Welcome to our resource for MG Car Information.



MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB GT V8 Factory Originals Technical - Swirl pots / Pumps for Fuel Injection

I have just added a swirl pot to my 3.9 fuel injected V8 (14CUX) which seems to work well BUT-----

The swirl pot has a return to the tank (the fuel rail return now goes to the swirl pot) - the LP pump which keeps the swirl pot topped up (electronic version of the factory unit) which would normally stop pumping when the carb pots are full - keep pumping because there is no back pressure to stop this.

Is there any way around this - the LP pump sounds very busy!

R Walker

If you are not using the later type of sender unit with the outlet on it,use one of those for the return to tank.If you are already using one,I would run a return pipe along side the tank under the boot floor & enter the boot at the very rear end & then tap it into the filler pipe.I think this is the safest way to go about it without interfering with the tank. I've never tried this so I don't know if there are any issues regarding pressure or breathing. I just thought this is the way I'd go if I ever fitted EFI again. Barrie E
B Egerton

I also have a swirl pot and twin pumps in my RH battery box. The low pressure pump (Pierburg) feeds the pot, and the high pressure pump (Bosch) feeds the fuel rail from the pot, and returns what's not used to the pot. The pot has a top union that takes excess fuel back to the tank. I used the early style tank with the separate fuel union as the feed, and fitted a late style fuel gauge unit with a pipe incorporated. This became the return. If you take the tank filler cap off while the pumps are running you can clearly hear the fuel splashing back into the tank when the tank level is low.

Mike Howlett

And here you can see the fuel tank arrangement.

Mike Howlett

I'd be tempted to fit a restrictor in the return line from the pot back to the tank - Really this line is only there to vent the pot it doesn't really have to return heaps of fuel- It really only needs to be a pinhole.
Just a suggestion Willy

That's not my understanding or my experience. The pumps push far more fuel up the fuel rail than the engine can use. They must, or you wouldn't have enough fuel when you were accelerating hard. The excess has to go somewhere and so it overflows the pot back to the tank. My swirl pot is a professional item bought from a respected supplier and the top union is a 1/4" tube. If it were a pin hole it would be impossible to control the pressure in the fuel rail as it would rise and rise until something blew. These pumps are not like the old SU which stalls under back pressure. These ones just keep going.
Mike Howlett

The LP pump feeds the swirl pot
The swirl pot supplies the HP pump
The HP pump supplies the HP fuel to the rail
The pressure regulator (usually on the rail)controls the fuel pressure in the rail and returns the excess to the pot where the HP pump picks it up again and repumps it to the rail.
If the engine is at low speed/load the fuel goes round and round from pot to HP pump to rail and back to the pot and the lift or LP pump does very little work - If you have a large return from the pot to the tank all the LP pump is doing is pumping fuel to the pot and straight back to the tank - The LP pump is going flat out for no reason With a small restrictor in the return to the tank the LP pump can supply the pot and then almost stop just like an ordinary carby. SU pump
Under heavy load exactly the same thing happens except that more fuel is required at the rail and less returns to the pot and the LP pump is asked to supply more fuel to the pot
The return from the pot to the tank only needs a tiny hole to bleed off any trapped air when using a SU type pump as a LP pump

OK Willy, I understand what you're saying, and you are right, on the HP side the regulator controls the pressure on the fuel rail and what I said earlier was wrong.

But on the LP side my LP pump doesn't work like an SU. It doesn't shut down when it detects back pressure, so like you say it is running flat out all the time (like the HP pump) and is designed to do that. As back pressure could harm it, I reckon I need a decent sized overflow. If I was using an SU as the LP pump, then I would have to agree with you.
Mike Howlett

Hi Mike
As you say it all depends on the type of pump being used as the LP supply -
We were each talking of different types of pumps, I can see that with your pump that yes you need the free return. I love this forum for just this reason It doesn't seem to matter what subject that comes up, there always seems to be someone that gets the same result by a different method It's fantastic to be a part of it all.

Cheers Willy


Sorry, I totally misread the question. The simple answer is no, unless Willy's idea of a restrictor works with an SU pump. Most people use the type of pump that runs constantly so it's not an issue (once you get used to the noise they make). The big advantage of the SU pump is that it's quiet. I did think of changing the LP pump for an SU at one stage ,not realising it would be running constantly. Fortunately the salesman at the local SU Midal agency told me that an SU was an overkill for what I was going to use it for & that I should save my money & use a cheap recirculating type of pump.Barrie E
B Egerton

Willi, Mike Barry thank you for your thoughts - I suspect I shall have to live with the LP pump running continually although I am tempted to experiment by constricting the return.

For the record, the conversion was done some years ago and the two pump system ran well with a large filter acting as swirl pot. The return went straight to the tank. The LP pump was a factory original and the difficulties only arose when that was changed for an electronic version.

R Walker

Roger& Willy
if the Low Pressure pump you have is an SU pump or some variety of diaphragm pump (it makes clicky click noises) you will have to replace that with a 'low pressure good suction' rollervane pump. EFI needs roller vane pumps that are designed to run all the time. I use the same pierberg pump that Mike mentions. They cost about $90. There are a couple of versions of it if you download their catalogue.
I run a setup as follows

Tank-->Big filter-->LP pump-->HP pump-->engine-->tank

filters and pumps under the back corner of the boot. I've never had a fuel starvation issue with this.

Since you want to use a swirl pot you would run two loops

Tank--Filter--LP (rollervane)pump--swirlpot--Tank
Swirlpot--HP pump--engine--Swirlpot

Diaphragm pumps (MG SU pumps) are not designed to run flat out all the time and will wear out quickly trying to keep up with the high pressure (and high flow) rollervane pump. Plus, as you point out, they are noisy. You need a high flow, run all the time, rollervane LP pump.
Unfortunately you can't fit a pressure restrictor to the top of the swirlpot/anti surge tank. This gets a bit involved so please bear with me.
The fuel in the swirlpot will get really hot because it isn't being diluted out with a good flow of fresh fuel from the tank. It will start to boil. Or boil harder in fact because petrol is a mix and has a boiling range that starts at around 17 degrees C and goes up to around 128. That's at atmospheric pressure. In any event a gas/vapour bubble will start to fill the top of the anti-surge tank as that restrictor is overloaded. When there is enough pressure the SU will stop pumping, then kick in again as the level of fuel drops. But not for long as the vapour bubble in the swirlpot/antisurge tank will will continue to grow until it reaches the bottom of the tank and the HP pump feed. The HP pump will then deliver (very noisily as it cavitates) a whole lot of petrol vapor/gas, no liquid fuel. This means the injectors will deliver allot less volume of fuel. The fuel air mix will be way too lean and the engine will immediately stall. The fuel line/rail will still be pressurised and so the SU won't start up again to supply more liquid petrol. The pressure will gradually drop as petrol condenses, or bleeds off (about 10 to 15 minutes) then you can start the car, and the process will repeat.
In fact the as the bubble grows the hot fuel will give up it's lighter ends, and the octane number will fall significantly so there would be some bad knocking.

One of my initial attempts at fuel delivery had an SU pump feeding a closed HP pump loop that included a big fuel filter as a anti-surge tank. Very close to what you are proposing. Took a little while to figure out what was going on, then I went to the flow though system above.

Conceivably you could adjust the restrictor so that it lets the petrol vapour out and keeps the swirlpot filled however the petrol will get hot in there (a little less than the engine manifold which is heating it) and the octane number would drop. It would have to be adjusted differently for summer and winter. Also I think you'd find that the HP pump would get noisy as it sucks boiling fuel with bubbles in it. Also those bubbles would effect the fuel to air mix. Perhaps the fuel rail might fill with vapour as well. A Bad scene.
Peter Sherman

Peter, Thanks for your input. I don't think I am going down the restrictor line. As you say, I am obviously concerned about the durability of the present diaphragm LP pump - which of the pierburg rollervane pumps do you and mike use.
R Walker

The low pressure pump is made by Pierburg
and is called a "universal autosuction rollervane pump
Part # KP 7.21440.53.0 or KP 7.21440.51.0 there doesnt seem to be any difference. The guys I got mine off (petroject) called it part #12001.
here is a catalogue you can download.

They've gone to PDF so you'll have to find it in
"fuel supply" page 17, looks like mine.

really any pump that is rollervane (run all the time) and low pressure good suction. Jag's (and many GM) use a two stage set up, low pressure feeder to the High pressure pump. You might find one on a wreck. Any roller vane pump absolutely must have a good filter (whatever size) before it. The fine vanes clog up very easily. I tried running without a filter when I was just using the HP pump. I was trying to ovoid using a feeder pump. I lasted about 30 or 40 minutes before destroying the pump. And that was with a new tank and the usual MG nylon filter on the late model fuel tank pick up. I suggest you go though the yellow pages to find an EFI fuel pump supply place for the best price. Auto parts places will be buying there also and slapping on a %
The issue is that the High pressure rollervane pumps presumably have lots of small vanes rotating rapidly to develop the pressure. This however, makes for very poor suction in petrol. Petrol is not anything like water, a much higher vapour pressure. I'm guessing that the low pressure rollervane pumps (eg peirberge) have larger vanes that suck reasonably well, but cant develop the pressure. You can have one or the other but not both in one pump.
so two pumps are required.
I experimented with my LP pump and found that it could suck though the big filter to a height of about half a meter before it began to cavitate. Makes a loud buzzy noise because the suction is such that the petrol (which has a very high vapour pressure, about 77 KPa at 39.7 degrees C) will form vapour. Tricky stuff petrol.
None the less, I did locate the LP pump as low as I could ( boot floor level) because I knew that in the summer the petrol tank would get warm. I would estimate about 50 degrees on a really hot summer day.

An 'in tank pump' is what many car manufacturers have gone to to get around these issues. The VW jetta is a good one for MGB's, comes with it's own swirl pot. But that does mean altering your fuel tank. Which can be expensive/risky to do and if you ever need to replace it , due to damage (I did) it's not an off the shelf item and you have alter it all over again.

Peter Sherman

Roger I used the same Pierburg LP pump as Peter - it was his advice I followed. But I couldn't find a supplier in the UK and eventually bought it from Germany. If you find a UK supplier, let me know please.
Mike Howlett

Some one certainly must sell them. Here is an article about Glenn Towerys EFI. He quite independently settled on the Jag set up. He doesn't say which year, but someone must supply the Jaguar parts!
if you scroll down a bit you can see his in line Jag pumps. Looks like the same feeder pump.
I prefer to have them more accessible after the no fuel filter episode. Mind you, 60,000 plus km now on those pumps, so quite reliable. I've got filters and pumps coiled up in a cut down box (old fibreglass glove box infact) just behind the rear right wheel. Used elbow joints as you mustn't crimp/pinch the fuellines of course. Pumps will get noisy. Did have them in the boot but changing the big filter in there was messy.

Peter Sherman

This thread was discussed between 05/10/2010 and 18/10/2010

MG MGB GT V8 Factory Originals Technical index

This thread is from the archive. The Live MG MGB GT V8 Factory Originals Technical BBS is active now.