Fuel Pressure Variation

[sedd]

I am seeing over 5 psi variation in my fuel pressure and I assume that is quite a bit more than desired. What do others see in fuel pressure variation?

Is there a number I should aim to get to, for instance, less that "X" psi variation?

I attached a short megasquirt file, fuel pressure is shown by sensor01. I have other files but they are too big to attach. The pressure pattern varies and knowing potential reasons for this are needed. I assume the regulator could be an issue? The fuel pump sucking vapor? Maybe the pump isn't performing to specifications?

I was hoping to get some ideas for fixing this should the variation be too great. IE could the AFR/EGO control make up for this variation? Would it be possible to reprogram MS3 to change the fuel injected based on a factor added into that equation in the program?

[jsmcortina]

What units are your fuel pressure in and is it relative or absolute?
Are you allowing for the MAP referencing element of fuel pressure?

James

[sedd]

Thanks for response. I think I got it, please check and let me know if I am off on the theory and next steps.

Pressure is not absolute on my pressure transducer. Units are psi, pounds per square inch. I screwed in a pressure gauge while doing calibrations and the pressure transducer matches that. 0 = 0 and 45 = 45psi

I think I see where your going with this. I know the pressure regulator is referenced to the manifold. I just ran the numbers, 100 kpa = 14.5 psi. I didn't recognize there was that much pressure change!

So if I am at 100kpa when I set my regulator, the pressure will adjust with variation lower in KPA?

When i turn on the MS3, the fuel pump runs and I get approx 48 psi when the engine isn't running. I will supposedly need that pressure to get the needed fuel flow for the horsepower expected at full throttle and injectors being used.

So at full open throttle, kpa = approx 100. Difference is approx 0 kpa and there should not be any adjustment in fuel pressure.

As soon as I start my engine my map goes down, mine is approx 50 kpa at idle and 35 to 40 kpa at higher rpm and low loads. So when the kpa number does down my gas pressure should also go down? If my map is around 50 kpa there will be approx 7.5 psi reduction?

I just looked at the gas pressure and map using megalog viewer. they follow fairly closely.

I am worried a bit at higher rpm's, the pressure is falling off....I need to evaluate more if the pump keeps that 48 psi at full open throttle. If it can't, I think I should try to set the regulator lower? What I want is to always have a steady and reliable number at full throttle? I am seeing pretty high VE numbers, around 120 at full open. I figure VE analyzer is compensating for the drop in pressure.

[jsmcortina]

I am worried a bit at higher rpm's, the pressure is falling off....I need to evaluate more if the pump keeps that 48 psi at full open throttle. If it can't, I think I should try to set the regulator lower? What I want is to always have a steady and reliable number at full throttle? I am seeing pretty high VE numbers, around 120 at full open. I figure VE analyzer is compensating for the drop in pressure.

If on a WOT pull with MAP ~100kPa you should have fairly constant fuel pressue. If it does drop off at higher rpms, then you need a higher flow pump. Change it now!

James

[sedd]

James:
I understand the serious issue with loosing gas pressure. I am not at all certain this is happening. Very likely I am overdoing the analysis but I want to be certain so I dont kill my engine. I am just starting to understand the megalog viewer, awsome program by the way. My air fuel ratio is well under 14.7 and close to my desired afr from my table.

I have attached a screen shot of the section based on a short burst at higher rpms. Notice that the time span in seconds is pretty short. the average pressure is ok but there is much more peak and valley that in other parts of the viewer file. So is that normal or do I need less peak and valley?

[dontz125]

Your regulator should be plumbed into your intake manifold downstream of your TB; if you're running ITB, it should be plumbed into one of the intake runners. The job of the fuel pressure regulator is to maintain the rail pressure at some given value (typically 3 bar / 43 psi) above the manifold pressure. As the MAP goes up and down, the rail pressure goes up down - ideally in perfect synch. Those ragged spikes in the rail pressure are bad news. Either the pump or the regulator isn't up to the job, and this needs to be sorted quickly!

[jsmcortina]

Your regulator should be plumbed into your intake manifold downstream of your TB; if you're running ITB, it should be plumbed into one of the intake runners.

Surely it should take an average reading across all runners?

James

[sedd]

I changed my fuel pressure and started my tune all over. I had made a fairly big mistake at setting the fuel pressure while running. I ended up with approx 51 psi instead of 44 psi desired. I adjusted it again and it appears to be much closer to 44.5 psi at open throttle.

I am running the auto tune again and have just started to analyze the data. So far the fuel pressure is fairly steady at normal driving conditions, but I noticed at a couple of full throttle efforts and mid rpm range the pressure will vary. The pressure varies around the correct pressure with spikes of pressure. There is 0.15 sec between the spikes and the AFR never varies much or too far out of range/away from my desired settings.

I would like to see some others logs at full throttle to see if they have a similar situation. I wonder if I am trying to fix a problem that isnt really a problem. If the pressure varies 2 to 3 psi I calculate it would only mean around a 2.5 to 3% change in flow over a very short time span?

I checked my fuel pump and the curves show I should have 20% extra flow, maybe more.

I would like to use a number that would show the injector flow %. Is that Pulse Width PW or is this duty cycle? If I know that couldn't I estimate the gas flow rate thru the injectors?

[dontz125]

Your regulator should be plumbed into your intake manifold downstream of your TB; if you're running ITB, it should be plumbed into one of the intake runners.

Surely it should take an average reading across all runners?

James

I'm ... suddenly not sure. Perhaps the FPR reference tap should be plumbed into the manifold immediately UPSTREAM of the ITBs. The tract pressure at idle / low load is several psi LOWER than manifold, whether NA or boosted; the tract pressure at WFO is near-as-dammit identical to manifold pressure - again, NA or boosted. This ensures good and sufficient flow at full-honk, while the increased flow due to greater dP at idle can be handled with reduced VE.

[E4ODnut]

On the 351 Windsors in my boat, I'm using Walbro GSL392 high pressure fuel pumps feeding from an accumulator. I have pressure transducers (Measurement Specialties Inc. model MSP300100OP4-N-1) on the discharge line to the fuel rails. The rails, regulator, and injectors are stock Ford. When I first got the engines running I noticed quite a bit of high frequency pressure fluctuation as well as "jumpy" MAP, but the average pressures tracked MAP like they should. I got the MAP smoothed out by moving the vacuum line attaching point on the intake plenum and installing a .060" restricting fitting in the line. This line also tees off to the pressure regulator. This helped the fuel pressure a bit. I then installed a pulsation dampener in the discharge line which helped a bit more, but not a complete cure. Other than it's irritating, I don't think it causes any problems as the average pressure still tracks MAP and it shows no anomalies on EGO. Perhaps because the frequency of the variations is so high it really doesn't affect the fuel delivery enough to matter, I don't know for sure. Both engines show the same symptoms.

I suspect that it has something to do with the sensitivity of these particular transducers. I also have them on engine lube oil pressure and they show similar fluctuations, but not as severe. The analog pressure gauges don't pick this up. In another application I have them on line pressure for an automatic transmission. In this case there are no fluctuations.