Re: [DML] fuel sending units

[Nick Kemp <nkemp@xxxxxxxx>]

Lots of interesting discussion about the new DMCH unit.  I have played 
with some GM units and they are fascinating (from a fuel sump 
perspective).  You can be assured that GM put a lot of effort into the unit.

Now if you are looking for a flaw here is one potential one.

To get the most fuel from the tank, the pickup must be on the bottom of 
the tank.  Regardless of the sump/ baffle design, if the fuel pickup is 
a 1/2 inch above the tank bottom you cannot get to the bottom 1/2 inch 
of gas.  When I converted to a straight down model (a long time ago), 
the problem was figuring out the correct dimension so that the pickup 
sat on the tank bottom.

The new design picks up from inside the sump.  From the models I have, I 
estimate that the pickup about 1/2" off the bottom of the tank (it could 
be as much as 3/4").  If my calculations are correct, for every square 
foot at 1/2" deep you loose about a .311 gallon (US gallon wet) capacity.

If someone can measure the width and length (not height) of the tank, in 
square inches/feet the tank area will be about equal to 1/2*width * 
length.  As such we can estimate the lost capacity.  If for example the 
tank is 3 square feet, then we could loose about a gallon of capacity.  
If the low fuel light comes on at 2 gallons remaining, then you really 
only have one available to the pump.

 From looking at the module designs that GM offers, there is a number 
that use a pickup screen that rests on the tank bottom.  Some use a 
pickup inside the sump which as I understand it is the DMCH design (per 
James).  The ones with the pickup screens outside the sump on the tank 
bottom would be preferred from a tank capacity standpoint.  Our tanks 
are not that big to begin with, every gallon helps.

Nick



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