What's the point of varying fuel pressure?

[Scott-m]

The fpr watches manifold vacuum, and raises fuel pressure at low manifold vacuum. I get that high vac means light throttle/cruise conditions, and low vacuum means more throttle, needing more fuel. But changing the fuel pressure doesn't make sense to me. The injectors will flow more of course, but the tuning data changes at the same time. So the ecm has to juggle pulse width with the changing pressure. Why not just leave fuel pressure constant, say 50 psi? What is the point of reduced pressure at cruise? To save wear on the fuel pump?

[blindsquirrel]

You're thinking about pressure only at one point, the rail/injector inlet, and ignoring pressure at the other important location, the injector outlet. Varying the fuel rail pressure is actually keeping pressure on both sides of the injector constant, so that the flow rate always remains the same.

[Scott-m]

I get it now, thanks! 43 psi plus 15 inches vacuum is the same differential across the injector as say 50 psi and 2 inches of vacuum.

[blindsquirrel]

Right. I was trying to avoid using the term 'differential', but that is the only thing that causes fluids/gases to flow from one place to another. If you have a returnless system at 50 PSI, and 50 PSI boost, how much fuel flow do you get when the injector opens? None! But if there's a regulator that always keeps rail pressure 50 PSI higher than whatever's in the manifold then IFR is always the same because the differential between inlet & outlet is always the same.

[kingtal0n]

Lower fuel pressure makes idle tuning easier with large injectors. I run around 38psi fuel pressure with 800rwhp ID1050 gasoline LS application for example.
Low fuel pressure takes stress away from fuel system components, less stress on hoses, pumps, longer pump life.
Lower fuel pressure also helps fuel pumps flow more fuel, e.g. the maximum pump output volumetric rate is achieved at the lowest possible pressure.

The main reason to raise fuel pressure is when the injectors are too small and you are willing to sacrifice pump flow rate max capability to make up for small injectors. Like if I have 55lb/hr(@58psi) injectors 8x gasoline and trying to squeeze 600rwhp I might be forced to approach 67psi of fuel pressure provided the pump can still meet the demand.


Once injector duty passes roughly 30% duty cycle the fuel spray is guaranteed to smack the back of the intake valve so atomization isn't important.
With some engines There may be some benefits to spray from particularly well designed and positioned injectors (good spray patterns) when duty cycle is in the 15-25% range and the injection timing is poised to spray into an open intake valve, the kinetic energy of fuel flowing from injector to cylinder through peak piston velocity may benefit cylinder fill and provide extra torque.

I would argue that with my dying breath Boosted applications especially it is a VERY keen player - I have gained up to 120hp with injection timing alone on twin turbo setups below 3000 rpms Not to mention if when done correctly and at a linear retard rate with rpms IT WILL increase torque enough to rip the tires loose whereas it may not have before... Just takes some playing and getting right for the particular application.

https://forum.hptuners.com/showthrea...l=1#post686907

Thus the question of fuel pressure has many facets. Does the injector spray pattern and position and size of the injector able to benefit the cylinder fill? If so, will increasing fuel pressure and therefore kinetic energy input (fuel pump supplies kinetic energy to fuel spray) increase possibility for power gains during the time frame of low duty cycle while intake valve is open? It is worthwhile to visualize the process and consider energy sources as you gain education in tuning.

[blindsquirrel]

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[kingtal0n]

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