Why do we need a fuel pressure regulator for boosted cars? The marren for example increases pressure for every PSI of manifold pressure it detects from the intake via vacuum tube. So you get a low to high pressure for low to high boost.

Why don't you just run your full fuel pressure all the time? Why do you need the FPR to regulate fuel pressure via boost pressure? I mean when your running full boost you obviously want to have the injectors spit out their designed amount of fuel with high fuel pressure, but why can't you have high fuel pressure on low rpms or no boost?

thx


Questions up the *** lately, but im just tryin to learn

 

Optimum fuel atomization occurs around 35-45 psi. By upping the fuel pressure with respect to boost, we maintain this pressure differential at the fuel pressure nozzle.
IE: 40 psi of base fuel rail pressure. At 10 psi of boost, the intake manifolds have 10 psi trying to push everything out, including fuel back into the rails when the injectors open. You will have 50 psi of fuel pressure. The difference between the two, at the nozzle head, is 40 psi, so atomization is still good.

When you go into vacuum, the FPR also drops fuel pressure, as the intake manifold vacuum will be drawing fuel in through the fuel injectors. So, for roughly every two inches of vacuum, the rail pressure drops, to keep the nozzle pressure roughly a constant.

 

So its a ratio then of fuel pressure to boost? thats what the FPR controlls, the ratio?

OK got that. I still don't understand why optimum fuel atomization is best at 35-45psi, like I mean I have no idea why fuel pressure helps with atomization of air.

 

Its not the atomization of air.....its the atomization of fuel. Fuel atomizes well with around 40psi of pressure. Like scathcart said, you have to increase the rail pressure when you increase the manifold pressure so you net a 40psi (or however much) difference.

Stephen

 

ok got it. thx guys.

 

It's easier to tune if you have a constant difference of pressure between the fuel rail and the manifold. The FPR (with a 1:1 manifold reference) provides this constant "effective" fuel pressure.

If you hold the injector open for 50ms at 0 psi of boost and 40 psi fuel rail pressure, it will flow the same amount of fuel as it does when you hold it open for 50ms at 15 psi boost pressure and 55 psi fuel rail pressure. In both cases the "effective" pressure is 40 psi [(40 - 0) = (55 - 15) = 40 psi].

Without this, you may have a hard time controlling the injector under vacuum, since the effective pressure would be so high (and since people already have trouble running rich at idle when running larger injectors). And it would also be hard to get enough fuel delivered under boost, since the effective pressure would be so low.

Now, this doesn't mean it isn't possible to run an FPR with no manifold reference, but things just tend to work out better if you do keep the "effective" pressure constant.

You always need some kind of fuel pressure regulator (for fuel injected cars) -- if that is your question, we can cover that, too. It seemed you were talking more about the manifold reference, though.

-Max

 

Together with the above , all fuel pumps are COOLED by the fuel they pump , at low load the fuel isnt being used by the motor so it will be returned to the tank via the reg. and return line .If you eliminate the return system (regulator and return line ) you will "dead head" the forward flow, the pressure will rise but the flow will fall , resulting in an inadequate cooling flow through the fuel pump . Eventually the pump will over heat , cause the fuel to vapourize which will cause majour problems with the motor and the pump will itself be damaged. The clearances within these little pumps are VERY close , any overheating and/or low flow will cause premature wear and tear.
Systems designed without a regulator usually employ a variable speed control for the pump and /or an internal regulator that will open up and allow a MINIMUM flow through the pump at elevated pressures.