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MG TD TF 1500 - Fuel pressues

For what it's worth..........
Finally got around to installing a pressure gauge on a TD, to measure an old SU type and a Facet cube fuel pump. First gauge is good quality 0-15psi glycerine filled presure gauge, but the old pump didn't budge the needle and the Facet registered about .1psi, yes, 1/10th psi. Another gauge showed the same disappointing results. Obviously, these bourdon tube/rack&pinion units are inaccurate down around 0-2psi. Don't have anything on hand in the 0-5psi range either.
OK, time to "see" the real pressure. Found a vinyl tube nearly 4' long and plugged that in and held the end up somewhat higher than 6' and the old diaphragm contraption ran gas up as high as 75" (and back down to 69" before kicking back on). Here are the other pertinent elevations: fuel level in tank ~22"; pumps 14" (under fuel tank in the rear); gauge 28"; single carb on supercharger 32". Finding 1 psi=33.75" column for gas, it looks like the old pump puts out around 1 3/4psi. After patching another vinyl hose onto the first one, I cut the test short as it was approaching 8' and about to run out the end.
and curiously, the gauge started registering 1 psi for the Facet. On today's outing, it was fascinating seeing the gauge read 1psi at idle, and drop back down to near zero on the run.

Jim - You are correct, it is exceedingly difficult to measure the pressure of an impulse pump into a Bourdon tube style gauge. I manage to get a fairly accurate measure of the pump with a Bourdon tube gauge on my test stand by having it connected in parallel with the fuel flow restricted to a set flow rate. I am not really hung up on the pressure of the pump as long as it is not excessively high (nearly impossible to be high since the pressure is determined by the tension of a volute spring pressing against the diaphragm). The important parameter in the pumps is the flow rate. I use a measured container where I run the pump wide open and time how long it takes to pump 1 pint. The low pressure pumps for the T series cars the maximum time is 46 seconds (1.3 pints per minute). The operating pressure is no more than 1.5 psi.

There is a gentleman in England who is modifying his test stand with a turbine that will read out in GPH. When he gets the system perfected, he is going to send me information on how to set one up. Cheers - Dave
D W DuBois

This thread was discussed on 22/06/2014

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