A4 TSCHUSS

Hey guys, I don't even own a Mustang, but there has been some heated debates going on and I am trying to find answers. I own a turbo charged Audi A4 and we have a "returnless" fuel system where there is no fuel return coming from the fuel rail and no regulator to increase fuel pressure (it runs static at one pressure).

There is an Audi tuner wanting to compare the Cobra (supercharged) fuel system saying they have the same deal (only a fuel feed to the rail and a constant fuel pressure).

What I want to know is what is the fuel pressure that the supercharged Cobra runs at?

And next do you guys need to increase your fuel pressure when you put on larger blowers when you are making the big power (600-800whp)?

The way this tuner was talking is that the big hp Cobra's are running on stock fuel systems (meaning stock fuel pressure, of course with bigger injectors and better fuel pumps). So do you guys need to increase your fuel pressure when making the big power (when running high boost) or not?

Thanks in advance for any help you can give me.

Eagle2000GT

We don't have a fuel pressure regulator. We have a fuel pressure sensor. And while it appears that we have a constant fuel pressure that is hardly the truth.

On our GTs (I assume Cobra is the same since we can run the Cobra fuel system), the computer controls the fuel pressure to keep a constant "adjusted" 40-43 psi. This is not the "raw" fuel pressure that you would see on an aftermarket gage. The computer calculates the adjusted fuel pressure by factoring in vacuum/boost and air tempature. A rough conversion from "adjusted" to "raw" is to subtract vacuum divided by 2 from 40 psi or add boost to 40 psi. For example. at idle the vacuum is approximately 18 inches. Subtract 9 from 40 and the aftermarket gage should read about 31 psi "raw" fuel pressure (mine actually reads 30). With 8 lbs of boost the aftermarket gage should read 48 psi (mine does). The only time the computer and the aftermarket gage will agree is at zero vacuum/boost. You can test this by turning the ignition key on but not starting the car. After a brief surge to 43 my aftermarket gage reads 40. Same as the computer.

So while the computer reports a constant fuel pressure (this is what you see when you data log) the real fuel pressure varies considerably. My aftermarket fuel gage swings wildly up and down when I go WOT (boost) and then back off (vacuum).

The Mustang's computer also uses air inlet tempature as an approximation for fuel tempature. It will raise the "raw" fuel pressure as air tempature increases to prevent vapor locking in the fuel rails. Where normally the "raw" fuel pressure is 30 psi at idle, on a hot day in heavy traffic I've seen imy aftermarket gage as high as 50-53 psi. When I had it checked the computer reported 43 psi. When the computer is making a tempature adjustment like this the vacuum/boost adjustment is overruled. The aftermarket gage will show a steady 50 psi even if I go WOT and back off.

When installing a blower we don't increase fuel pressure but we have to increase fuel flow. When I had the blower installed I had to have the fuel injectors changed from 19 to 42 psi and the fuel pump upgraded to a 255 lph pump. Going to higher level of boost usually requires a dual pump system. (The '03-04 Cobra already has this.)

cliffyk

The spec for the absolute pressure across the injectors is 39 psia, same as the GTs.

As explained quite well above, the psig (gauge) pressure in the fuel rail varies depending on the intake manifold vacuum/pressure, so as to mainatain a constant absolute pressure between the rail and the intake--I.e. "across" the injector.