<html><head></head><body><div style="color:#000; background-color:#fff; font-family:verdana, helvetica, sans-serif;font-size:16px"><div id="yui_3_16_0_ym19_1_1471989460895_7731"><span id="yui_3_16_0_ym19_1_1471989460895_7735">I think the FPR is composed of a spring, a diaphragm and a valve. Fuel pressure exerts force </span>to overcome the spring. Ambient air pressure is not a factor in this.</div><div id="yui_3_16_0_ym19_1_1471989460895_7732"><br></div><div id="yui_3_16_0_ym19_1_1471989460895_7733">I would think the pressure gauge is also just a spring and a diaphragm.</div><div id="yui_3_16_0_ym19_1_1471989460895_7734"><br></div><div id="yui_3_16_0_ym19_1_1471989460895_7209">Anyways, the injectors should deliver the same amount of fuel for the same duty-cycle (percentage of open time) as commanded by the ECU.</div><div id="yui_3_16_0_ym19_1_1471989460895_7208"><br></div><div id="yui_3_16_0_ym19_1_1471989460895_7208">But, the amount of O2 for a given volume of air is less at altitude because of the change in air density.</div><div id="yui_3_16_0_ym19_1_1471989460895_7208"><br></div><div id="yui_3_16_0_ym19_1_1471989460895_7208" dir="ltr">I don't know how the stock FI system deals with this. </div><div id="yui_3_16_0_ym19_1_1471989460895_7208"><br></div><div class="signature" id="yui_3_16_0_ym19_1_1471989460895_7094">Max Welton http://www.maxwelton2k.net/</div><div class="qtdSeparateBR"><br><br></div><div class="yahoo_quoted" id="yui_3_16_0_ym19_1_1471989460895_6751" style="display: block;"> <div style="font-family: verdana, helvetica, sans-serif; font-size: 16px;" id="yui_3_16_0_ym19_1_1471989460895_6750"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;" id="yui_3_16_0_ym19_1_1471989460895_6749"> <div dir="ltr" id="yui_3_16_0_ym19_1_1471989460895_6769"> <font size="2" face="Arial" id="yui_3_16_0_ym19_1_1471989460895_6768"> <hr size="1"> <b><span style="font-weight:bold;">From:</span></b> Daniel K. Du Vall <dduvall@1peter4-10.org><br> <b><span style="font-weight: bold;">To:</span></b> "type3@vwtype3.org" <type3@vwtype3.org> <br> <b><span style="font-weight: bold;">Sent:</span></b> Tuesday, August 23, 2016 4:02 PM<br> <b><span style="font-weight: bold;">Subject:</span></b> Re: [T3] Fuel pressure and elevation<br> </font> </div> <div class="y_msg_container" id="yui_3_16_0_ym19_1_1471989460895_6748"><br>I am no expert but could the altitude change being referred be not with fuel pressure but more with the readings?<br clear="none"><br clear="none">Depending on the type of gauge maybe its possible to change the accuracy of the gauge used depending on atmospheric or barometric pressure change?<br clear="none"><br clear="none"><br clear="none">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?<br clear="none"><br clear="none"><br clear="none">Like said I am no expert and just making a wild guess.<br clear="none"><br clear="none"><br clear="none">Daniel Du Vall<br clear="none"><br clear="none">________________________________<br clear="none">From: type3-vwtype3.org <<a shape="rect" ymailto="mailto:type3-vwtype3.org-bounces@lists.vwtype3.org" href="mailto:type3-vwtype3.org-bounces@lists.vwtype3.org">type3-vwtype3.org-bounces@lists.vwtype3.org</a>> on behalf of William J <<a shape="rect" ymailto="mailto:catnine09@dslextreme.com" href="mailto:catnine09@dslextreme.com" id="yui_3_16_0_ym19_1_1471989460895_7820">catnine09@dslextreme.com</a>><br clear="none">Sent: Tuesday, August 23, 2016 2:47:05 PM<br clear="none">To: <a shape="rect" ymailto="mailto:type3@vwtype3.org" href="mailto:type3@vwtype3.org">type3@vwtype3.org</a><br clear="none">Subject: Re: [T3] Fuel pressure and elevation<br clear="none"><br clear="none">I don't get how air pressure affects the FPR since it's a closed sealed<br clear="none">system . The only way air pressure may have an affect is on the fuel in the<br clear="none">tank which is vented. . It's not like the MPS where it is open to adjust the<br clear="none">pressure.. What affect does air pressure have , does it collapse the rubber<br clear="none">fuel lines . Wouldn't it also affect oil pressure it's still a pump and open<br clear="none">by the vent .<br clear="none"><br clear="none"><br clear="none">----- Original Message -----<br clear="none">From: "Jim Adney" <<a shape="rect" ymailto="mailto:jadney@VWType3.org" href="mailto:jadney@VWType3.org">jadney@VWType3.org</a>><br clear="none">To: <<a shape="rect" ymailto="mailto:type3@vwtype3.org" href="mailto:type3@vwtype3.org">type3@vwtype3.org</a>><br clear="none">Sent: Tuesday, August 23, 2016 10:47 AM<br clear="none">Subject: Re: [T3] Fuel pressure and elevation<br clear="none"><br clear="none"><br clear="none">On 22 Aug 2016 at 21:51, Daniel Nohejl wrote:<br clear="none"><br clear="none">> So a little over a month ago, we were in Yellowstone NP and had to<br clear="none">> diagnose a running problem. In the process, we checked the fuel<br clear="none">> pressure and discovered that it was only 25psi even though we´d set it<br clear="none">> to 29psi a few months prior. Clearly elevation has an effect on fuel<br clear="none">> pressure. We live more or less at sea level, but in Yellowstone we<br clear="none">> were anywhere from 6000TM-9000TM above sea level.<br clear="none"><br clear="none">I think you're asking a question that no one has ever asked before,<br clear="none">and it's extremely interesting.<br clear="none"><br clear="none">My short answer is that I believe the fuel pressure should be<br clear="none">adjusted (to 28 lb/sq in, in your case) at something close to sea<br clear="none">level and then just left alone. I'm sure Bosch never intended for<br clear="none">this to be something that had to be re-tweaked every time you drove<br clear="none">up or downhill.<br clear="none"><br clear="none">The longer answer requires that we look at how the pressure regulator<br clear="none">works. There's a diaphram inside that feels fuel pressure from one<br clear="none">side and spring force plus air pressure on the other side. When the<br clear="none">system is working, the forces from each side are balanced (equal.)<br clear="none">Because the outlet tube is so small, we can consider the effected<br clear="none">areas on each side to be equal. So we have 28 psi on one side and<br clear="none">14.7 psi plus spring force on the other side.<br clear="none"><br clear="none">I just measured a pressure regulator, and the diaphram has a diameter<br clear="none">of about 1.5" or an area of about 1.75 sq in. Thus the pressure<br clear="none">difference, ~14 lb/sq in, over 1.75 sq in means that the spring force<br clear="none">must be ~24 lbs.<br clear="none"><br clear="none">At, say, 5000 ft, the air pressure falls to ~12 lb/sq in. so a<br clear="none">reduction of ~2.7 lb/sq in. Since the only things that can vary are<br clear="none">the 2 pressures, and they work on effectively equal areas on the 2<br clear="none">sides of the diaphram, this means that the fuel pressure must also<br clear="none">drop by ~2.7 lb/sq in when driving at 5000 ft altitude.<br clear="none"><br clear="none">At 8000 ft, air pressure is ~11 lb/sq in, so the fuel pressure should<br clear="none">have dropped by ~3.7 lb/sq in.<br clear="none"><br clear="none">Our variations would be higher at higher elevations and lower at<br clear="none">lower levels.<br clear="none"><br clear="none">It's reasonable to assume that the folks at Bosch knew all this and<br clear="none">this was integral to the design of the FI system. It's not like<br clear="none">Germany was some flat, sea-level place where no one ever thought of<br clear="none">the existance of hills and mountains.<br clear="none"><br clear="none">Perhaps the larger question is what should we set our pressure to if<br clear="none">we are not at sea level. Sounds like we should decrease our settings<br clear="none">depending on our altitude (AND the current barometric pressure!) An<br clear="none">additional complication is that very few pressure gauges measure<br clear="none">absolute pressure. Our gauge readings are also somewhat dependent on<br clear="none">the ambient air pressure surrounding them. Given all the<br clear="none">complications, I recommend setting your fuel pressure when the car is<br clear="none">at the lowest elevation you can conveniently reach. Anything under<br clear="none">1500 ft should be reasonable.<br clear="none"><br clear="none">I DEFINITELY DO NOT recommend tweaking this as you drive into<br clear="none">different regions. It should be a set once and forget item.<br clear="none"><br clear="none">--<br clear="none">*******************************<br clear="none">Jim Adney, <a shape="rect" ymailto="mailto:jadney@vwtype3.org" href="mailto:jadney@vwtype3.org">jadney@vwtype3.org</a><br clear="none">Madison, Wisconsin, USA<br clear="none">*******************************<br clear="none"><br clear="none">_______________________________________________<br clear="none">VWType3.Org mailing list - <a shape="rect" ymailto="mailto:type3@vwtype3.org" href="mailto:type3@vwtype3.org">type3@vwtype3.org</a><br clear="none">To unsubscribe or change subscription options, visit:<br clear="none"><a shape="rect" href="http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org" target="_blank">http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org</a><br clear="none">If you need more help, contact: <a shape="rect" ymailto="mailto:gregm@vwtype3.org" href="mailto:gregm@vwtype3.org">gregm@vwtype3.org</a><br clear="none"><br clear="none"><br clear="none">---<br clear="none">This email has been checked for viruses by Avast antivirus software.<br clear="none"><a shape="rect" href="https://www.avast.com/antivirus" target="_blank">https://www.avast.com/antivirus</a><br clear="none"><br clear="none"><br clear="none">_______________________________________________<br clear="none">VWType3.Org mailing list - <a shape="rect" ymailto="mailto:type3@vwtype3.org" href="mailto:type3@vwtype3.org">type3@vwtype3.org</a><br clear="none">To unsubscribe or change subscription options, visit:<br clear="none"><a shape="rect" href="http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org" target="_blank">http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org</a><br clear="none">If you need more help, contact: <a shape="rect" ymailto="mailto:gregm@vwtype3.org" href="mailto:gregm@vwtype3.org">gregm@vwtype3.org</a><br clear="none">-------------- next part --------------<br clear="none">An HTML attachment was scrubbed...<br clear="none">URL: <<a shape="rect" href="http://lists.vwtype3.org/pipermail/type3-vwtype3.org/attachments/20160823/9fa7e93b/attachment.htm" target="_blank">http://lists.vwtype3.org/pipermail/type3-vwtype3.org/attachments/20160823/9fa7e93b/attachment.htm</a>><div class="yqt9535736036" id="yqtfd92680"><br clear="none">_______________________________________________<br clear="none">VWType3.Org mailing list - <a shape="rect" ymailto="mailto:type3@vwtype3.org" href="mailto:type3@vwtype3.org">type3@vwtype3.org</a><br clear="none">To unsubscribe or change subscription options, visit:<br clear="none"><a shape="rect" href="http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org" target="_blank">http://lists.vwtype3.org/listinfo.cgi/type3-vwtype3.org</a><br clear="none">If you need more help, contact: <a shape="rect" ymailto="mailto:gregm@vwtype3.org" href="mailto:gregm@vwtype3.org">gregm@vwtype3.org</a></div><br><br></div> </div> </div> </div></div></body></html>