I have a JDM 3 Bar MAP sensor and a bunch of Defi Advanced CR gauges and sensors, including a Defi MAP sensor and a knock-off Defi Fuel Pressure sensor (the rest of my Defi CR gauges and sensors are genuine). I am able to display the differential pressure between fuel and MAP on the fuel pressure gauge and straight away noticed that there are small fluctuations. While off the throttle the pressure would display around 3.3 Bar / 48 PSI, while on some throttle 3.2 Bar / 46~47 PSI and with enough boost 3 Bar/43.5 PSI.

I have drilled out my siphon return several times (because I was nervous and obviously I didn't want to end up in the position of drilling a hole smaller!), going to 2mm, then to 2.5mm / 0.1", then to 3.2mm / 1/8". I did notice a reduction in fluctuations going from 2.0 to 2.5 mm, but not from 2.5 to 3.2mm. Maybe there was a small effect.

I even more recently realised that I could input the Defi analogue input into my ECU and log it, and therefore compare its data trace to that of my JDM 3 Bar MAP sensor. After coming up with a good enough equation, I plotted this:

The vertical scale is in PSI but please ignore the exact numbers as they depend on my equation for the Defi fuel pressure signal and because there is clearly a lot of noise in the signal.

Despite the noise, you can see that the darker blue dots are while the TPS is at a minimum, teal and green are with some throttle and red is only during 100% throttle. I suspect that some of the lowest dips in differential fuel pressure are during my transition from low voltage to high voltage on the fuel pump as that's the only time I've noticed it while watching my gauge while boosting, so please ignore that.

Before I forget, I don't have too many relevant mods: Walbro 255 fuel pump and an AEM fuel rail (with the Defi pressure sensor mounted after injector 1 on the fuel rail). Fuel pressure regulator and injectors are stock.

What I want is super consistent fuelling, so before I start chasing ghosts do I have a real issue that I need to deal with? Does anyone else experience small fluctuations like this and are they expected?

 

Does anyone else log their fuel pressures? Should the pressure be perfectly stable?

BTW the x axis on the graph above is effectively time, in case that wasn't clear.

 

Hopefully someone with a log can chime in but I am not too surprised about the fluctuations. The system is reactive, not proactive so the fact that it dips/pulses and kind of oscillates around makes sense to me.
I mean there are even parts made to dampen these pulses.

 

Thanks, yeah I am talking myself into getting a damper, especially since reading through a thread on the topic here and a good one on an Mx5/Miata forum.

Having looked at my chart again, I think I can actually see the pulses in the noise. See the ~4 vertically stacked light and dark blue waves? I think they're corresponding with 4 cylinders. Of course, my data rate is too low to be sure but the pattern is very consistent - how else could I have such distinct waves given that there is a lot of potential noise present. It's just too perfect to be noise...

To be clear, I don't think I have any issues with my car but having seen data logs from mrfred on this forum, I know that my AFR traces should be a lot better than they currently are.

Maybe I'll do some DMA logging of pressures to dig a little more.

 

If you get a damper it would be really cool to see a similar graph to compare if they actually help. I have read both that they do and that they don't make a difference so it would be cool to see some actual data.
Yea, I noticed that wave as well. It would make sense if those are the injectors opening up and dropping pressure. It's also fitting that the more throttle you give it, the larger the pulses; more fuel leaving the rail via the injectors vs the return.

 

Well, I finally did some DMA logging today. You can very clearly see the waves that must surely be correlating to the injector pulses.

I'm quite surprised that the fuel pressure is falling away (in red) while on boost. I have a Walbro 255, so it can definitely supply enough juice. I am using the factory fuel lines, so maybe that's making it hard for the pump to keep up.

Below, you can see that the fluctuations in fuel pressure increase dramatically higher up the RPM range while on boost.

You can possibly see that the big downward spikes (troughs?) in fuel pressure are followed closely after by small lean spikes in the WB02 reading.

BTW yes, I do need to sort my boost curve. Sorting the fuelling first