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Correct pressure

EFI Tuning Fundamentals

Discussion and questions related to the course EFI Tuning Fundamentals


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I'm sure I will learn this further into the course, but what is the correct fuel pressure? Reason for my question is that I am running a return system on a supercharged LsX and we are having fuel issues. The pressure is set at 58psi but after watching this, I am wondering if it should be decreased?

If we wind pressure out of the rail, we are reducing flow but do we have too much flow now? My current tuner has tuned the car to match my pressure but I'm wondering if this should be altered to see a better result?

Thanks

If you have a return system your base fuel pressure is set without vacuum applied to the regulator. So for example, it could be 4 bar (which is about 60psi). Then the actual fuel pressure in the rail varies according to manifold pressure. If there is 15 psi vacuum (about 1 bar), your fuel pressure drops from 4 bar to 3 bar in the rail. If there is 15 psi boost (about 1 bar), your fuel pressure rises from 4 bar to 5 bar in the rail. The pressure difference between the rail and the atmosphere stays the same due to the action of the regulator.

See attached image from Toyota technician training on return fuel systems.

If you have a return fuel system without a regular that is being referenced to manifold pressure, you have a problem with your build.

Attached Files

There is no 'right' fuel pressure as such. It's common to run 3 bar differential pressure in a manifold pressure referenced fuel system and 4 bar in a return-less system but that is not a written in stone rule you must follow. Depending on your ECU though you may need to adjust the injector flow data to match the actual pressure in your fuel system.

Brock, In my opinion it's better to be thinking in terms of having the correct AFR for the operating conditions, rather than the correct fuel pressure. As long as you have the right AFR it doesn't matter too much what pressure you used to get there - for example, the Nissan GT-R that raced in Bathurst in 1991 used a mechanical fuel pump and over 100psi pressure to get the required fuel flow, because the largest injectors available at the time simply weren't big enough (I believe they were Bosch Motorsport 550cc units).