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I've recently discovered Mega Log Viewer and now I have finished my studies I actually have time to look at this stuff that interests me. And the question I'll get to is: How low is "too low" for fuel pressure?

So every time i hear people talk about boosting their car and asking what's required I always hear "Fuel pressure regulator, upgraded fuel lines etc etc"!
I have a built SE at 20psi with 1050cc injectors, 255lph (or 225?) fuel pump. Everything else is the stock setup (So non-return). If the internet is to be believed I should have blown up in a cloud of smoke and internet shame years ago for not have converted to return fuel lines and a manifold referenced fuel pressure regulator.

So according to injector dynamics: When an engine is idling, there is a vacuum in the intake manifold. This vacuum pulls fuel out of the injectors, and increases the effective pressure across the injector to a pressure higher than the rail pressure itself, boost does the opposite and lowers fuel pressure proportionate to the amount of boost in the manifold so I understand why return lines and a manifold reference FPR makes sense to keep fuel pressure at a constant rate regardless of manifold pressure.... But is it needed?

SE/NB2 runs 60psi fuel pressure standard with a regulator at the tank. At idle I read 68.1 psi on the sensor after the pulse damper on the frame rail (I assume this is the upgraded fuel pump over powering the stock regulator?) with a Differential pressure of 76.9psi. Makes sense:
68psi plus 8.8psi (vacuum) = 76.9psi.

At full tilt in the below 3rd gear log at the point to get off the throttle I measure 60psi FP at MAP of 19.6psi at 5540rpm. Diff FP is 40.4psi. Makes sense:
60psi FP less 19.6psi MAP = 40.4psi FP

Seeing fuel pressure dip to 40psi in the log initially made my heart skip. Oh no danger to manifold!!
But looking at AFR's and injector duty cycle....everything is in check. I'm able to reach boost target and not leaning out. Injector Duty cycle is around than 50%

So if everything is in check at 40psi fuel pressure, do I need to worry about upgrading the fuel rail with return lines and FPR or am I all good?
I do get to 21psi momentarily in 3rd gear at 6500rpm but even then that would still be a low of 39psi differential fuel pressure without leaning out AFR.

Is there an issue running fuel pressure down that low at redline? Is there a danger even long term? Or is everything all good and not something to worry about?

I am not an expert on fuel injectors, but I do understand pressure, absolute, and gauge/reference and how engines work.

Bosch give the flow data for their (port style) injectors at 43.5psi differential pressure. This seems to be an industry standard. 39psi is close enough I think, to ensure that they work reasonably well. I think the fact that your engine runs, makes power and doesn't lean out is the answer in and of itself.

My only question, and I wonder this about my own engine, is whether the atomisation is ideal at the lower pressure and whether more power could be made, or less fuel used, with either:
a) more fuel pressure and less injector duration,
b) an injector that flows less but may have better atomisation for the same pressure.

But I personally wouldn't worry. If it works, it works.

Yes you are right. If it works, it works. Just wanted to make sure my logic was sound and wasn’t putting the engine in danger.
You do raise a good point re: higher pressure with short duration vs lower pressure with longer duration in relation to fuel consumption and power.
Could more power be made? That I don’t know, but I’m sure a constant fuel pressure from vacuum through to full boost would make tuning easier.

*I’m not a tuner!*

You can tune around the limits of the stock FPR.

I understand there are two issues with the stock FPR which you’ve already named:

1) it’s not rising rate (matched to boost); and
2) an upgraded fuel pump can over power it.

You’ve observed the results of both of these in your data. the first issue means you can’t get the most out of your chosen injector, and the second can make control of fuel when demand is low more difficult. Both require the fuel side of the tune to account for extra variables that are not there with a RRFPR.

If neither results in you running out of injector or messes too much with fueling at idle, then perhaps you’re fine.

Return vs return less is the other thing you’ve mentioned.

People talk about fuel temp rising because it has more time to spend in the dead head waiting to be used. They also talk about changes in fuel pressure across the rail. Do either of these play a role in real world 300hp mx5 fueling? No idea!

That’s all very helpful information. Thanks.

Can a manifold referenced FPR be added to a non return fuel rail? I thought it had to be a return system to be able to do this?
So I guess my only concern now would be, is overpowering the stock regulator at idle/cruise going to cause it to fail over time? Or are these things pretty rock solid?

Yes you can.

In your car, you actually have a return circuit already, it’s just very short.

The stock fPR is inside the fuel tank on the ‘return line’ from the T that comes off the fuel filter. One side of that line connects to the filter, the next connects to the feed line that goes up to the rail, and the last one goes back up to the tank where it dumps through the stock FPR.

If you replace that ‘T’ with your own lines and add a FPR in the line back to the tank (and remove the stock FPR from the hanger in the tank) you will have an adjustable FPR. If you add a vacuum line up to the manifold, you can make it referenced.

You probably don’t need to even replace the lines if you use the right fittings to connect with the stock flexible lines at the filter.