Inconsistent fuel pressure & Radium pulse damper
 

OK, so as documented in my build thread (https://www.miataturbo.net/build-thr...5/), I've recently converted my 99 from the stock NB pressure regulator setup to use an FM "BFK". AEM regulator mounted where the filter normally goes, -6 AN lines, deleting the fender-mounted pulse damper. Walbro pump, older FM dual-port rail, ID1000s, MS3.

With the stock regulator & pulse damper, the Walbro was overwhelming the FPR at idle, but under boost the injectors leaked enough that it worked well. Logging the fuel pressure, the stock system kept it between 60 and 61 psi.

With the AEM, I now have a reasonable idle fuel pressure (plus I can vacuum reference it), but it's giving me very noisy fuel under boost, wavering by 15 psi at 270 kpa MAP (white trace in graph #3):

https://www.miataturbo.net/attachmen...ine=1435618104

So I'm thinking I need a pulse damper again. I've still got the rail mounted one, but the fender mounted one would need a bunch of fittings and adapters to go into the AN lines, plus it's 5/16 rather than the -6 AN lines. So poking around on the net I found this:

https://www.radiumauto.com/Blog/Post...lse-Dampers-90

Anyone know anything about it?

--Ian

Question: When you logged the stock system, was your wiring and fuel pressure sensor the same? Just to rule out noise affecting the readings.

Same sensor, same sensor wiring. Different plumbing, obviously. :)

--Ian

Hmmm. Now I'm gonna go look, I don't remember seeing a damper on my fuel rail, it's a 2003 fuel rail though, let me check. I know I had a fender one and it's gone now.

Also given the fact that you're running 1,000 cc injectors, I'm sure some pressure drop is going to be unavoidable but I guess the damper would still help.

Are you using the dual port functionality of the rail? I run a stock rail and it's single feed. I wonder if your experience would change by feeding it only from one end? I dunno how easy or difficult that would be for your to test.

IIRC, the rationale for using double-ended fuel rails is that by supplying both ends it cuts down on pressure inconsistencies going down the length of the rail. Speculating at random, I would think a single-ended rail would increase the communication time between a rear-mounted regulator and the furthest point of the fuel system, thus making it less consistent.

I'm "using" the dual port functionality in the sense that it's hooked up at both ends. There's a -6 AN line coming forward from the tank, and in the vicinity of the intake manifold it goes into a tee with a line that feeds the pressure sensor. The other outlet of the tee is attached to a second tee that then has lines feeding each end of the rail.

The rail looks like this:

https://www.miataturbo.net/attachmen...ine=1435620255

The elbows on each end are the two feed lines. The pulse damper is mounted underneath, in the center, and isn't visible.

The tee fittings look like this:

https://www.miataturbo.net/attachmen...ine=1435620255

The feed line is the one with the straight, blue hose end and the the plastic ribbing covering up the braided line. The right-angle fitting at the top of the photo goes to the sensor. The silver tee at the bottom goes to the two feed lines which branch off right and left.

--Ian

<p></p><p>On the 2003 fuel rail the damper is where the regulator is on NA cars.</p>

The AEM regulator has known issues in the Honda community, I'm sure the form and function of your regulator is similar. I have never owned an AEM regulator so I cannot comment on the quality or issues.

In an idealsystem you would have the regulator plumbed AFTER the fuel rail, I believe the dual feed rails is a step down and still acts as a dead end rather than a true regulated system.

Do you know what the issues that the Honda community encounters are?

--Ian

Imagine the carb is your fuel rail


https://www.miataturbo.net/attachmen...ine=1435622441

The AEM universal regulator is a 3-port regulator, it is designed to be used in the fashion labelled as "incorrect" in that diagram.

--Ian

From what I have been told the diaphrams can fail prematurely and the springs have been known to lose tension and not operate in a linear fashion.

Cap one end, it's universal so it's made to fit a dual rail engine as well. Think about how a regulator works and it will make sense.

Neither of these is likely to be the cause of the noisy fuel pressure that I graphed.

As for the routing, yes, the FPR could be used as a 2-port regulator if desired, but it is specifically designed to be capable of being used as a 3-port regulator as well.

--Ian

If your fine regulating the pressure at the line and not the rail expect problems. Your not using the regulator properly, I don't care how many ports it has.... Or how many dampers your using.

He does have a point- the regulator that regulates pressure WOULD work better the closer you can get it to the fuel injectors. I'm not saying it's the cause of your spikes, it's probably not, but it would regulate and dampen better being closer. I never liked where FM put the regulator in their fuel kit. I guess it makes for an easy install but not ideal.

Mazda puts it there and the pressure stays within 1 psi of the set point.

--Ian

Here's a datalog of the factory regulator, with the range on the fuel pressure forced to the same delimiters as the ones in the previous image. Once again it's the white line in the 3rd graph, look at the part where it's in boost, in the center under the plateau of the duty cycle.

https://www.miataturbo.net/attachmen...ine=1435643893

Yes, it's moving around outside of that range, that's because the Walbro pump is overwhelming the factory regulator, which is the whole reason I put in the AEM.

--Ian

I would at least try moving the regulator to the correct spot. That's absolutely not how that's supposed to be used, and that's definitely not how the Mazda return systems works. :)

I don't use a pulsation damper on the MX6 and don't have these problems. My regulator is mounted "after" the rail, though. I do run upwards of 80psi rail pressure at full boost.

The factory system is a well engineered SYSTEM designed to work well within its parameters, not various off the shelf parts thrown together and expected to work.

Mazda put the fuel regulator where they did to meet EPA standards, it was not a performance minded change.

It looks to me that the difference between the 2 is that the "incorrect" version will regulate pressure at the outlet of the FPR; the pressure at the rail will be the pressure at the FPR minus the pressure drop in the line from the FPR to the rail.

This pressure drop will be proportional to flow (power) and inversely with the size of the line. If the line is big, then the pressure drop will be small. This pressure drop characteristic will be consistent for a given line, and will be baked into the fuel table during tuning.

I don't see how the "correct" version will reduce pressure spikes in the line, unless the FPR can absorb them (i.e. act like a damper), and is mounted close to the fuel rail (in either the "correct" or "incorrect" version).