Let’s talk about adjusting the air-fuel ratio with an adjustable fuel pressure regulator, and whether it is even possible.

In closed-loop (cruising), the ECU injects fuel based upon it’s programmed tables, but feedback from the primary O2 sensors tell the ECU whether it is rich or lean, and the ECU adjusts the fuel calculation based on the O2 sensor feedback via the short-term and long-term fuel trims.

As I understand, the ECU adjusts the fuel based on the fuel trims, even in open-loop operation. If so, you could make a fuel pressure adjustment, and initially see a corresponding change in open-loop (WOT) air-fuel ratio, but over time, driving in closed loop operation, the ECU would see that the base fuel calculation is running richer or leaner than expected and adjust the short, and eventually long, term fuel trims to compensate, effectively cancelling out your adjustment of the air-fuel ratio via fuel pressure change.

Thoughts?

 

Yes this is what happens with our ECU's. Unless you fake every fuel and air signal to the ECU it will eventually adjust it's fuel trims to cancel out any AFR modifications.

 

Adjust your fuel pressure to achieve ltfts near zero. Your open loop AFR can then be set via your "tune" and will stay.

 

Thanks guys. I have the AFPR set where the stock one was at - 58 psi - and I will just leave it there. AFRs don't look bad anyway. The AFPR was free to me, so nothing wasted there. With the AFPR relocated to the engine bay, much closer to the fuel rail, new fuel lines, and Walbro pump, I should have nothing to worry about regarding fuel delivery.

A tune is the next mod on my list.

 

Why 58 psi and not a little higher to compensate for drop at high rpm's?

 

Anyone???????

 

I was running 66 psi ,afr's were 11.7 to 11.9 ,added exhaust, afrs went to 12.5 to 12.8 jacked up the fuel to 70 , now the afrs are back to 11.6 to 11.8 and have been that way for a while, with my tune, I can make minor fuel adjustments to ajust afr.. don't forget my system is not like the stock srt6 system., I am answering the question can it be done,,,yes,,
if you add a pulley (adding boost) you need to add fuel,

 

Yes, this is why I am asking. A reliable source has told me the stock SRT6 pressure is actually 53 to 56 psi range.

 

Yes, 53-56 is my understanding also. At less than WOT, I wouldn't expect the adjustable regulator to make any difference as the ECU compensates but BTTW when the injectors are 100% duty cycle the added pressure should get you a little more fuel, which should be enough to keep us from leaning out. At least, that's what it looks like to me but I've been wrong before.

 

If it would stop snowing here I could at least get the car to the dyno and finish my tune and I'd have the answer. May take till spring to know.

 

This is a good conversation. I am still lost with my system running rich with stock fuel system and added boost.

 

My fuel pressure was dropping off at 5700 rpm withy the SL55 pump. I'm hoping the fuel pressure regulator with unregulated filter will correct this issue. Only have the SL55 Intake and 62mm Karman pulley. Time on the dyno will tell.

 

What regulator? Going to use the high flow fuel filter Mann Mercedes High Flow Fuel Filter

 

I have been doing a little research and I am not at all sure that we don't need the larger 550 injectors. The injectors should be working at not more than 85% duty cycle, so just adding the adjustable regulator and hi-flow filer, while it helps may not be enough. In the spring, I will try to get some datalogging in and see where I'm at but as I understand it, if the injectors are operating above 85% duty cycle for extended time they can fail. Rob, NW, fill us in here on this.

 

This one : NeedsWings SRT6 Adjustable Fuel Pressure Regulator

it is also noted in the link for the filter you posted that you will need this regulator.

 

Hi,

I'm running:

550cc injectors
Closed fuel rail
sL55 pump
High flow fuel filter
AEM fuel pressure regulator

Custom manis
90mm TB
61mm pulley

The fuel pressure stays constantly at 60-62PSI without any dipp Even at 6750rpm

And yes, the fuel pressure regulator was needed to achieve this.

/Adam

 

The fuel pressure adjustment is for open loop operation only, closed loop is that, and compensates for the changes in operation. If I left a different impression, the fault is mine. It does assist the closed loop when boost is high by compensating for the lower relative fuel pressure due to the boost retarding fuel flow in the pintal. Woody

 

How is your regulator set? Static pressure setting or adjustable through the inputs of boost and Zeitronix? nevermiond I see you are static at 60-62 psi

 

Interesting topic for me right as I am working with my system to resolve rich AFR at low end and lean AFR at top end. I like others that are posting here and other threads are trying to get this topic nailed down. I also realize that this may be a dead horse for some of our members that have worked through these issues on their cars in the past. I would like to write what I think I know and hope to solidify my understanding and hopefully problems.

Fuel system factors;
1. Fuel pump - Fuel pressure and/or volume available for regulator
2. Fuel regulator – Controls what fuel pressure and/or volume is available for injector
3. Injector – Fuel volume capability, volume over a time frame
4. ECU – controls injector duty cycle (IDC) and timing based off of OEM input parameters (narrow band O2, boost) and modifies with short term fuel trims and ultimately long term fuel trims as well as real time timing advance/retard.
5. ECU tune – modifies what the ECU does with the injectors and timing due to limited ability of OEM inputs and no real time wide band inputs and automated adjustments.

I have read two ways to correct lean AFR here on our forum and both seem to have work based on feedback and tests people have shared.

Adjustable fuel regulator system by Needswings developed by Woody controlled by increasing boost through a Zt-2. This is described well on Robs website. I read that if the regulator is adjusted based on the boost that it should not run rich at lower rpm's and the stock fuel system has plenty of volume, pressure, and 450cc injectors would not run above optimal IDC.

or

The 550cc injectors (looped rail to even pressure) and high volume pump (SL55). I have also read that if you don't use the larger injectors and just install a higher lph pump then a tune can't have an effect on the AFR because the IDC is already maxed and would just be relying on increased pressure/volume behind the injectors since they would be opening for a longer period of time and more frequently at sustained higher RPM's (top end). Essentially a larger pump by itself would just make a larger reservoir of fuel available so that the injectors don't run out of volume or pressure especially with increased boost that the injector has to overcome. Adding larger injectors with a larger pump would give the ability of a tune (the ECU) to control the injectors and modify the IDC.


Once again, I am not an expert and this is only what I think I know based on what I have read and understand.

What I can’t figure out is why would the system run rich at low end in either scenario? Adjustable regulator with OEM injectors and pump shouldn’t cause anything to run rich at the lower end. Even if there was not an adjustable regulator and there was a larger pump the ECU should be able to keep it from running rich at low end. The larger volume injectors with larger pump shouldn’t either because the ECU could modify the IDC and timing as well.