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MG MGB GT V8 Factory Originals Technical - Why do i need a swirl pot in fuel tank.

I amcurrently converting from a Hulley carb to Rover SD1 fuel injection on my 77 215 V8 conversion. Why do i need a swirl pot . What does it do. Thanks in advance.
David Cousins

You either need a swirl pot or a pressure return line fromt the fuel rail back to the tank. A fuel injection system utilizes higher pressures and requires a constant flow pump which must have pressure regulation which bypasses any excess fuel and pressure back to the tank. A swirl pot just allows for the extra fuel to be returned to the swirl pot instead of the tank. In most cases it's easier to plumb in than a return line.
Bill Young

Use the early model tank with the built in pick up line and the late model sender that has the pickup built right into it. That combination gives you a send and return line to your fuel tank without having to add anything extra.

Been there, done that.

p.s. Why do you want the SD1 fuel injection? It's problematic and VERY VERY old. I'd go with the range rover hotwire fuel injection and machine the plenum so it fits under the hood. It's a much more modern system. I bought a full used system (computer and all) for $350.00 several years ago, so I know it's available cheaply.

Can this swirl pot be bought commercially or does it have to be fabricated?
CW Strong

First, the purpose of a swirl tank or swirl pot is to maintain a supply of fuel for the high pressure pump. This is important when the fuel supply is low and you are doing hard cornering.Fuel starvation burns pistons.

If you never expect to experience these crcumstances, you do not need a swirl pot.

Regardless of whether you have a swirl pot, you will need a fuel return line to return unused fuel to the main tank. This is because the FI system requires fuel at approximately 40 lbs pressure & usually has a high pressure pump producing 70-100 lbs of pressure. The pressure regulator, which is part of the FI system bleeds off the excess into a return line. At idle, lots of return. At top end, not so much, and at this point, you need that high pressure pump or you will fuel starve the engine & probably burn pistons.

There was a long thread about swirl pots a few months ago, so check the archives.

Justin's soluion is very workable. Having a new tank modified is not expensive, & will give you a large feed line & a swirl pot built in. Other solutions involve a swirl pot outside of the fuel tank & a second low pressure pump, or a very large filter, doubling as a swirl pot.
Jim Stuart

I do know about the need for a fuel return line and have the parts to fabricate that. In addition to the fuel return line i'm planning to put a big filter before the high pressure pump. Hopefully this obviates the need for a swirl pot.
David Cousins

I had a 1980 SD1 with the fuel injected V8 sitting in my yard for a while. I cut open the fuel tank:
Dana Wilson


Look for a filter for a large diesel truck. Some of these are over 1 qt capacity.
Jim Stuart

The large filter works (go along with Jim's advice) - the level of the swirl pot filter is critical and if you can not get it to top up with gravity you might consider keeping your OE SU pump to top it up.

I can only write with experience of the Bosch fuel injection pump which relies on the fuel for lubrication - if it runs dry, it will very quickly pack up. It pays to get the arrangement right first time !


You can use the 1 qt filter to supply the reserve fuel and you can either mount it low to fill by gravity, or you can use a low pressure pump to keep it filled. There are many generic pumps for ~$30 to do that job, some as small as a 2" cube with fittings on the ends. Pick one for a V8 application. The filter and high pressure pump are often mounted in the unused battery location. A plastic battery tub is good here but should have adequate vents and drains, and plumbing the lines to the opposite side of the car and back can be tricky. A vapor lock in the filter can occur, so the system should be designed to remove any vapor. On a separate surge tank this can be done by running a vent line back to the main tank, with an orifice sized to quickly vent vapor but restrict liquid flow in order to reduce the duty cycle of the low pressure pump. With the large filter, if it is vented on fill up and plumbed to take fuel from the top so that any vapor is removed, any vapor would then go through the fuel rail and there would not be any fuel reserve, which sort of defeats the purpose. Instead, the other option here is to run a vent line from the top of the filter and draw fuel from the bottom of it, and this could work particularly well on a gravity feed system if the tank was high enough, which is questionable with the MGB. The vent line may be fitted by drilling and tapping a hole for a fitting in the filter mount housing if it is a screw on filter, and then plumb it for gravity, or the same as the remote swirl tank. The problem here is that you now need one more return line to the tank unless you ran the regulator return to the swirl tank, in which case you may have an issue with heating the fuel from pumping it continuously in a loop. Running a "T" to the regulator return is not a great solution either as you are combining a low pressure return with a high pressure return and could backfeed the swirl tank, cutting off that vent and filling the swirl tank with vapor. But if done correctly, so that the regulator return flow generates a vacuum on the swirl tank vent line it can work. This requires a transition in the regulator return line from a smaller diameter or orifice to a larger diameter with the vent line fed in on the immediate downstream edge of that transition. This fitting should be located right against the tank return fitting, which needs to be at least as big as the larger diameter above. A better option is to remove the tank filler neck and braze a fitting into the side of it in an unobtrusive location, put it back in and run the vent line to that fitting. The orifice can be as simple as a crimp in the line or as complex as a separate precision orifice with it's own housing. Carb jets should work well here, perhaps with standard plumbing fittings. I crimped my vent line with vise grips until I had the flow I wanted, but this leaves a flattened and therefore weak spot in the line and I may change to an orifice later. Wouldn't want it to break and pump gasoline into the trunk!

I think this addresses most of the issues, other than mounting, plumbing and wiring. Dan, if you want to put this in the newsletter feel free, or if there is a tech tips or similar section here it could go there as a reference.

Jim Blackwood

Take a look at this site for an external filter type solution.
Tony Bates

Tony - that is the solution I fastened on. Ingenious use of stock parts.

The fuel comes in the outside port on the filter(either gravity or a small pusher pump - don't know that SU pumps are intended to run constantly, but a little 'cube' pump would work fine)

The pick-up to the high pressure pump is at the bottom of the filter - an extended centre tube. Any air sits at the top of the filter, and there is a T in the fuel return line that is tapped into the filter at the top so this air simply returns to the tank. As for actually returning it to the tank, this is how I am doing it:
Bill Spohn

Excuse me, I have to go out to the garage now and kiss my carburetor!
Carl Floyd

Carl, you just go right ahead and do that! And the best of luck with it too. I mean that with all sincerety, I really do like your car.

That Jegs filler tube adapter is a nice find. Should do the job right well. The CB Bronco's system is just about right too, aside from that noisy Carter fuel pump and the low hanging filter accumulator. But I'm a little concerned about venting the filter to the return line. It might work OK, but it might not. It depends on whether or not there is enough positive pressure in the filter to overcome the pressure gradient between the tie in point and the fuel tank, considering that the line is already pressurized by the high pressure return. A 5 psi drop from there to the tank would be a real problem. If you do it that way I would recommend that you immediately step up to 3/8" line for the run from the "T" to the tank and also use a low pressure pump to pressurize the filter. Most likely that will do the trick, but it doesn't give you any ability to restrict the vent flow if you mount the "T" directly to the filter vent fitting, whereas a separate line would. Which would also allow you to put the "T" closer to the filler tube adapter, and the closer the better.

Jim Blackwood

Well david, I'll put my two cents worth in.
In pursuit of a quiet reliable fuel supply I’ve tried just about every approach and oddly, the easiest to set up has been by far the most successful.
The definitive answer to EFI fuel supply (for me) is simply 2 pumps and a big filter.

No swirl pot required, just a very late MGB sender (around $50 Australian, check exchange rate for comparison), which has a fuel uptake line in it, which (via fuel line) connects to a large cheap (soup can) filter (Kmart $18) , which connects to a feeder roller vane pump, which connects to a high pressure rollervane pump. This connects (via fuel line under car) to the fuel rail on the engine and thence back to the old pick up point on the tank.

The big filter acts as an external antisurge tank (swirl pot) as well as a filter for your system. The feeder pump solves the problem of fuel vapour pressure (BIG problem, see below) and the fact that ALL high pressure pumps suction very very badly.

Obviously all of this can be set up very quickly since it’s all just push on hoses and clamps. Because you’re using a ‘feeder pump’ both pumps and the filter can be put in the boot.
The feeder pump is made by Pierburg in germany and is called an Auto-suction vane cell pump. stock no. 12001. it can deliver 0.5 bar when used as a primary pump. It cost $95 Australian (apply exchange rate to get an idea what it might cost locally. This is a small high volume rollervane pump able to keep up with the larger high pressure Bosh pump ($150) but more able to draw petrol, This main high pressure pump is a Bosh one pt no 0580464070 and cost $140.

Why go to this trouble?
You’ve got two problems to deal with. Vapour pressure and surge starvation.
Petrol has a high vapour pressure which will increase as it is warmed up (around 100Kpa @ 38 degC). Vapour pressure is regulated by government, this regulated value has increased over the last few years.
High vapour pressure means that warm petrol doesn’t siphon too well (or at all), it’s not like dealing with water which siphons very well. Also you cannot allow this vapour pressure to build in any part of your supply or your pump will be pumping gas/vapour , not fluid, will cavitate, become very noisy or maybe even vapour lock, Certainly it will be irritatingly loud and reduce the life of the pump. You need a pressure release, which all petrol tanks have, so any system must have a return line back to the tank, no closed loops. The other problem of surge starvation has been covered above. Most modern cars solve these problems by having a small “inner tank” in the main tank, and submerge the pump this. This is expensive to do for MG converters since this has to be custom made by people who specialise in welding petrol tanks. If you try this your self you’ll probably die.

I think this inline approach this is actually better because both pumps draw through a fine filter and are protected, Both pumps (and filter) can be easily got at for replacement should the need arise. I carry a spare bosh pump so should the 'in use' one fail (hasn't yet) I'm not stuck in the middle of nowhere. Should the feeder fail then I can rely on the main (albeit noisy) pump to get me to a replacement/home. All bases covered.
Even with less than 5 litres of fuel in the tank, I have never encounted fuel starvation.
These pumps can be heard when the motor is off but very definitely not when the motor is running, unless you press your ear to the side of the car. Not something you are going to do voluntarily at any reasonable road speed ;)


Just a thought.

Why don't those who manufacture replacement fuel tanks for MGs offer versions with a swirl pot built-in?

Nigel Steward

Okay, what I'm gathering is due to 'vapor pressure', the large filter/swirl pot has to be mounted as low as possible to guard against vapor lock? Also, couldn't the low-pressure pump be before the large filter, to feed it and keep it full? That seems to be the best way to me, rather than a pump feeding another pump, or am I misinterpreting this? The carb guys must be laughing their butts off at all this complication! Bill

"Carl, you just go right ahead and do that!"

Jim, y'all keep on brainstormin' & fine-tuning the EFI. One of these days (years) I'll take the plunge.

BTW, I luv your car, too! :)
Carl Floyd

Bill, that is the way it should work - regular 'priming' pump dumps into the big filter and the high pressure pump picks up from the bottom of the filter and sends it to the engine.

Any air that happens to get sucked up from the gas tank pick-up just rises to the top of the big filter and gets sent back to the tank.
Bill Spohn

Bill the advantage of the low pressure pump is that its suction is strong enough to keep the filter filled and that filter acts as the swirl pot ie you get your swirl pot for free, plus filtered fuel of course.
Fuel injection roller vane pumps must have a filter before the inlet, otherwise they don't last too long. I tried running without a filter, lasted about 4 hours.
If you plan to use an existing pump as a feeder it may not be able to keep up (volume wise)with the high pressure pump in an "in line" arrangement. If this is the case then you need to buy (or make) a separate swirl pot.
This is just a tin can with 2 seperate inlets at the bottom and two at the top. The feeder pump feeds in at the bottom and out at the top, back to the main tank. The main high pressure pump, which is located at a lower level than the swirl pot, is gravity fed from the bottom of the swirl pot (via a half inch outlet)and returns (via fuel rail motor etc) at the top of the swirl pot. Any vapour bubbles formed exit the top of the swirl pot and go back to the tank. You can put this anywhere in the car, so long as the high pressure pump is under the swirl pot(gravity feed).Also at some point in the system you will need a filter prior to the main high pressure pump and probably should also have one prior to your feeder pump. Don't be tempted to just use some T pieces like I did initialy. Bubbles get pulled through to the main pump, very noisy.
All this is going to cost you lots and lots more time and money than a new sender and that pierberg auto suction vane cell pump, at least thats what I figured at the time. You might save $ on the sender and put a pick up into the old one, since you can safly remove this to a distance and weld/solder it. Personaly I opted for a new one, since it's an easy swap and an off the shelf part should you need to replace it. Minor note- you will need to turn the float part around so it faces forward and doesn't catch under the old intank pick up (now a return) line. takes about two seconds to do, clip out, clip in.

Actually if you are not using the standard MGB rear axle and hand brake cable there might be room infront of the petrol tank to have a big filter and a high pressure fuel pump down level with the bottom of the tank, and you won't have to buy a feeder pump. With the standard MG set up, like I've got, there's not a chance of fitting anything anywhere around the tank. On one side you've got the spring, the other side the exhaust and in front the handbrake cable sissoring up and down.
Peter Sherman

Yeah, thanks, Bill, the simpler the better, eh?
Peter, thanks for that description; I can follow that, I think. Along with the Bronco website and some other threads from the archives, I should be able to come up with a system. When I first got the new gas tank, I had a 3/8 return fitted into the neck, and I also 'picked up' the newer style pickup, so it looks like both should be useful in the system. Good tip on reversing the float. I am using all MG under there, and a Panhard rod is in the works as well. I'll be snaking the fuel system thru there somehow, soon as the brake system is finished. Best, Bill

wrt the neck pickup. Unless you've got a vented cap British style, then the vent on the neck will have to be used as a pressure release through your charcol cannister set up. My was American and that's the set up I've adopted for petrol tank venting.

Yes, Peter, I was also thinking along those lines. I do have a vented cap, '68 GT, but am considering how to put a charcoal canister in there for venting. Just in case the pollution requirements eventually require it, and also seems like a good idea, as the air around here is noticeably crappy the last few years. Although most of it seems to be diesel, by the odor. Bill

You might consider this kind of setup

Werner haussmann

This thread was discussed between 17/11/2005 and 27/11/2005

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