[T3] Fuel pressure and elevation

[Jim Adney]

On 23 Aug 2016 at 22:21, Max Welton wrote:

> 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. 

The spring only about half the force needed in this case. The other 
half is provided by the outside air pressure. So, in this case, 
ambient air pressure IS a factor in this.

> I would think the pressure gauge is also just a spring and a diaphragm.

Well, sort of, but there's really no spring in a typical gauge. If 
you put a typical pressure gauge in a pressure chamber, it will also 
read just the difference between the chamber pressure and the 
"measured" pressure. It will show the pressure difference.

That certainly makes this problem harder, since you're trying to 
compensate for both the change in the gauge and the change in the 
fuel pressure regulator. The fact that there is a spring in the 
regulator but there isn't in the gauge means that the two errors 
don't cancel out.

I'll have to think about this some more....

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