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MG MGB GT V8 Factory Originals Technical - Fuel pump cutoff ... question ...

It is my understanding that, unlike SUs (where you can hear the clicking slow and then stop), modern solid state electric fuel pumps do not stop when they have hit the target pressure ... they just keep pumping. I would like to pull out my SU and put in something more powerful, as the SU isn't keeping up with the V8, that's pretty clear. On the new pump, though, do I need to plumb in one of those switches that cuts power to the fuel pump when there's no oil pressure? ... otherwise, if/when the motor stalls, won't the pump just flood the motor?

Please comment!

There is also an inertia switch that you can install that cuts power to the FP in the event of an impact to the car, etc. Moss sells one called a "First Inertia" switch. I don't know if that's more or less useful than an oil pressure sensing switch. I also don't know how good it would be on bumpy surfaces, especially if your suspension is fairly stiff. But one or the other, or maybe both, would probably be a great idea. I guess the OP sensing switch would be better because then you wouldn't be pumping gas in there if the car's just sitting with the ignition on. Interesting issue.

Depends on whether you use carbs or fuel injection, for the most part. My experience of carbs is that they have a float chamber with a valve that shuts off fuel ingress when the appropriate level is reached, and lets more fuel in according to how much is being sucked out through the jets. Anything else would mean that the engine would be flooded at anything other than full-throttle, let alone ignition on but engine stopped for whatever reason.

FI systems in my experience use a recirculatory system that pumps a high volume around a closed circuit, the injectors taking out as much as they need and the rest returning to the tank. These have to have an inertia switch as they can pump out vast quantities in the event of a fuel line rupture in an accident.

An SU pump, by comparison, will pump far less in a pipe-rupture situation but North America still required them to have at least one if not two cut-off switches.

A non-OE pump may well pump more in the same situation, but that would probably mean a higher presure at the carbs, which if SU, could well overwhelm the float valves which *will* cause flooding, but out of the carbs onto the floor, which in itself is a serious fire hazard.

Paul Hunt

The solid state pumps continue running, but do not produce flow beyond their cut-off pressure, which should be below what your carb float valves can handle. If you install an oil pressure sensing switch, you may wish to have over-ride capabilities in the event you run out of fuel to aid in restarting. I believe the oil pressure switch is usually bypassed when the ignition switch is in the start position, but it could still requre several seconds of cranking to get fuel to the engine. The purpose of the inertia switch is to prevent fuel from pumping out of damaged lines in the event of an accident. The bottom line is, I don't think you will need the oil pressure sense switch.
George B.

You might consider using a pair of S.U. pumps ala Mark II MGTD's. They should have enough flow for the V8 plus give a good limp-home capability in the event of the failure of one. Just a thought.
George B.

A Holley or AFB (Edelbrock) carbs do not need more than 4 1/2 lbs. of fuel pressure at wide-open throttle.
Cut off SW's are used to protect against pumping fuel in an event the car is in a accident or if it looses oil pressure, the cut off valve it will shot off the fuel pump to stop or minimize damage to the engine.
Make sure floats are set correctly in the carb.
Bill Guzman

This thread was discussed between 22/01/2001 and 23/01/2001

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