Fuel pressure at idle??

[Rawze]

Fuel rail pressure mostly determines fuel penetration into the cylinders. The higher and faster the fuel penetrates the cylinder, the more volatile it becomes and indirectly, slightly less timing advance is needed.

- From factory, the rail pressure in the newer engines is definitely a bit low (5-8k PSI). It is amplified by the higher compression ratios these engines have too. It does however coincide with the 50-70+% positioning of the VG turbo and the heavy EGR gas too. Bigger fuel droplets to go along with the longer burn, lower penetration, added inert gases all add up to meeting the emissions standard while advancing the timing to slightly get better economy. - Disconnect those cans and you can see it and smell the unburnt fuel it at idle.

- Running the engine with all the cans off it reveals that raising the rail pressure slightly increases penetration and improves efficiency,.. but injection timing must be changed to accommodate it. They kinda go hand in hand in an indirect relationship but not exactly. For example,... excess injection timing alone causes detonation, internal liner friction and vibration, excess combustion temperatures, sharp combustion peaks and other related problems. Excess fuel pressure will cause these same symptoms,.. but not as badly. Excess fuel penetration also adds the problems of fuel impingement, deposits, etching, and other factors that eventually take their toll in other ways.

The general consensus thus far that I have seen for those who have been doing custom tuning after a de-mandate is to raise the fuel rail pressure slightly at idle to reduce the unburnt fuel smell/quantity, but not so much as to cause penetration issues. The problem with this though is that it takes energy away from the flywheel to produce those pressures (the load placed by the pump onto the gear train), so any more pressure than necessary, and it takes away from overall engine efficiency. It is a delicate balance.

BTW: Older engines like the CM870/871's idle at about 23-28k PSI injection pressures and peak at about 32k injection pressures based on the information I have seen in the past. This is much higher than newer engines at idle, but then again, they do not have the compression ratio, nor the high injection spray angle, nor the finer spray that the newer engines have. It is like comparing apples and green-beans, they are two different things.

That is my own understanding of it anyways.