Effect of FPR mounting location on AFR's (long)

[Nigel]

Hi Guys!

 

I few weeks back I replied to this post:

 

http://forums.hybridz.org/showthread.php?t=105219

 

regarding my observations with respect to the effect of the fuel pressure regulator mounting location on air/fuel ratios. I said that I would do a writeup and post my logs, so here it is (finally).

 

First of all, here are the specs on my '73 240Z:

 

L28 Turbo engine with T3/T04B turbo (set to approx 9 psi boost), ScottieGNZ downpipe, 3†mandrel exhaust, ZCarCreations fuel rail, RX-7 440cc injectors, Walbro 255lph pump, SDS EM3-6F engine management, and a Techedge DIY 2A0 Wideband meter. I have been using a stock Z FPR up until this past September, when I switched to an Aeromotive adjustable FPR.

 

Now, earlier this year, I finally got around to making use of the datalogging capabilities of my Techedge wideband meter. I made a short interface cable using discrete wires that plugs in between the ECU and the 25 pin harness that connects to the MAP, throttle position, and air and water temp sensors. I then tapped into the wires on my interface cable that had the signals I wanted to log. The Techedge only has 3 user inputs in addition to RPM, so I used MAP, TP, and air temp.

 

For reference, the pinouts on the DB25 connector are as follows:

MAP sensor

Signal – pin 6

+5V – pin 4

Com – pin 15

 

Throttle Position Sensor

Signal – pin 21

+5V – pin 2

Com – pin 25

 

Air Temp Sensor

Signal – Pin 7

Com – Pin 16

 

As much as I like the SDS system, I realized very quickly that datalogging is invaluable. I made significant improvements to my AFR’s as a consequence. The log that follows is the best tune that I got with the stock FPR.

 

Here is the key for the graph (and all that follow)

Green: Air/Fuel Ratio

Light Blue: RPM

Red: Manifold Air Pressure

Blue: Throttle Position

Pink: Air Temperature

 

 

All of these runs were done at full throttle in third gear, from around 2600 RPM to about 6000 RPM. And yes, my turbo takes a long time to spool up.

 

Now, for reasons I won’t get into here, I became suspicious that my stock FPR was not functioning properly. So, I decided to bite the bullet and order a new adjustable FPR from Aeromotive. I didn’t have a lot of time for the installation, so I mounted it to the inner fender, near the bottom of the passenger side strut tower in the engine bay. This meant that the vacuum and fuel lines to the FPR were probably around 4’ in length. I set the base fuel pressure to match the stock FPR. The graph that follows is the first run with the new FPR, using my maps for the stock FPR.

 

 

You can clearly see how the AFR’s lean out dramatically while boost is building after the accelerator pump is done, then drop as peak boost is reached, only to lean out again as RPM’s build.

 

After significant re-tuning, this is the best I could come up with:

 

 

As you can see, it still leans out considerably while boost is building. I couldn’t find a viable solution to this strictly through tuning. My MAP FUEL values almost ended up being the same around 4 psi as they did around 8 psi, and consequently, 4 psi at part throttle ended up being way too rich. I could have raised the RPM FUEL values in that range, but that would have created other problems which I’ll explain if asked.

 

I hypothesized that this lean spike while building boost was due to the location of the FPR, so I remounted it directly to the end of the fuel rail. Here’s the first log I did with the FPR on the fuel rail, but using my maps for the old mounting location:

 

 

The effect on AFR’s is again dramatic. The lean spike while building boost is completely gone, and overall the AFR’s are much richer, dropping to as low as 10:1 at 6000 RPM. I didn’t have time to do much tuning after that, but here is where I left off. It still needs some work, but it’s almost there.

 

 

So, I think the logs speak for themselves, but I’ll gladly answer any questions. The bottom line is that the distance from the FPR to the fuel rail has a significant impact on AFR’s. Whether this is due to the vacuum line length, the fuel line length or both, I don’t know at this time. My car is off the road for the winter, but next year I may try using a really long vacuum line to see what the effect is.

 

Nigel

[Thumper]

Wow I had no idea. I too have a wideband 02 sensor but no datalogging ability just yet. I too noticed it lean when building boost and then a gradual leaning out at high rpms. I also have the fpr installed on the passenger side strut tower. Did you hook the fpr directly to the fuel rail or did you just install the fpr alot closer. Thanks

[Nigel]

Here's where I originaly had it mounted:

 

 

And here's where I have it mounted now:

 

 

So, yes it is mounted directly to the end of the rail...

 

Nigel

[Thumper]

That is where I have it located also. I will relocate it to the fuel rail. Thanks

[proxlamus©]

now this is very interesting!!

 

thank you so much for all of that very usefull info !!

 

I had no idea that would make a difference...

 

 

*psst psst maybe while your car is "winterized" you can install an intercooler!*

 

whoo hoo!

[Nigel]

 

*psst psst maybe while your car is "winterized" you can install an intercooler!*

 

whoo hoo!

 

That's what I've been telling myself every winter for the past four years! But the $$$ always seems to get divereted to some other part of the car that needs attention. Unfortunately, my tires are shot now, so...

 

Nigel

[Nigel]

I forgot to mention one other observation I have made. Previously, with the stock regulator and with the Aeromotive regulator mounted to the inner fender, the needle in the liquid filled fuel pressure gauge on my fuel rail would flicker significantly. It would pulse in time with the injectors firing. Much to my surprise, with the FPR mounted directly to the rail, the needle is now rock steady. It doesn’t flicker at all anymore. However, I have to admit that at the same time I relocated the FPR to the rail, I fiddled with the gauge a bit. The needle was pointing about 6psi too high (I over pressurized it by accident a couple of years ago), so I tweaked the mechanism through the fill hole in the back of the gauge to zero it. Maybe I screwed it up. But the needle seems to be moving freely. I'm going to replace it over the winter.

 

Nigel

[240hoke]

Do you think that the differences were related to the position or the signal going to the FPR. Maybe you could mount a 4" vaccum hose to the FPR in the position that it is in now and re-datalog. Ive also heard of the mapp sensors flickering and giving strange readings without a restrictor in the line. Maybe something similar is happing at teh FPR I have no idea.

 

Anyway thanks for posting up these charts, its interesting Info. I ended up mounting mine to the firewall. We will see how that goes.

 

here is an idea of how it is mounted ill have to take a better picture. The fuel comes into the firewall side of the rail and loops back around to the FPR which is mounted to the firewall. i dont understand how the placment from the rail would effect how the FPR works. But anyway we will see what happens when i get on the dyno in a month or so..

 

 

-Austin

[Nigel]

Do you think that the differences were related to the position or the signal going to the FPR. Maybe you could mount a 4" vaccum hose to the FPR in the position that it is in now and re-datalog. Ive also heard of the mapp sensors flickering and giving strange readings without a restrictor in the line. Maybe something similar is happing at teh FPR I have no idea.

 

Well, my theory is that because air is compressable, it delays the responsivness of the FPR in some proportion to the length of the vacuum line. In other words, a long vacuum line means it takes longer to compress the air in the line to a point that it can provide sufficient pressure to actuate the FPR. An easy way to test this would be to try different lengths of vacuum line to the FPR. But like I stated, my Z is in hibernation for the winter now...

 

Nigel