Board logo

LP Pump feeding HP Pump query
keybaud - 1/6/12 at 11:01 PM

I have a Facet Competition pump feeding the carbs at the moment, but I'm going to upgrade to fuel injection. Is it possible to use the Low Pressure Pump to feed a High Pressure Pump for fuel injection without a swirl pot?

blakep82 - 1/6/12 at 11:42 PM

if there's no swirl pot, theres no point in having 2 pumps really. just use the HP pump from the tank.
if you've got a small sump in the bottom of the tank, then that would be even better

a LP pump would only really be used to keep a swirl pot full (kinda like filling a 2nd smaller fuel tank) and the HP pump send fuel from the swirl pot to the fuel rail

[Edited on 1/6/12 by blakep82]

mark chandler - 2/6/12 at 07:30 AM

When I dumped carbs I used the existing LP pump to fill a swirl pot up front.

You still need a fuel return on the swirl pot to the tank so you do not save any extra plumbing, it just clicks away gently and works a treat.

keybaud - 3/6/12 at 05:11 PM

Thanks for the replies, but I'm trying to see if I can do it without a swirl pump and without having to mount the HP pump below the tank. I know the Bosch 044 HP pump is meant to be fed by another pump for the best performance and I assume it uses a LP pump. I don't have a HP pump yet, but if the Bosch 044 logic is correct, I could just fit a HP pump in the engine bay fed from the existing LP pump and just sort out the return to the tank; however, I'd like to confirm that this would work before I try it.

lotusmadandy - 3/6/12 at 06:16 PM

No that wont work. As already said,you will need
a swirl pot fed by your lp pump. Then a hp pump
feeding off your swirl pot.
Or do what i have done and fit a hp pump in place of
the lp pump.

Andy

keybaud - 3/6/12 at 10:49 PM

quote:
Originally posted by lotusmadandy
No that wont work.

Andy


Why not?

My LP pump is rated at 128 l/hr. A fuel rate calculator reckons I need approx 64 l/hr at max power. A HP pump will pressurise the fuel higher, but will not increase the rate at which the fuel is used. Any excess l/hr is returned to the tank via the regulator.

A swirl pot fed by a LP pump will not increase the rate of fuel flow to the HP pump over a LP pump on its own. HP pumps are not good at pulling, which is why they usually need to be below the tank, so they are gravity fed, or fed by a pressurised source (swirl pot and LP pump). Gravity feeding will be even lower rate than a LP pump.

A swirl pot is a stored volume of fuel, so you could call the length of hose between the LP and HP pumps a tiny swirl pot. Surely, the LP pump feeds a swirl pot with fuel at the same pressure, or higher, than it would feed the HP pump.

I know I don't need a swirl pot to run fuel injection. They just help if the fuel in the tank is low and you have no baffles and you corner hard causing the fuel to move away from the outlet. Personally, they just add weight and complexity to a car that isn't intended to be used with low fuel levels on a track.

The maths implies that you can feed a HP pump with a LP pump, hence my original question.

[Edited on 3/6/12 by keybaud]

britishtrident - 4/6/12 at 07:28 AM

You need a swirl pot or a sump. The swirl pot bleeds air out the fuel before it gets to the HP pump.
The problem is the shape of the standard lotus Seven style tank.
Apart from the fact air in the system will cause a missfire HP pumps don't take well to running dry.

rusty nuts - 4/6/12 at 07:42 AM

quote:
Originally posted by keybaud
quote:
Originally posted by lotusmadandy
No that wont work.

Andy [/quote

The maths implies that you can feed a HP pump with a LP pump, hence my original question.

[Edited on 3/6/12 by keybaud]


If the maths were the only part of the equation that would be the case but it isn't, you need to bleed the air out of the fuel as BT states. If a swirl pot wasn't needed then nobody would be using them but an awful lot still are.I wonder why?

britishtrident - 4/6/12 at 10:43 AM

Because of the shape of the tank spirited anything much below 1/2 tank will draw some air into the pump, get below 1/3 tank and you pump a lot of air.

cliftyhanger - 4/6/12 at 12:45 PM

Not only that, but the HP pump will want a lot more fuel than the engine uses. 044 is 300+LPH, so way more than the LP pump will supply. Even if your engine uses 60LPH (very unlikely continuously) the other 240L gets returned to the fuel tank or swirl pot.

The idea is that a swirl pot acts like a buffer, and the LP pump is only needed to keep it topped up, the rest is "recycled"

That is my take on it anyway.

EDIT

A better solution is an in-tank pump, plenty to choose from, many have built-in swirl pots.

[Edited on 4/6/12 by cliftyhanger]

britishtrident - 5/6/12 at 01:33 PM

Looking back on the early posts in this thread I should clear up a misconception HP pumps will happily pull fuel on the inlet side but only when already primed with fuel but they are not designed to create a suction head when dry.

To have a initial positive head on the suction side the hp pump inlet should be below the fuel level or be supplied from a pump fed swirlpot/header tank with return line at a higher level than the HP pump.

[Edited on 5/6/12 by britishtrident]

keybaud - 5/6/12 at 03:04 PM

quote:
Originally posted by britishtrident
Looking back on the early posts in this thread I should clear up a misconception HP pumps will happily pull fuel on the inlet side but only when already primed with fuel but they are not designed to create a suction head when dry.

To have a initial positive head on the suction side the hp pump inlet should be below the fuel level or be supplied from a pump fed swirlpot/header tank with return line at a higher level than the HP pump.

[Edited on 5/6/12 by britishtrident]


Hence my question, can they be primed with just a LP pump?

britishtrident - 5/6/12 at 03:21 PM

Hence what you have already been told
Connecting and lp pump outlet to an hp pump inlet is pointless. The hp pump dosen't need the lp pump, the inlet of the hp pump just needs to be sligtly lower than the fuel level to give the fuel supplied at inlet a small positive head of pressure and no slugs of air in the fuel.

With a Seven style tank either a sump in the tank with the HP pump mounted very low or a swirl pot fed by lp pump with hp pump below the the swirl pot.

motorcycle_mayhem - 5/6/12 at 04:07 PM

If the car isn't going around a track, then a tube into an open bucket will happily supply the HP pump, forget the LP. If you're going around corners, forget sumping a main tank, you'll need the LP feed to a swirl pot and then the HP feed from that to the injectors.

My experience, worth nothing, is based upon my Westfield. Back in the days when Justin Bieber was in diapers, Westfield were peddling the Megabusa with the usual '7' style tank, but with a sump at one end feeding the HP pump..... when the tank went down to about 1/3 full, the pump would starve on right-handers. Discovering this entering the 'Mountain' climb at Cadwell proved a very expensive experience.