jeffw
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| posted on 7/4/09 at 08:48 PM |
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Swirl Pot to tank return
My Phoenix is off on it's holidays getting a Suzuki TB set up fitted. The question has arisen as to if it needs a return from the Swirl Pot to
the tank or not.
To fit an inlet to the tank will involve dropping the rear axle and the dropping the tank or removing the bodywork....both of which are large jobs so
the answer to the question will cost me money if it is required.
What do we think?
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tomgregory2000
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| posted on 7/4/09 at 09:12 PM |
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What about a return into the fuel filler hose?
I coud be wrong but it would be cheap
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mark chandler
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| posted on 7/4/09 at 11:07 PM |
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I tried without a return and found the swirl pot emptied itself after a while so ran in a return, I have a see through filter before the HP pump and
this had always contained a bit of air + I could see bubbles coming in.
When I dug the car out of hibernation in February the HP fuel pump was leaking a bit of fuel so I changed it for a new one, the see through filter is
now always full with no air bubbles in sight so I suspect air entering the system was my downfall and the return was not really required.
I would suggest you try without in the first instance, in theory the engine is always drawing fuel and the HP pump is just circulating so not strictly
required, in my case I was adding air at a higher pressure than the low pump could provide fuel so eventually emptied the pot.
Regards Mark
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DaveFJ
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| posted on 8/4/09 at 08:21 AM |
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if i read your question right then yes! dependiong on how your swirl pot is filled......
my swirl pot is filled by a LP pump and in turn feeds the HP pump...
if I didnt have an overflow return on the swirl pot back to the tank what happens when the pot is full???
if your swirl pot is gravity fed then i assume it will just back up through the feed from the tank....
Dave
"In Support of Help the Heroes" - Always
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jeffw
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| posted on 8/4/09 at 10:26 AM |
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If you use that logic how does a Carb system with a LP pump work with no return ?
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DaveFJ
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| posted on 8/4/09 at 10:35 AM |
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fair comment...
i think that you also have to take into account the amount of fuel coming back from the hp side which should also be feeding the swirl pot so
potentially you have far more fuel going into the swirl pot than is is just coming from the lp pump.....
of course i could be talking total crap... just what i picked up when designing my system... 
Dave
"In Support of Help the Heroes" - Always
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Doofus
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| posted on 8/4/09 at 10:36 AM |
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quote:
Originally posted by jeffw
If you use that logic how does a Carb system with a LP pump work with no return ?
The float chamber vents the air to atmos until the inlet valve stops the fuel rising.
In the same way you could put a float and inlet valve in a swirl pot but that seems like more work than a return to the main tank.
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paulf
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| posted on 8/4/09 at 01:42 PM |
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The point of a swirl pot is to keep a head of fuel that is not affected by movement of the car.If there is no return to the tank it could be partially
filled with air or fuel vapour and would not serve its purpose correctly.
I have a return consisting of 8mm fuel hose with a .8mm restrictor in it to allow air out but give a resistance against the low pressure pump.You
could connect this into the filler neck if the tank is difficult to access.
Paul.
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jeffw
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| posted on 8/4/09 at 04:18 PM |
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This actually quite interesting...
In a LP Carb system the fuel pump works all the time to feed the carbs (dependant on the pressure regulator). I presume when the pressure rises
beyond what is set on the regulator the fuel pump can't increase the pressure in the carbs and hence no additional flow.
With a swirl pot and a return we remove that restriction and the LP pump will work all the time pumping fuel round the circuit. The HP pump then
takes the fuel from the swirl pot and feeds the injectors dependant on the HP pressure regulator in the circuit.
If the above is correct what is to stop me using the existing lp pressure regulator and restricting the flow to the swirl pot?
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MikeRJ
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| posted on 8/4/09 at 04:50 PM |
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quote:
Originally posted by jeffw
If the above is correct what is to stop me using the existing lp pressure regulator and restricting the flow to the swirl pot?
As Paul has said; how do you ensure the swirl pot is actually full of fuel rather than mostly air? The low pressure pump will be trying to pump fuel
in at a greater rate than it's being removed by the injectors, so any air in the swirl pot will tend to remain there with out a return to the
tank.
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SPYDER
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| posted on 9/4/09 at 08:24 PM |
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It sounds like you will need a return from your swirl pot. Run it into the fuel filler connection as suggested by TOMGREGORY200.
I did this years ago on a fuel injected GT6 with no problems. Ran like that for years.
Use the existing LP pump to fill the swirl pot, no regulator required. The LP pump will run like crazy though without any resistance. I like the idea
of a small restrictor in the return as per the earlier post. This will give the LP pump something to work against but will necessarily create a
little pressure in the swirl pot. Will the HP pump tolerate this slight pressure? I suspect it will.
It might raise the fuel rail pressure by a corresponding amount , I think. Only a couple of PSI at most. Maybe less than one psi. Am I thinking
straight on this one?
If the swirl pot is higher than the main tank put the fuel entry pipe above half way up the side of it so it can't back siphon itself empty.
And consider wiring the LP pump into the bike loom standard HP pump circuit with the necessary relay. It will then come under ECU control. Both
pumps will be switched off if the engine stops during an accident. I would also recommend some sort of inertia switch incase the engine doesn't
stall. The thought of TWO fuel pumps still merrily blasting away with the car on it's roof.......
Geoff.
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andrew-theasby
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| posted on 10/4/09 at 10:04 AM |
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How about cutting the line from lp pump to swirl pot and inserting a piece of 8mm bar with a 2mm (maybe more?) hole in it to give the pump some
resistance, then a 0.8mm restrictor coming out of the swirl pot to slow the pump down even more, then you would have less pressure in the swirl pot
(obviously only when its all running, when engines off, it will always pump up to the pressure of the pump cutout, just slower as the flow would be
restricted) you only need to supply the fuel pump with enough flow for the maximum consumption for your engine. I bought the lowest pressure facet i
could find which i think cuts out at 3.5psi if i remember correctly. The restrictor at the top of the swirl pot is only there to let the air bleed
off quickly it hardly has to pass any fuel, so it cuts the pump out but a return line is still needed.
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MikeRJ
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| posted on 10/4/09 at 01:27 PM |
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What's the point in restricting the low pressure pump? All it's doing is circulating the fuel between the tank and swirl pot, which is
desirable as it keeps the fuel cooler. Restricting it will just make the pump work harder and slow the flow down.
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andrew-theasby
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| posted on 10/4/09 at 03:22 PM |
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It wont make it work harder as they are designed to work at pressure. They slow down when there is back pressure, to virtually stopped when they reach
the pressure your regulator is set at. A completely unrestricted pump will pump as fast as it can permanently and wear out quicker.
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matt_gsxr
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| posted on 19/6/09 at 09:53 PM |
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Interesting question whether a low pressure pump will be doing more work/wearing out faster if running fast at no pressure or slow at higher
pressure.
One test on working hard would be to measure the amps.
My gut feeling is that restricting the return flow would make the pump behave more like it is designed for.
Interesting discussion, I am in the same position and am trying to work out the best option.
Matt
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paulf
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| posted on 19/6/09 at 10:06 PM |
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My swirl pot is fed by a facet red top pump , I have a .6mm restrictor in the return from the swirl pot and it still pumps quite rapidly but has been
ok for the last few thousand miles.The red top is a higher pressure rated pump and may be over the top for feeding a swirl pot but at least I have a
good flow of fuel.
Paul.
quote:
Originally posted by matt_gsxr
Interesting question whether a low pressure pump will be doing more work/wearing out faster if running fast at no pressure or slow at higher
pressure.
One test on working hard would be to measure the amps.
My gut feeling is that restricting the return flow would make the pump behave more like it is designed for.
Interesting discussion, I am in the same position and am trying to work out the best option.
Matt
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DaveFJ
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| posted on 22/6/09 at 08:27 AM |
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this was a question that no-one seemed to want to answer for me!...
I have a facet (silvertop i think) feeding my swirl pot and unrestricted return to tank... I was worried that the pump may not supply enough to feed
the hp pump...
but reading the above i take it that isnt a problem? (btw my facet has to lift the fuel from the bottom of the tank through a dip tube type
arrangement, is that bad?)
I was thinking of trying to fit some kind of fuel level guage to my swirl pot to try and monitor the level for diagnosis.....
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fatbaldbloke
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| posted on 22/6/09 at 06:28 PM |
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I've written some fairly extensive notes on my website about converting from carbs to bike TB's including stuff on fuel pumps, swirl pots,
return lines, etc. Might be of interest.
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