webbsite Posted March 19, 2008 Share Posted March 19, 2008 Hello people, I am running a single tank with twin external pumps to feed my 3.5 efi V8. The reason is to avoid fuel starvation on side slopes in the dunes. The tank is approximately 100cm wide x 30cm height x 22cm front to back, and it is fitted across the vehicle and has an outlet from the bottom at each end. The current connectivity is:- tank to pump, to tee piece, to large single filter, to fuel rail and return to tank. I am using a pair of Bosch external EFI pumps, part #05804764070 as used in BMW's Vauxhall and Opel etc cars. I am currently waiting for a pair of one way valves to arrive which will be fitted between the pumps and the tee piece to stop feed problems if one pump is trying to pump air, due to the tank outlet being exposed. In the opinion of the committee does this make sense or am I barking again. Quote Link to comment Share on other sites More sharing options...
Hybrid_From_Hell Posted March 19, 2008 Share Posted March 19, 2008 Hmmmm Couple of things to consider 1st - some pumps are 2Pullers" and some "Pushers", and they don't like it if they are wrong. The main in Tank pumps RR have are normally up to the job, but main people shove a external pump on the system instead, and find that they burn out as they are not "Primed" and don';t like sucking up to get pressure - if that makes sense, just make sure the pumps are the right type. A one way valve will help with drain back, but has no effect if the pump is pulling in air.... 2nd - If the pump(s) suck air for any amount of time you will loose rail pressure and as such the engine will spit and possible die, one way around this is to have a swirl cham,ber inside the tank, this should always be full - but will allow the fuel pump to always have a short term supply of fuel, irrespective of where the fuel is in the tank at that time... HTH Nige Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted March 19, 2008 Share Posted March 19, 2008 It makes sense but if the pump runs dry for any length of time you're going to burn it out, and as Nige said unless the pump is a "sucker" type or is mounted below the bottom of the tank it may also burn out. Very few EFi pumps will like drawing fuel up. As for the design - yes it will work although I'm not sure I'd do it that way. One pump, sitting in a "sump" of fuel or fed from a central sump in the tank that can be relied on to stay full even at an angle would be preferable to avoid sucking air. Run the return line to the sump and the fuel in the sump will last quite a while. You could also do a "clunk" (jet-ski / chainsaw) style pickup that flops about on a bit of flexi tube and follows the fuel as it sloshes about. You can also get fuel-tank foam for not much, a bit of that strategically placed will stop the fuel running away too quickly. My setup is just a normal-style 109 rear tank with a central pickup, to a carb-type fuel pump to suck the fuel up, via a swirl-pot into the EFi pump. The swirl pot is teed into the return from the fuel rail, with a small filter in the tank return. That causes a bit of back-pressure which force-feeds the swirl pot. It's never suffered fuel starvation yet. Quote Link to comment Share on other sites More sharing options...
webbsite Posted March 19, 2008 Author Share Posted March 19, 2008 HFH & FF, Thanks for the feedback. The pumps are below the tank so the lift pump criteria doesn't come into the picture. To have a sump in the centre of the tank I guess would mean having a new tank made, but is not really feasible as there is a crossmember right under the middle of the tank. Also I have very limited space to fit a decent sized swirl pot as it would be under the rear floor. The idea of the one way valves is to keep the fuel rail under pressure if one pump starts to suck air. Quote Link to comment Share on other sites More sharing options...
Night Train Posted March 19, 2008 Share Posted March 19, 2008 The sump doesn't have to in the centre of the tank, it doesn't have to be a part of the tank at all, just lower then the tank. It could be a small capacity tank that is tall and thin that is joined to the base of the main tank by a large tube. Each time the main tank sloshed fuel around some will run down the large pipe and into the sump and that is where you pick up from. The sump tank just needs to be big enough to last as long as you are sideways for. Depending on the size of the joining tube you might need a breather from the sump tank to allow it to fill quickly. Quote Link to comment Share on other sites More sharing options...
simonr Posted March 21, 2008 Share Posted March 21, 2008 The way this type of system usually works is to have a pair of low pressure pumps, one on each outlet which feed into a single swirl pot. The pot has a vent with a tee feeding back to both tanks so that excess fuel or air can be returned. Your high pressure pump (for efi) then takes fuel from the bottom of the pot and delivers to the engine. If the pot can be positioned sufficiently low, you can avoid the two low pressure pumps - just use a gravity feed. This is normally used when one is working with a pair of fuel tanks, but the principle and possibility of one being empty is the same. Si Quote Link to comment Share on other sites More sharing options...
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