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tencentlife Samba Member
Joined: May 02, 2006 Posts: 10078 Location: Abiquiu, NM, USA
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Posted: Fri Oct 12, 2018 12:12 pm Post subject: Re: fuel pressure regulator |
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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. _________________ Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/
Please don't PM here, I will not reply.
Experience is kryptonite to doctrine. |
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FarPoint Samba Member
Joined: May 04, 2016 Posts: 58 Location: Santa Clara
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Posted: Sat Oct 13, 2018 10:10 am Post subject: Re: fuel pressure regulator |
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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. |
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FarPoint Samba Member
Joined: May 04, 2016 Posts: 58 Location: Santa Clara
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Posted: Mon Oct 15, 2018 4:23 pm Post subject: Re: fuel pressure regulator |
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tencentlife wrote: |
The ECU lookup tables are based on the rated 2.5 bar (36psi) differential pressure at the nozzle. At high manifold vacuum the FPR draws down ~7psi, because the absolute air pressure at the intake ports is also effectively ~15"Hg. (~7psi) under ambient, to maintain the same 2.5 bar differential pressure. That change isn't for high load enrichment, as many claim, it's for normalising the injection rate.
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So at idle, absolute pressure in the intake manifold is ~8 psi. Then to maintain 36 psi across the nozzle, absolute fuel pressure needs to be 44 psi. And at full throttle, manifold abs press is ~15 and abs fuel press is 51, again pressure difference across nozzle is 36 psi.
Is this right? |
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tencentlife Samba Member
Joined: May 02, 2006 Posts: 10078 Location: Abiquiu, NM, USA
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Posted: Tue Oct 16, 2018 7:25 am Post subject: Re: fuel pressure regulator |
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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. _________________ Shop for unique Vanagon accessories at the Vanistan shop:
https://intrepidoverland.com/vanistan/
Please don't PM here, I will not reply.
Experience is kryptonite to doctrine. |
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MarkWard Samba Member
Joined: February 09, 2005 Posts: 17122 Location: Retired South Florida
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Posted: Tue Oct 16, 2018 9:42 am Post subject: Re: fuel pressure regulator |
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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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