MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Measuring fuel pump pressure

I pulled off my little fuel Facet pump and rigged it up so I could measure the pressure it delivers since the label has fallen off and I don't know if it is a low or high (well, higher) pressure one.

I am just wondering of my test is valid. All I did was hook the pump up to two short hoses. one in a container of fuel and the other attached to a pressure gauge. The outlet pipe was only about 12 inches long.

I powered it up from a 12 volt battery (on long leads so it was well away from any vapour. I measured about 4 - 4.5 PSI on the gauge. I believe the pumps come in 3 - 4.5, 4.5 - 7 and 7 - 10 PSI versions.

I figure with such short hoses I was seeing the maximum I would expect from that pump and with the length of the fuel pipe on the car and hoses to the carbs it might drop a little. I figure then the pump is the lowest pressure one and should be OK to use then without needing a regulator?

Sound sensible? I'll put the pump back and once I remember to actually get some fuel to fill the tank I will measure the pressure at the carbs.

Simon
Simon Jansen

Hi, Simon - theoretically - since fuel is incompressible - static pressure on a horizontal line should be the same whether you had one foot, or 100 feet of line attached to the pump. And, if you had pressure gauges "T'd" into the line at multiple points, they would all read the same. Of course, uphill or downhill would show a big difference, and there would be another big difference if there were any flow of the fuel happening, as through an orfice, where friction within the line would come into play. This is called dynamic pressure, and will change according to many factors, such as the size of the tubing, the viscosity of the fluid, and even the ambient temperature.

Dave DuBois and Paul Hunt are two of the fuel delivery gurus who monitor these boards, and I'm sure they can explain this better than I can, but I think you'll be all right, though a little high on the pressure.

Cheers - - Alec
Alec Darnall

SSimon - I am not a fluid dynamics expert, so I will defer to Alec's explanation. I have never been entirely satisfied with the pressure measurements that I have taken on the pumps as they spike way up and then drop quite low as the float bowl that I am pumping into drains down to the point where the pump will cycle again. I have always just used the average of the low and high points on the readings. The Facet works a bit differently than the SU pumps do. In addition to check valves, the Facet pumps also have a bypass valve which sets the maximum pressure and is the reason that the Facet continue to click away after the float bowl is full and the needle valve cuts off the fuel flow. Based on this, I would have to say that the maximum pressure your pump is putting out is 4.5 psi. While that is a bit on the high side, you should be able to run with that with no problem. You can check this out by reinstalling the pump, turn on the ignition and watch for awhile. If you don't get any overflow from the float bowls, you should be ok. Cheers - Dave
David DuBois

The original workshop manual has several pages on pump performance and details of the SU and Churchill test jigs that were available. Both of these measure both flow and pressure. The SU does it with a go/nogo approach of a tube with a small hole in the side, the pump has to be capable of filling the tube to overflowing while sustaining the flow rate at the pressure height of the test hole. The Churchil rig was an all singing all dancing unit with flow and pressure metering over the full range of pump performance. There is a lot of other useful stuff about fault diagnosis and other perfomance checks (e.g. 20s between clicks with no flow). Let me know if this would be helpful to you.
Stan Best

Simon, when fuel is pushed through a line, there will be very little pressure loss - less than .25 psi by the time the fuel gets to the carbs, especially in a short vehicle like the B. You would be more likely to lose fuel volume due to restrictions in the fuel line such as bends and friction.
This issue becomes more problematic in, say, a long pickup truck with a front mounted fuel pump pulling the fuel. Your test is definately valid!
Jeff Schlemmer

This thread is getting too technical. Just connect it up and if you don't get fuel spilling over the carb overflow pipes with good needle valves fitted then you'll be fine.
Iain MacKintosh

"This thread is getting too technical. Just connect it up and if you don't get fuel spilling over the carb overflow pipes with good needle valves fitted then you'll be fine."

I'll do just that I think! I just need to get some fuel.

Where exactly are the overflow pipes meant to run? I'll just use some hose for the test but eventually I need the proper hard lines. The bottom end is attached to the sump I think? I guess as long as I rin them away from the hot exhaust it doesn't really matter?
Simon Jansen

Right, done! Thanks Iain, everything works fine. I put in 10L or so of petrol, a couple of old plastic pottles under the overflow pipes and switched on. RATTLE RATTLE RATTLE until the pump started sucking gas then it quietened down. I went and pulled the fuel pipe off the front carb and gas was getting through OK. I reconnected it and left it for a wee while and nothing came from the overflows so I think this pump will be fine as it is.

Only thing stopping me from starting the thing now it the wiring loom. I have a friend who visits NZ every summer from New York and this year he is down with his fiance (actualyl visiting her ein 3 hours). Would have been nice to at least be able to run the engine for him but it won't be this year.

Simon
Simon Jansen

Sorry to have missed your first post Simon. Great you've done it and all seems well so the next stage is to get the electrics connected and then fire it up.

Just opposite the carb fuel inlet connectiond there are a couple of short brass stubs and it is to these that you connect the copper overflow pipes by means of a short rubber sleeve and then lead then down to beyond the sump joint level clipping them to P Clips attached to the fuel pump blanking plate. You can see the top part of these connections in the Clausager but the part behind the manifolds if difficult to see
Iain MacKintosh

Simon, I think I w-mailed you some pics of my overflow pipes which while not being concours, are original.
Stan Best

Hi Stan, I never received the email. Could you please resend?

Iain, thanks for the tip about the fuel pump blanking plate. I thought they went further back down the engine. Mine has a little L bracket on the sump which is just behind the rear exhaust manifold pipe. I might move that forwards and drop the pipes down near the blanking plate instead.
Simon Jansen

Stan, I found them. I was looking at this a year ago and you sent me the pics then. Sorry, it was a while ago and it took me a while to remember! Those are exactly what I needed, thanks.

Simon
Simon Jansen

A year, that's scary! Glad you pressed on to the point wher you need them.
Stan Best

Stan, I found them. I was looking at this a year ago and you sent me the pics then. Sorry, it was a while ago and it took me a while to remember! Those are exactly what I needed, thanks. I've put the pipes in place now.

Simon
Simon Jansen

Oops, don't know what happened then. The pipes are on now.

I didn't think a year is too bad. It is almost 2 years to the day that I bought the car actually!
Simon Jansen

This thread was discussed between 04/02/2006 and 05/02/2006

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.