no the mech pump does not block the flow - at least not by much - see below...... (and NO - i dont have shares!

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The revolutionary designed Davies Craig Electric Water Pump (EWP) is a performance accessory that improves engine cooling and temperature control, giving increased power and/or improved fuel economy
The EWP can either assist the original mechanical pump, or replace it.
As a replacement it
outperforms the old pump by saving power and matching coolant flow to engine needs.
The EWP80, with its flow rate of up to 80 litres/min, is suitable for vehicles with an engine capacity of up to 5 litres
The recently launched EWP110 with its greater flow rate of up to 110 litres/min
is suitable for larger engines, and also smaller capacity more highly tuned engines,
4WD's and medium sized trucks
The EWP benefits classic cars where the
original cooling system is inadequate for modern day traffic, or for modern day fuels which burn hotter. And running the pump after shutdown minimises heat-soak problems.
The EWP is ideal for competition cars where racing engines at high revs need maximum cooling efficiency, as does
cooling the engine after the race when heat-soak can do its worst.
There are three installation options available:
- Firstly, the EWP80 can be installed as a complete replacement for the existing mechanically driven pump, in which case an EWP Controller (separately available) should also be fitted. The existing pump is deactivated or, in many cases may be removed, in which case belt runs are shorter. The EWP Controller governs water pump speed to permit a wide range of temperature settings so that installed benefits may be varied between power gain and fuel economy.
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Secondly, the EWP80 may be fitted into the existing cooling system wired to either a manual or a thermal switch to provide a very significant boost to the original system. The thermal switch we recommend for this purpose is the mechanical switch, part number 0401.
- Thirdly, the pump can be fitted for continuous running, either as the sole pump or to assist the mechanical pump. This method of control is most suited to competition engines, running sustained high revs. Normal use running may result in engine temperatures below ideal operating levels. This option is also useful for cars running on LPG, which tend to run at higher operating temperatures.
It is important that, in all cases, care should be taken by the installer to select the most appropriate option for each application. Maw Solutions Ltd are happy to advise!
The ‘Smart’ Controller, which is used in conjunction with the EWP80, electronically senses engine heat and adjusts the rate of coolant flow accordingly, even after engine shut down.
see also
http://www.mawsolutions.com/documents/ewpinfo.PDF