#559, "Has anyone figured out why our cars lean out at 4000 RPMS?"
i know other people have had this problem. when the boost is up around 8 PSI, and i watch my A/F gauge, it leans out for a little bit right around 4000 RPMS. then it goes rich again. Anyone figured this out yet?
#561, "RE: Has anyone figured out why our cars lean out at 4000 RPMS?" In response to Reply # 0
Basically at 4000rpm's when you floor it boost comes on almost instantly.. When that happens, your FMU which is mechanical, cannot raise fuel pressure fast enough. Thats why you lean out for a second or two.
If you had a stand alone programmable computer, this would never happen because all fueling is controlled by the ECU. with the fmu however there is a small amount of lag before it can pressurize enough fuel the specified boost..
#562, "RE: Has anyone figured out why our cars lean out at 4000 RPMS?" In response to Reply # 0
LAST EDITED ON 02-Jan-01 AT 05:46 PM (PST)
<[br /> Do you have the FMU running off the intake manifold??
I noticed the same thing too. As with any mechanical device, response time is going to suffer vs. electronically. What I did before I went the with SAFC, is I plumbed the pressure tubing for the FMU to the compressor side of the turbo. I didn't have my wastegate connected there anymore, since I had my MBC running to it that I T'd directly off the intake manifold....better boost response. Anyway, connecting the FMU tubing direclty off the turbo will give the FMU a higher reading than what is actually present at the intake manifold b/c of the pressure loss through the intercooler. For instance, I think at the time I was boosting 7psi. The FMU was probably sensing roughly 9psi, figuring a 2psi loss through the IC. Since I did that, I didnt notice any lean spots in the A/F mixture once running into boost as I had before. Another reason why this might've helped in that the FMU will immediately sense boost. Meaning that while our cars are still in turbo lag when we intially hit it, the FMU is already getting a boost pressure and is already X-amount psi ahead of your right foot. So by the time that amount of boost pressure is coming into the intake manifold, with the original setup, the FMU would just be begining to sense a boost signal. By connecting it directly to the turbo the FMU has now already sensed that amount of boost coming into the intake and has already beefed up fuel pressure....(whew)
#563, "RE: Has anyone figured out why our cars lean out at 4000 RPMS?" In response to Reply # 3
>Do you have the FMU running >off the intake manifold??
That's a good idea. Use turbo lag to our benefit! Very good suggestion.
> >I noticed the same thing too. >As with any mechanical device, >response time is going to >suffer vs. electronically. What I >did before I went the >with SAFC, is I plumbed
So what are you doing now with the SAFC? Hooked the FMU to the turbo or the intake manifold?
#564, "RE: Has anyone figured out why our cars lean out at 4000 RPMS?" In response to Reply # 4
i think i remember reading that our fuel injection system switches from sequential operation to simultaneous operation at 4000 rpm. in other words, the injectors open one at a time just as the intake valve is opening for that cylinder below 4000 rpms. above 4000 rpms the injectors all open and close at the same time. perhaps the point when they switch causes a momentary lean condition. i dunno... just a thought.
#565, "RE: Has anyone figured out why our cars lean out at 4000 RPMS?" In response to Reply # 4
95Esi, I've connected the FMU back to the manifold now that I have the SAFC. I've only installed the SAFC though. I haven't put in my 440cc's so all my settings are at zero. I just got my car back from my build-up not more than a month ago so I'm still taking it easy. Want to be extremely sure I do a good break-in of my engine and new clutch. I've got a recalibration kit for the FMU to drop it from 12:1 to 6:1 though once I drop in the 440's and start to really turn up the boost.
