[Healeys] Why carry spares

[Oudesluys]

Bob,

The pressure that a pump delivers is the difference in inlet and outlet 
pressure, say 3psi, so pump one will raise the pressure from 0 to 3 psi 
and pump two from 3 to 6psi. So if the pumps in series are both 
energised/running the end pressure is 6psi. If only one pump is running 
the pressure will be slightly less than 3psi as it has to overcome the 
resistance of the valves in the second pump.

Kees Oudesluijs
NL



Bob Spidell schreef op 19-3-2014 17:01:
> OK, I'm not a theoretical physicist, and don't play one on TV, and we're not talking electronics so I don't see how running two pumps in series doubles the output pressure. AFAIK, the output pressure of an SU pump is determined by the strength of the spring that pushes the diaphragm (presuming the solenoid is strong enough to compress an even stronger spring). There may be some internal 'regulation'--there is a small hole between the input and output chambers on BN1/2 pumps that may provide a pressure limit by bypass--but no real pressure regulation. So, if the spring is capable of producing, say, 3PSI when the solenoid is switched off, then the output to the second pump will, at most, push the spring back some but it wouldn't add to the 3PSI.
>
> IOW, a spring capable of pushing a diaphragm at 3PSI won't be able to push at 6PSI, just because the incoming pressure is greater. Anyone actually measured the pressure of two SUs in series?
>
> Bob
>
> ----- Original Message -----
>
>
>
> Two pumps in series might not be suitable for a racing Healey, but they have
> worked just fine on my street BJ8 without any problems for a long time.
> It's just simpler to plumb them in series than in parallel, in my opinion.
> If you have two pumps in parallel with separate switches, then you can run
> both pumps at the same time and get twice the flow rate at the same output
> pressure. Running two pumps in series at the same time will provide the
> same flow at double the output pressure - not good for the carbs. So, for
> pumps in series there should be only one switch to prevent running both
> pumps at the same time.
>
>
>
> Steve Byers
>
> HBJ8L/36666
>
> BJ8 Registry
>
> AHCA Delegate at Large
>
> Havelock, NC USA
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