[TR] What is the correct way to measure fuel pressure

[Michael Porter]

On 10/21/2019 3:12 PM, Randall wrote:
> Don't forget to account for the difference in height between the pump and
> carbs.  The spec in my book is 2.7 psi measured at the pump; and if I've
> done my math right, each 3.75" rise will reduce that by 0.1 psi.  So
> assuming the carbs are about 2' above the fuel pump, you should see closer
> to 2 psi at the carb inlets.

Umm, this might confuse a couple of principles.  Yes, lifting a column 
of liquid by atmospheric pressure limits the column to 34 feet (of 
water, or the equivalent), and the figures above reflect that for the 
lower density of gasoline.  But, the fuel system is not pumping against 
atmosphere.  It's a closed system (at least when the float valve is 
closed), and with the gauge directly connected to the fuel line, it's 
definitely a closed system from pump outlet to gauge.  And, by 
definition, the gauge reads PSIG, i.e., pressure above atmospheric, 
regardless of ambient air pressure.

In a closed system, pressure equalizes at all points in the system, so, 
2.7 psi at the pump would be 2.7 psi at the gauge.

That said, taking the measurement might be time-dependent due to the 
construction of the pump.  It may build to the correct pressure and then 
quickly leak down past the pump inlet check valve.  Dunno about the 
construction of this pump.  But, leaving the ignition in the "on" 
position while taking the reading and keeping it on to determine if the 
pump is building to rated pressure, then shutting off and leaking down 
would be advisable. If peak pressure never gets above 1 psig, then the 
pump's defective and needs replacement or rebuilding.

>
> But I don't trust measuring tools from HF, especially used ones.  Check it
> with another gauge; or if you don't have one, build a simple manometer.  A
> 6' length of pipe or tubing could make a simple "go/no-go" gauge; hold it
> vertically and if the pump can push fuel out the top, you've got enough fuel
> pressure.  Or for temporary use, some cheap vinyl tubing from Home Depot
> tied to a piece of wood will let you read the height.  Vinyl isn't rated for
> fuel of course, but should do for a quick test.  (Don't use it for anything
> else afterwards, though.)  37.4" should equal 1 psi of gasoline (using
> numbers I got from Google).

I generally agree with the comment about measuring tools from HF, 
especially if one does not know the percent error at full travel. And, 
if one doesn't know the extra energy required to move the needle off 
zero (overcome inertia), it gets more complicated, especially for 
measurements on the low end of the scale.

As for the manometer, yes, a handy way to verify pressure, and in that 
case, because the tube is open to atmosphere, all the above comments 
about height of the column apply.

>
> Not at all relevant, but the book also calls the pump "high pressure", which
> I find amusing.

As compared to gravity feed, perhaps?  :)


Cheers.


-- 


Michael Porter
Roswell, NM


Never let anyone drive you crazy when you know it's within walking distance....