[T3] Fuel pressure and elevation

Tue Aug 23 15:21:08 PDT 2016

I think the FPR is composed of a spring, a diaphragm and a valve. Fuel pressure exerts force to overcome the spring. Ambient air pressure is not a factor in this.
I would think the pressure gauge is also just a spring and a diaphragm.
Anyways, the injectors should deliver the same amount of fuel for the same duty-cycle (percentage of open time) as commanded by the ECU.
But, the amount of O2 for a given volume of air is less at altitude because of the change in air density.
I don't know how the stock FI system deals with this. 
Max Welton http://www.maxwelton2k.net/

      From: Daniel K. Du Vall <dduvall at 1peter4-10.org>
 To: "type3 at vwtype3.org" <type3 at vwtype3.org> 
 Sent: Tuesday, August 23, 2016 4:02 PM
 Subject: Re: [T3] Fuel pressure and elevation

I am no expert but could the altitude change being referred be not with fuel pressure but more with the readings?

Depending on the type of gauge maybe its possible to  change the accuracy of the gauge used depending on atmospheric or barometric pressure change?

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?

Like said I am no expert and just making a wild guess.

Daniel Du Vall

________________________________
From: type3-vwtype3.org <type3-vwtype3.org-bounces at lists.vwtype3.org> on behalf of William J <catnine09 at dslextreme.com>
Sent: Tuesday, August 23, 2016 2:47:05 PM
To: type3 at vwtype3.org
Subject: Re: [T3] Fuel pressure and elevation

I don't get how air pressure affects the FPR since it's a closed sealed
system . The only way air pressure may have an affect is on the fuel in the
tank which is vented. . It's not like the MPS where it is open to adjust the
pressure.. What affect does air pressure have , does it collapse the rubber
fuel lines . Wouldn't it also affect oil pressure it's still a pump and open
by the vent .

----- Original Message -----
From: "Jim Adney" <jadney at VWType3.org>
To: <type3 at vwtype3.org>
Sent: Tuesday, August 23, 2016 10:47 AM
Subject: Re: [T3] Fuel pressure and elevation

On 22 Aug 2016 at 21:51, Daniel Nohejl wrote:

> So a little over a month ago, we were in Yellowstone NP and had to
> diagnose a running problem. In the process, we checked the fuel
> pressure and discovered that it was only 25psi even though we´d set it
> to 29psi a few months prior. Clearly elevation has an effect on fuel
> pressure. We live more or less at sea level, but in Yellowstone we
> were anywhere from 6000TM-9000TM above sea level.

I think you're asking a question that no one has ever asked before,
and it's extremely interesting.

My short answer is that I believe the fuel pressure should be
adjusted (to 28 lb/sq in, in your case) at something close to sea
level and then just left alone. I'm sure Bosch never intended for
this to be something that had to be re-tweaked every time you drove
up or downhill.

The longer answer requires that we look at how the pressure regulator
works. There's a diaphram inside that feels fuel pressure from one
side and spring force plus air pressure on the other side. When the
system is working, the forces from each side are balanced (equal.)
Because the outlet tube is so small, we can consider the effected
areas on each side to be equal. So we have 28 psi on one side and
14.7 psi plus spring force on the other side.

I just measured a pressure regulator, and the diaphram has a diameter
of about 1.5" or an area of about 1.75 sq in. Thus the pressure
difference, ~14 lb/sq in, over 1.75 sq in means that the spring force
must be ~24 lbs.

At, say, 5000 ft, the air pressure falls to ~12 lb/sq in. so a
reduction of ~2.7 lb/sq in. Since the only things that can vary are
the 2 pressures, and they work on effectively equal areas on the 2
sides of the diaphram, this means that the fuel pressure must also
drop by ~2.7 lb/sq in when driving at 5000 ft altitude.

At 8000 ft, air pressure is ~11 lb/sq in, so the fuel pressure should
have dropped by ~3.7 lb/sq in.

Our variations would be higher at higher elevations and lower at
lower levels.

It's reasonable to assume that the folks at Bosch knew all this and
this was integral to the design of the FI system. It's not like
Germany was some flat, sea-level place where no one ever thought of
the existance of hills and mountains.

Perhaps the larger question is what should we set our pressure to if
we are not at sea level. Sounds like we should decrease our settings
depending on our altitude (AND the current barometric pressure!) An
additional complication is that very few pressure gauges measure
absolute pressure. Our gauge readings are also somewhat dependent on
the ambient air pressure surrounding them. Given all the
complications, I recommend setting your fuel pressure when the car is
at the lowest elevation you can conveniently reach. Anything under
1500 ft should be reasonable.

I DEFINITELY DO NOT recommend tweaking this as you drive into
different regions. It should be a set once and forget item.

--
*******************************
Jim Adney, jadney at vwtype3.org
Madison, Wisconsin, USA
*******************************

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