What is the purpose of varying the pressure based on vacuum? Wouldn't tuning VE make more sense if you had constant fuel pressure? Maybe I'm just totally missing something, but it seems to me like the vacuum modulated fuel pressure regulator, and the MAP/RPM VE tables are trying to accomplish almost the same thing. Thanks, and please do let me know if I'm way off base here.

Thanks!
Scott

 

On a port injected engine a vacuum referenced fuel pressure regulator maintains the same differential pressure across the injector.

IOW, the fuel pressure between the input of the injector and the discharge nozzle of the injector is held constant. The lower the manifold pressure the lower the fuel pressure is.

This keeps the fuel flow throguh the injector constant even as the manifold pressure changes.

RBob.

 

Ha, I have wondered that myself, but that makes complete sense. Thanks for the enlightenment!

 

At WOT when the vacuum is essentially zero, there's atmospheric pressure in the intake. If the fuel pressure is say 60 psi above atmospheric, the pressure difference between the fuel and the intake is 60psi. At cruise there might be 20 inches of vacuum in the intake, which is about -10 psi. The pressure difference between the fuel and the intake is now about 70 psi, unless the fuel pressure is reduced.

 

Ahh, makes total sense now... Thanks for the input!

 

btw, this is also why TBI and carbed cars dont NEED vacuum modulated regulators.

they're above the throttle blades, so the pressure is not effected by vacuum. so theres no need to compensate.


athough some TBI guys are running them, but for diffrent reasons..

 

Right, in a carbed car the fuel in the bowl isn't pressurized at all. It's the difference between the atmospheric pressure on the fuel in the bowl and the reduced pressure in the venturi that moves the fuel.

 

It still doesn't make it easier to tune part throttle when you have the fuel pressure dropping and increasing due to vaccum fluctuations.

If the fuel pressure was constant at all times with-out any negative effects, it would make VE and other tables easier to tune, IMO.

 

ahh.

but it IS constant.


constant to the relative pressure of the intake manifold.
this means that a 30ms pulse will spray the same amount of fuel at any MAP point.

if you left it constant to the relative pressure of the atmosphere, at WOT it would be spraying less then at idle.
so your idle pulsewidths would be absolutly tiny.. too small.. and your injectors would ahve to be larger to spray the correct amount of fuel at WOT.

 

Its to make the control output of the injector "Linear" so that increase of the pulse width by a certain amount will result in a proportional amount increase in fuel delivery.
Without a straight line output of fuel Vs pulse width, the control would be much more difficult to get the desired amount.
Trying to tune a non linear device curve is a real PITA.
It can be done via lookup tables, and specific calibration points, etc but if conditions change the lookup is shifted and the accurracy is lost.
Not to be confused with repeatability, devices can be non-linear yet have excellent repeatability.
Jp

 

I keep forgetting this myself! I need to drill it into my brain!

So, anytime the vacuum fluctuates, so does the fuel pressure, but at any point, the pressure differential IS THE SAME. Really, when the fuel pressure rises due to low vacuum, it really isn't increasing, even though it looks like it to a gauge.

Many people running Turbo's and SC's tend to forget this, and it is much more important then!

 

Makes perfect sense now!