| Sparing the entire story, which I don't think is important at the moment: I'm putting my car on the dyno next wed to diagnose a lean condition (~13.5:1) over ~3.4krpm that was detected the last time it was on the dyno, about 2 months ago. Last time a tailpipe sniffer was used, which I know are notoriously inaccurate, so this time we'll be measuring AFR at a bung I have (I am actually hoping this is all there is to my "problem", but that might be wishful thinking). While it's on the dyno I also want to confirm that fuel pressure is okay. I have read the 6Ps and I know that if I track fuel pressure to boost, I should see 1psi increase in fuel pressure for every positive psi of boost pressure. My original plan was to hook up a boost gauge side by side a fuel pressure gauge and record them during the runs so that I can check this. The shop with the dyno, a respected tuner, has suggested that instead of trying to track boost to fuel pressure, we just disconnect the vacuum from the FPR. They are saying that in this state the fuel pressure will be "consistent at 51psi" and that they have boosted up to 18psi in this condition without issue. They are saying that if the car cannot maintain 51psi in this state, that will indicate a fuel delivery issue (though I guess it also eliminates the FPR from the test to some extent, since it's not fully hooked up...right?) I feel like there must be something I am not getting about how the fuel pressure system works, or I must be misinterpreting what "consistent at 51psi" means, because in my mind 51psi fuel pressure will only work up to about 7psi of boost, at which point I'll be making an effective 44psi of fuel pressure. We have gone back and forth on this in email a few times today, and although the responses seem to be missing critical parts that would help me understand why "consistent at 51psi" would be okay, it is clear that the shop is absolutely certain fuel won't be a problem with the vacuum disconnected from the FPR. So...is there something that I am not understanding about why what they're suggesting is safe? Or does it sound like they're wrong? It sounds to me like they're wrong but that's kind of hard to swallow given their experience (tons) vs mine (little). The car will be on the dyno either way so if things lean out they'll stop the run...but I'd feel a lot better if I understood what was going on here. Thanks.
Andy
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