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<p>I am no expert but could the altitude change being referred be not with fuel pressure but more with the readings?</p>
<p>Depending on the type of gauge maybe its possible to change the accuracy of the gauge used depending on atmospheric or barometric pressure change?</p>
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<p>Oh just had a thought my fuel regulator has an extra port to sample external pressure mostly for use with a turbo but could the regulator in this situation be affected by outside influence?</p>
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<p>Like said I am no expert and just making a wild guess. </p>
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<p>Daniel Du Vall</p>
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<div id="x_divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" color="#000000" style="font-size:11pt"><b>From:</b> type3-vwtype3.org <type3-vwtype3.org-bounces@lists.vwtype3.org> on behalf of William J <catnine09@dslextreme.com><br>
<b>Sent:</b> Tuesday, August 23, 2016 2:47:05 PM<br>
<b>To:</b> type3@vwtype3.org<br>
<b>Subject:</b> Re: [T3] Fuel pressure and elevation</font>
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<div class="PlainText">I don't get how air pressure affects the FPR since it's a closed sealed
<br>
system . The only way air pressure may have an affect is on the fuel in the <br>
tank which is vented. . It's not like the MPS where it is open to adjust the <br>
pressure.. What affect does air pressure have , does it collapse the rubber <br>
fuel lines . Wouldn't it also affect oil pressure it's still a pump and open <br>
by the vent .<br>
<br>
<br>
----- Original Message ----- <br>
From: "Jim Adney" <jadney@VWType3.org><br>
To: <type3@vwtype3.org><br>
Sent: Tuesday, August 23, 2016 10:47 AM<br>
Subject: Re: [T3] Fuel pressure and elevation<br>
<br>
<br>
On 22 Aug 2016 at 21:51, Daniel Nohejl wrote:<br>
<br>
> So a little over a month ago, we were in Yellowstone NP and had to<br>
> diagnose a running problem. In the process, we checked the fuel<br>
> pressure and discovered that it was only 25psi even though weŽd set it<br>
> to 29psi a few months prior. Clearly elevation has an effect on fuel<br>
> pressure. We live more or less at sea level, but in Yellowstone we<br>
> were anywhere from 6000TM-9000TM above sea level.<br>
<br>
I think you're asking a question that no one has ever asked before,<br>
and it's extremely interesting.<br>
<br>
My short answer is that I believe the fuel pressure should be<br>
adjusted (to 28 lb/sq in, in your case) at something close to sea<br>
level and then just left alone. I'm sure Bosch never intended for<br>
this to be something that had to be re-tweaked every time you drove<br>
up or downhill.<br>
<br>
The longer answer requires that we look at how the pressure regulator<br>
works. There's a diaphram inside that feels fuel pressure from one<br>
side and spring force plus air pressure on the other side. When the<br>
system is working, the forces from each side are balanced (equal.)<br>
Because the outlet tube is so small, we can consider the effected<br>
areas on each side to be equal. So we have 28 psi on one side and<br>
14.7 psi plus spring force on the other side.<br>
<br>
I just measured a pressure regulator, and the diaphram has a diameter<br>
of about 1.5" or an area of about 1.75 sq in. Thus the pressure<br>
difference, ~14 lb/sq in, over 1.75 sq in means that the spring force<br>
must be ~24 lbs.<br>
<br>
At, say, 5000 ft, the air pressure falls to ~12 lb/sq in. so a<br>
reduction of ~2.7 lb/sq in. Since the only things that can vary are<br>
the 2 pressures, and they work on effectively equal areas on the 2<br>
sides of the diaphram, this means that the fuel pressure must also<br>
drop by ~2.7 lb/sq in when driving at 5000 ft altitude.<br>
<br>
At 8000 ft, air pressure is ~11 lb/sq in, so the fuel pressure should<br>
have dropped by ~3.7 lb/sq in.<br>
<br>
Our variations would be higher at higher elevations and lower at<br>
lower levels.<br>
<br>
It's reasonable to assume that the folks at Bosch knew all this and<br>
this was integral to the design of the FI system. It's not like<br>
Germany was some flat, sea-level place where no one ever thought of<br>
the existance of hills and mountains.<br>
<br>
Perhaps the larger question is what should we set our pressure to if<br>
we are not at sea level. Sounds like we should decrease our settings<br>
depending on our altitude (AND the current barometric pressure!) An<br>
additional complication is that very few pressure gauges measure<br>
absolute pressure. Our gauge readings are also somewhat dependent on<br>
the ambient air pressure surrounding them. Given all the<br>
complications, I recommend setting your fuel pressure when the car is<br>
at the lowest elevation you can conveniently reach. Anything under<br>
1500 ft should be reasonable.<br>
<br>
I DEFINITELY DO NOT recommend tweaking this as you drive into<br>
different regions. It should be a set once and forget item.<br>
<br>
-- <br>
*******************************<br>
Jim Adney, jadney@vwtype3.org<br>
Madison, Wisconsin, USA<br>
*******************************<br>
<br>
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