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fuel pump wiring
pekwah1 - 6/2/14 at 06:13 PM

Hi Guys,

I'm going to be using a swirl pot with a HP and LP pump in my car.

The HP pump is going to be controlled by the ECU which is obvious.

For the LP pump, i was planning to control this via a relay running off an ignition live feed so as soon as i turn on the ignition, it will effectively be running all the time. Would this be the preferable way to wire up?

Regards,
Andy

owelly - 6/2/14 at 06:18 PM

I have my LP and HP pump, runing off two seperate relays, but switched from the same ECU pump feed.

big_wasa - 6/2/14 at 06:58 PM

I would go as above or even both on one relay will be well with in the limits of a relay

pekwah1 - 6/2/14 at 07:02 PM

so is it better not to run the pump continuous but run off the ECU?
What if the flow rate from the HP pump is more than the LP, surely there's a risk of the swirl pot running dry?

big_wasa - 6/2/14 at 07:19 PM

Check the rating of the lift pump, it will be way more than your engine will ever need. So once everything is primed it will look after itself.

Smoking Frog - 6/2/14 at 07:19 PM

I did as suggested above. The LP pump shifts more fuel than the HP pump. That's why you need a return from the swirl pot to the tank.

SteveWallace - 6/2/14 at 07:49 PM

I run both off the one relay which is triggered via the ECU. I didn't have to turn the engine over for very long for it to prime and, as said above the flow rates are such that the swirl pot always overflows back into the tank.

When you think about it, once the whole system is full of fuel, all that the LP pump has to do is to is to replace the petrol that's burnt by the engine as the excess from the injector rail in the HP pump loop also returns to the swirl pot. In practice, the LP pump is shifting much more fuel than that.

Paul Turner - 7/2/14 at 09:03 AM

quote:
Originally posted by owelly
I have my LP and HP pump, runing off two seperate relays, but switched from the same ECU pump feed.


Exactly what I did. One small difference was my loom already had a fuel pump relay in it so in truth the 2 new relays I fitted were switched by the loom relay.

Paul Turner - 7/2/14 at 09:07 AM

quote:
Originally posted by Smoking Frog
That's why you need a return from the swirl pot to the tank.


The return to the tank only needs to be very small, it only there to bleed any air trapped in the swirlpot. My return pipe has a 1.5 mm restrictor in it and has worked perfectly for 10 years. Mate built his without a restrictor, sounded like someone pissing into the fuel tank all the time and the LP pump was runing constantly at fullchat. My LP pump runs just like it did when I had carbs in the car.

SteveWallace - 7/2/14 at 10:44 AM



My return pipe has a 1.5 mm restrictor in it and has worked perfectly for 10 years. Mate built his without a restrictor, sounded like someone pissing into the fuel tank all the time and the LP pump was runing constantly at fullchat.


That sounds like a good idea. Just so that I understand, the restrictor is creating a back pressure that slows the LP pump down so that you don't get as much flow back into the tank - freely lets air through but not much liquid? Where did you get the restrictor from?

britishtrident - 7/2/14 at 11:45 AM

You also would be advised to fit an inertia switch aka "bump switch" to shut off the power to the relay coil in the event of a major accident.

For a restrictor all you need is a short length of brake pipe or similar inside your return line, the return line can also be smaller bore (say 6mm) than the feed.

[Edited on 7/2/14 by britishtrident]

Paul Turner - 7/2/14 at 01:24 PM

quote:
Originally posted by SteveWallace

That sounds like a good idea. Just so that I understand, the restrictor is creating a back pressure that slows the LP pump down so that you don't get as much flow back into the tank - freely lets air through but not much liquid? Where did you get the restrictor from?


Mate turned me the restrictor from a piece of ally bar of a suitable diameter to fit into my retun fulepipe (1/4" diameter), its about 2" long in total. There is a 1.5mm hole drilled down the centre.

Smoking Frog - 7/2/14 at 01:38 PM

quote:

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