Contour Enthusiasts Group Archives Forums Modifications Forced Induction Fuel pressure confusion. Whats the deal?

So I hooked up a fuel pressure gauge to the schraeder valve on the fuel rail at SZ while at the dyno day just to see what was happening to my fuel delivery which would cause me to be running lean. I never did 13:1 afr on the dyno, (It did jump from 11:1 to 12:1 between the 2nd and 3rd pull) but what did surprise me was my fuel pressure readings. At idle my fuel pressure was 59 psi. And whats even more interesting, is at WOT the fuel pressure stayed right at 59 psi the whole time and didnt rise. Does walbro make a low and high impedance fuel pump and im running the high instead of low? Something isnt right. Im thinking my pump was getting hot and causing my afr to go from 12:1 to 13:1 from the 2nd to 3rd pull.

Btw, heres a picture of my dyno graph and afr.

Sounds to me like you have no vacuum applied to the FPR.
59 is reasonable from a stock fuel system with a walbro at WOT but at idle it should be pulled off to around 45psi, maybe 49psi with your numbers. That is what the pcm is expecting, an automatic enrichment when you hit the gas. The boost will make it rise a little bit but not significantly.

I had mine hooked up to vacuum/boost and I was always able to dial in my AFR pretty easily.
Nice round torque curve you got there.

What boost level?

Boost was at 14 psi.

Tom, any thoughts as to why the afr jumped from 11:1 to 12:1 from the 2nd and 3rd pull? This was the same thing that happened on the mustang dyno about a month ago. We made about 3 pulls that were fine. The last 2 pulls had to be cut short since they hit 13:1 afr.

Well I don't think it is the fuel pump, it runs all the time at full tilt and is regulated by the FPR...which should be plugged into vacuum/boost as I said. That will take some error out.

You have several other heat related factors however:
- injectors themselves run at a lower duty cycle normally, in back to back runs they could be getting hot though this is a small possibility to me.
- oxygen sensors could be reading differently as the heat sets in.
- IAT sensor could be getting too hot and not cooling down on its own between runs if it is mounted before the intercooler but after the turbo.

More like the O2 sensor and or exhaust temp the cause:
If the O2 was read with a tailpipe sniffer then it could be affected as the turbine and exhaust components heat up. Hotter exhaust components can at the least help the exhaust gasses continue to combust for a longer period of time, maybe lowering the measurable air fuel ratio at the tailpipe.
On top of that, do you have a catalyst? If so then that may be more of the issues. The catalyst would be especially sensitive to the hot/cold exhaust factors in terms of 'light-off' time; as in if you did the first run cold without a warm cat, and then the others as the system progressively warmed up.
And even if you don't have a cat, if you did the first run cold (without a good 15min drive on the road shall we say) then the heat the exhaust system is leaching out of the exhaust stream will result in less complete burn.

If you did like I do, drive to the dyno 100miles on the highway, pull in and do a run while the car is still quite hot, then I don't know what to tell you other than look at your FPR and your O2 sensors.
On my runs, all of them where I didn't dyno at ADC were made by driving 60-100 miles to a dyno to meet an appointment and then doing the runs. I am impressed that my engine was able to set down 350+ HP at the settings it did, right off the street with a full catalyst exhaust. It also agrees with your system running about 14psi anbd with lower compression producing around the same numbers.
What I can't wait to see is what that setup you have will do on 18-20psi.

From what I saw in your dyno graphs I wouldn't worry too much about it as long as your fuel pressure isn't dropping, as long as your MAF voltage isn't near maximum during the run, and as long as your injector duty cycle isn't over 85%.
With those factors kept in check then you can just keep going up two-three psi at a time.

Hmm...i wonder if ADC sent me 39 lb injectors instead of 42 lb? If my calculations are right, 39s would be good for ~355 whp at 80% duty cycle. My Lightning IAT is mounted right after the intercooler. I want to say that Jerry just turned off my O2 sensors completely. I was throwing a lean code I believe and he just turned them off iirc. I do not have a cat. I purposely took the car down the road and beat on it pretty good before I dynoed. It probably had less than 10 minutes of cool down time before the dyno. Maybe I can send Jerry an email and see what he thinks.

Chris, I know your out there. What are your thoughts?

You definitely won't have 39# injectors as those are returnless only.

Rick

The 39#s are rated at 40psi anyway. At 59psi I think you would have plenty of fuel.

How would I check to see if vacuum is applied to the FPR?

T into the vacuum line and run it to a gauge