The MGA With An Attitude
DUAL FUEL PUMPS, in Parallel Or in Series -- FP-125

There is an issue (or discussion) that pops up occasionally related to fuel pumps plumbed in series or in parallel. In general, two pumps in parallel running at the same time can produce twice the volume at same pressure (if there is enough flow demand at the time). Two positive displacement pumps in series running at the same time would produce twice the pressure at the same volume. This condition would be a no-no for an MG running SU carburetors that need low pressure fuel delivery. However, SU reciprocating diaphragm fuel pumps are not positive displacement, and they will not double the pressure when plumbed in series. The spring behind the diaphragm in each pump sets a specific limit for output pressure. Two of these pumps in series will still produce the same original pressure. The flow volume will also not increase, except for a very small increase when there is very little back pressure with maximum demand (open output pipe). This is because the two pumps can at times tick out of sequence, and one can draw or push fuel through the other while the other is on the non-pressure intake (withdraw) stroke, and the first pump effectively supercharges the second one for quicker filling.

If you are using aftermarket electronic fuel pumps (like Facet or Airtex for instance) that are sealed with inlet and outlet ports but no atmospheric air around the pumping parts, then it's a different story. This type of pump will produce a fixed pressure differential from intake to outlet ports. If you plumb two of these in series and run them at the same time it will indeed double the pressure. That is a no-no for SU carburetors, as it may cause fuel to blow past the float valve when it should be closed. For this setup you need to wire the pumps separately with a toggle switch to select one or the other so the two pumps will never run at the same time.

In the prior article Bo Giersing reports of running two fuel pumps in series, one SU pump and one Facet pump. The trick here is to place the facet pump before the SU pump, so the spring loaded diaphragm of the SU pump can regulate the final output pressure. If the first pump in line is producing 3 psi, and the second (SU) pump will normally produce 2-psi, then the second pump may sit constantly with the diaphram under pressure in the withdrawn position, and it may never cycle. If you put the SU pump before the Facet pump, the Facet pump will take the 2-psi pressure from the SU output and boost it by about 3-psi for around 5-psi total (bad news), similar to using two Facet pumps in series.

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