fuel pressure regulator

[tencentlife]

Yeah there can be a slight visible pattern like that if you spray into liquid, but in the air no discernible pattern should be apparent, certainly nothing that looks like a stream.
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[FarPoint]

I just checked the fuel pressure and it's ~95 psi with the FPR return blocked. So the fuel pump must be ok.

The question is, why is the pressure only 24 psi at idle? Could it be that I got a bad FPR? The old FPR that was on before read 27 and 30 (with vac hose off). This new one is 24/30.

[FarPoint]

[tencentlife]

Yeah, that's right if you view it all on an absolute pressure scale.

The measured fuel pressure of 2.5 bar/~36psi is gauge pressure. That's pressure relative not to zero absolute, but to zero being whatever the ambient pressure is, so at the nozzle the pressure differential of 2.5 bar is relative to ambient, or 1 bar/14.5psi/~30"Hg. at sea level. That means the absolute pressure in the fuel ring is 3.5 bar/~51psi at sea level.

You obviously get this but for those confused about gauge vs. absolute, here's the first thing Google spit up, quoted from https://physics.stackexchange.com/questions/20460/gauge-pressure-vs-absolute-pressure

"Absolute pressure is zero-referenced against a perfect vacuum, so it is equal to gauge pressure plus atmospheric pressure. Gauge pressure is zero-referenced against ambient air pressure, so it is equal to absolute pressure minus atmospheric pressure. Negative signs are usually omitted."

If you put everything on one scale, whether it's absolute or gauge pressure, and introduce the 0.5 bar/7.25 psi pressure trim that the FPR applies with ~15"Hg. vacuum applied to its sensing port, the the numbers always add up, you always have a 2.5 bar/36psi differential.

Absolute is easier to do the math because it's on a single positive scale, where doing it in gauge pressure you have divergent numbers above and below ambient, and usually using inches of mercury as a measure of vacuum, so it can get confusing. But since most people take ambient pressure for granted and think of things relative to it, these things are more often discussed as gauge pressures, even though absolute is really easier to understand.

And of course it's not an on-off pressure trim, as you can see when you apply vacuum to the FPR the pressure changes gradually in proportion to the amount of vacuum applied, we're just talking about the limits here.
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[MarkWard]

Sorry to ask, but are the pressure values in the factory manual gauge pressure or a combination of the methods you mentioned in the last post? I have always looked at it from a gauge pressure point of view. Thanks.
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