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Fuel Pressure and Injector Parameters [EMU Black]

EFI Tuning Fundamentals

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Hi All,

I'm tuning a supercharged 2zz-ge which from factory has a deadhead fuel rail setup with an in-tank regulator. I've fitted a gauge/sensor to the fuel line just before the rail so I've got some idea of how much the pump is making and at idle it's sitting at 400kpa.

I'm now looking to tune in some new injectors, I have all of the flowrate and deadtime data but I'm not sure what settings I should be using. Should I use the settings provided by ID for 400kpa, or should I be factoring in the fact the car will be making around 70kpa of boost during WOT?

I just about understand the concept of a rising rate FPR on a 'returned' fuel rail setup and understand that the effective fuel pressure should remain static, but I think I'm overthinking the deadhead setup and confusing myself :)

For what it's worth, the car idles "fine" with the 400kpa settings used but I haven't done any pulls yet. Wondering whether I should be modifying the parameters to use 400kpa - (boost pressure) instead, but then wouldn't that throw out my idle settings?!

Many thanks in advance!

What the injector dynamics use is differential pressure. Without a fuel pressure regulator referenced to manifold pressure this will vary with the manifold pressure instead of remaining essentially constant. Your ECU will have to do a lot more work if it has to calculate the differential pressure in order to determine what the flow from the injector is going to be.

Here is an old article from the Injector Dynamics Library that gives some info on this subject:

http://injectordynamics.com/articles/fuel-pressure-explained/

Thank you David, that reinforces some of the reading I've been doing tonight.

If you don't mind, could you sanity-check my logic here?

The EMU Black logs Fuel Pressure Differential, which I assume is just absolute fuel pressure minus manifold absolute pressure. I can then use one of the fuelling correction tables (differential fuel rail pressure) to add/remove fuelling depending on this differential.

So let's say my effective fuel pressure (or differential pressure) is 4bar at idle. If I add one bar of boost, then the differential pressure will become 3bar. Using the DFPR correction table and a fuel injector: flow calculation I can alter the pulse width accordingly to add more fuel at 3 bar to compensate for the drop in differential pressure.

I would set the dead time latency to the figures closely matching idle/low load because this is where the dead times are likely to make the most impact. Setting them for the 3bar figures is a little less useful, because at high load the dead times are not quite so significant.

Sound like a reasonable approach?

I think you have a good understanding, and it sounds like you have the capability to compensate for the differential fuel pressure.

Good luck.

Excellent, thanks again for the pointers.