ghoonk

Found this on a local Evo forum:

Modern injectors are rated at X flow rate @ 3 bar fuel pressure. Typical values are in cc/min or lb/hr.

Fuel pressure should rise in accordance with boost pressure. This is done using a fuel pressure regulator which taps the vacuum source from the intake plenum, and ensures that adequate fueling is being supplied.

So if your car idles at 0.5 bar vacuum, fuel pressure should be 2.5 bar. If the engine is operating at efficiency levels at atmospheric pressure i.e. 0 bar boost, then fuel pressure should be 3 bar. Similarly, if you are making 1 bar of boost, then fuel pressure should be 4 bar, so on and so forth.

BTW, fuel FLOW RATE does not rise linearly with increased fuel pressure. It is in fact the square root of the new fuel pressure divided by the old fuel pressure, multiplied by the old flow rate.

[Evo]Cookie and cpoevo, thanks for the advice.

@ cpoevo -- how does closing the vacuum hose end of the FPR affect readings compared to if I had just left it open?

cpoevo

iTrader: (1)

When you pull the hose off the vacuum fitting on the FPR you will create a vacuum leak on the engine if it is not covered and the car will idle rough. Just pull the line off and put you finger on it while you adjust the pressure.

ghoonk

Understood, thanks!

viktor4g37

Greetings! Prompt please what correct pressure to adjust on FPR Aeromotive?
2 boost modes 0.8 and 1.2, ingectors 510, AEM 320 pump, Evo 8 ECU.
After I covered my native FPR, I found a similar one from some small car. The car drove normally, but the consumption is bigger then was before . After installing the RDT Aeromotive set to 2.7 bar or 40psi, as a result, the car at the bottom began to blunt, but after 4000 rpm seemed to go better, and somewhere in the 5th gear after a speed of 180 began to flash detonation.

RS200

iTrader: (1)

Your setup is quite unusual. With boost pressure that low, you do not have the original wastegate (or perhaps you've changed the whole turbo). Is it a 4G63?

The ECU's maps have to be tuned for the fuel pressure that the system operates at, since the fuel injector flow is directly proportional to the pressure of the fuel that they are flowing. If you set the base pressure lower than what the ECU was tuned for, it's going to be extremely lean. In your case, it sounds like it was running rich. I suggest you restart at 3-bar base pressure, make sure that the fuel pressure rises at 1:1 ratio with intake boost pressure, and retune your car. You can do serious damage otherwise.

viktor4g37

thanks for the clarification