[T3] Fuel pressure and elevation

[Jim Adney]

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.

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