[T3] Fuel pressure and elevation

[Jim Adney]

Okay, I said I needed to think about this some more, so here goes:

Recall the image of the pressure regulator with gas on one side and 
air + spring on the other. To keep everything in order I'll talk 
about each pressure in ABSOLUTE terms; that means we'll be talking 
about the pressures above zero rather than the pressure above 
atmospheric. Let's simplify things by imagining that the air pressure 
outside is 1 atm (atmosphere) and the fuel pressure is 3 atm (that's 
2 atmospheres ABOVE the ambient.)

When the air pressure drops to, for example, 1/2 atm, the fuel 
pressure regulator will regulate (balanced forces on the diaphram) 
when the fuel pressure drops the same amount. So the fuel pressure 
drops from 3 to 2.5 atm.

Now the air pressure is .5 atm and the fuel pressure is 2.5 atm, both 
absolute.

If we use a standard Bourdon tube pressure gauge, it will always 
measure the pressure DIFFERENCE between the measured fluid and the 
ambient. Since virtually all common pressure gauges are of the 
Bourdon tube design, this part is easy.  

So, when we started out at sea level the difference was:

3 - 1 = 2 atm.

When we ended up at altitude the difference is:

2.5 -.5 = 2 atm.

Note that this means that the pressure drop across the injectors 
stays constant regardless of altitude or barometric pressure.

So, Keith was right that his pressure gauge never changed, and Daniel 
was right that his pressure changes probably had more to do with a 
clogged filter than altitude, and I was wrong when I said that the 
changes in the regulator and the changes in the pressure gauge did 
not cancel out.

The really good news is that we DO NOT have to worry about any of 
these things when we set our fuel pressure. All you need is a gauge 
you can trust.

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