I'm really bored at work, and TTAngle was asking me about fuel pressure regulators the other day.

Fuel pressure is simply the fuel pump keeping pressure in the system. This is controled by the fuel pressure regulator located on the passenger side of the rear fuel rail.

Now, the way it works is, there is a spring and plate inside of it which blocks the flow to the return line. This is what gives you pressure in the fuel rails because the fuel pump is "fighting" the restriction which creates the pressure. The fuel pumps only task is to keep the system pressurized. That is it!

If you were to take a fuel pressure gauge and hook it up to your completely stock car, you should see around 36 + or - 2 psi if your car is a turbo, and about 41 + or - 2psi if your car is a nonturbo. Now, this is with the vaccum line connected to it and the motor running (this all depends on how much vaccume your motor draws and your elevation).

Vaccum from the motor is what pulls open the spring in the FPR (Fuel pressure regulator) and lowers the fuel pressure. The spring inside the stock turbo FPR is rated at 42.7 to 45.6 psi meaning if you disconnect the vacume line, you get 42-45 psi of fuel pressure.

On nonturbo cars, when you're accelerating, you have less vaccume (because the throttle plate is wide open, meaning there is no air restriction, and you get less vaccume) and this is how the fuel pressure is raised.

On turbo cars, it's a little different. Boost pressure pushes down on the FPR creating even more of a restriction in the fuel system which gives even more fuel pressure. Quite simply, the fuel pressure raises 1 psi for every 1 psi of boost. Meaning, a stock 2G running 12psi of boost, the fuel pressure will be around 54-57psi of fuel pressure.

Now, on turbo cars, our stock injectors are rated at 360cc/min @ 43psi All that means is, when there is 43psi of fuel pressure, the fuel injector will spray 360cc/min of fuel when it is open.

IF you raise the fuel pressure, it will force even more fuel out of the injector when it's open. See where I'm going with this? If you are happy with your stock turbos and are maxing the stock injectors at 14psi, you can simply try raising the fuel pressure. This will also lower your injector duty cycles which prolongs the life of the injector. You MUST also replace the fuel pump as the stock pump won't be able to keep the system pressurized. You may even be able to get away without using an a/f controler.

Let's do some math. I said earlier that our stock injectors are rated at 360cc/min at 43psi. Injector flow rate is proportional to the square root of the new pressure across the injector divided the injector's rated pressure. In this case: SQRT(53/43) = 1.11

Now, we take this number and multiply it by 360 = 399.67. Let's just simplify this by saying the stock injectors are now flowing almost 400cc/min when you raise the fuel pressure 10psi.

Be careful with this! IF you raised the base fuel pressure 10psi (to keep things simple) meaning you now see 64-67psi of fuel at 12psi of boost instead of 54-57. I don't know if I trust the stock rubber lines and 10 year old seals to hold this kind of pressure for too long.

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06 Blue EVO 9

Now, on my car, I'm running 550cc/min injectors, and I saw above 100% duty cycles around 19psi. On pump gas, I don't get knock at 19psi so if I had a larger injector, I could run more boost.

An adjustable fuel pressure regulator gives you full control over what you want your base fuel pressure to be. The stock ones are not adjustable. I was in a hurry to get to the track and didn't have time/$$ to buy larger injectors or a AFPR, so what I did was swap the stock NA FPR out of my SL into my TT. The stock NA FPR is 5psi higher than the stock TT one meaning I got 5 more psi of fuel pressure being forced thru the injector.

Why does this work? The only difference between the 2 regulators is the spring inside them. The NA one is a little heavier (higher psi). The effect is similar to heavier springs in the turbo wastegates to get more boost.

The results were I was able to run 20psi on race gas before the injectors were completely maxed. At 22psi, I ran out of fuel at the top of 3rd gear and the o2s went lean enough to cause knock. I do not have a fuel pressure gauge so I have no idea what my exact FPR was, but if I knew, I could calculate the rating of the injectors at that psi. If I had to guess, I was probably seeing 63psi of fuel pressure. Luckily, nothing blew.

Now, to confuse you guys even more, the pressure across the fuel injectors is always the same.

Example:

43psi in the rail at zero psi in the plenum is 43psi across the injectors.

33psi in the rail at -10mm/hg in the plenum is 43psi across the injectors.

53psi in the rail at 10psi in the plenum is 43psi across the injectors.

When under vacuum, the engine is effectively "sucking" on the injector, so a lower rail pressure results in the same total flow. When under boost, the plenum is "blowing on" the injectors, which would reduce fuel flow if you didn't correspondingly increase fuel rail pressure to keep the effective pressure the same.

The injectors are effectively "unaware" that pressure has been increased on either side or decreased on either side, as long as the increase or decrease is of the same amount.

If the FPR has troubles keeping up the 1psi for every 1psi of boost (because of a restriction in the return oraface for example), these numbers are scewed and your fuel pressure will be all over the place.