Swirl Pot

[greyV8pete]

While running my tank down to about 4 gallons to check the fuel gauge calibration I moved the car into the garage the other day and as I bumped up the step into the garage and hit the brakes the fuel was sloshing around in the tank. The engine nearly cut out and it seemed that the sound of the HP pump changed . As 4 gallons is only 2.5" fuel depth I am thinking that maybe the LP pump was possibly sucking air through for a few seconds, possibly starving the HP pump (bad news) and the drop in fuel line pressure caused the injectors to hiccup hence the engine. I thought that a swirl pot was supposed to avoid this situation until I looked carefully at my OE swirl pot fitted at POB. Photo attached shows the outlet to the HP pump (arrowed) is quite near the top of the swirl pot. I would have expected it to be at the bottom.


Does anyone else have the same OE swirl pot as mine?

As I don't wan't to run the HP pump dry at any cost I was thinking of replacing with the following which should just fit. Peter.

http://www.s-v-c.co.uk/product/1-ltr...t--base-mount/

[Mike]

Re: Swirl Pot

While running my tank down to about 4 gallons to check the fuel gauge calibration I moved the car into the garage the other day and as I bumped up the step into the garage and hit the brakes the fuel was sloshing around in the tank. The engine nearly cut out and it seemed that the sound of the HP pump changed . As 4 gallons is only 2.5" fuel depth I am thinking that maybe the LP pump was possibly sucking air through for a few seconds, possibly starving the HP pump (bad news) and the drop in fuel line pressure caused the injectors to hiccup hence the engine. I thought that a swirl pot was supposed to avoid this situation until I looked carefully at my OE swirl pot fitted at POB. Photo attached shows the outlet to the HP pump (arrowed) is quite near the top of the swirl pot. I would have expected it to be at the bottom.


Does anyone else have the same OE swirl pot as mine?

As I don't wan't to run the HP pump dry at any cost I was thinking of replacing with the following which should just fit. Peter.

http://www.s-v-c.co.uk/product/1-ltr...t--base-mount/

Before you buy a new one Peter, it may be worth checking whether the swirl pot fitted has outlets which go down into the pot. Obviously you cannot look inside, but you could test it before deciding to invest in a new one.
It does seem odd that the feed to the HP is not taken from the bottom of the swirl pot?

Mike

[dogoncrazy]

Re: Swirl Pot

Hi Peter
I've the same swirl pot and coincidently have also experience problems recently with the engine stumbling. On reflection the occasions during which it has misbehaved have been when I thought I had sufficient fuel. On checking the fuel gauge against the sender this evening the gauge was telling me I had a fraction under half a tank when in fact I had 3gallons so gauge was over reading. next job is to find out whether the supply outlet on the swirl pot 'reaches' into the bottom of the pot. I suspect as its a 'Marlin' supplied part it doesn't !! What do you think would be best way to check?

[CosworthCabrioLee]

Re: Swirl Pot

Here is a picture of my swirl tank located in the boot, not the same as yours Peter. The pipe goes all the way to the bottom. zero fuel and it runs fine

[dogoncrazy]

Re: Swirl Pot

Further to my above I've checked the outlet pipe to high pressure pump and it does appear to turn toward the bottom of the swirl pot. Exactly how far down it goes I can't make out via my simple examination but at least it doesnt seem to get its supply only if the swirl pot is full.

[greyV8pete]

Re: Swirl Pot

Here is a picture of my swirl tank located in the boot, not the same as yours Peter. The pipe goes all the way to the bottom. zero fuel and it runs fine

Thanks Lee. That's a very neat solution. Where is your HP pump? Under the boot floor or in the engine comparment? Peter.

[greyV8pete]

Re: Swirl Pot

Hi Peter
I've the same swirl pot and coincidently have also experience problems recently with the engine stumbling. On reflection the occasions during which it has misbehaved have been when I thought I had sufficient fuel. On checking the fuel gauge against the sender this evening the gauge was telling me I had a fraction under half a tank when in fact I had 3gallons so gauge was over reading. next job is to find out whether the supply outlet on the swirl pot 'reaches' into the bottom of the pot. I suspect as its a 'Marlin' supplied part it doesn't !! What do you think would be best way to check?

That checks out as this is the first time I have deliberately run the tank so low due to the gauge always under reading prior to my recent recalibration of the float arm.

I was going to poke a bit of brass wire or semi rigid poythene tube down the "HP pump feed" pipe but I guess it would have been as inconclusive as your check. My guess is that the pipe only just points vaguely downwards. Also if air gets in from the LP pump / low fuel condition there will still be air bubbles swirling down to that point. What is needed with that design is a baffle inside to keep any air bubbles away from the HP feed. Wish I still worked in an Xray dept! That would reveal everything.

As the new swirl pot is a lot cheaper than a new HP pump I will probably go for that as "insurance". Peter.

[Mike]

Re: Swirl Pot

That checks out as this is the first time I have deliberately run the tank so low due to the gauge always under reading prior to my recent recalibration of the float arm.

I was going to poke a bit of brass wire or semi rigid poythene tube down the "HP pump feed" pipe but I guess it would have been as inconclusive as your check. My guess is that the pipe only just points vaguely downwards. Also if air gets in from the LP pump / low fuel condition there will still be air bubbles swirling down to that point. What is needed with that design is a baffle inside to keep any air bubbles away from the HP feed. Wish I still worked in an Xray dept! That would reveal everything.

As the new swirl pot is a lot cheaper than a new HP pump I will probably go for that as "insurance". Peter.

Peter

Do you have access to a local engineering company who could weld a port at the bottom of your swirl pot?
Its a very easy job for them to do, if you can find one.

I can not understand Marlin supplying something like yours appears to be. The whole point of a swirl pot is to create a sump which will never run dry, and the only way to do that is have the HP feed from the bottom of the pot.

If you want to test your current arrangement, remove your swirl pot, empty petrol out, rinse it several times with water. Fill it with water, noting its capacity: then try to syphon the water out through the feed to the HP outlet int o a measuring jug. This will tell you exactly how low the pipe goes into the pot.

It might be worth testing the other outlets too, as your HP feed could be connected to the wrong outlet!

I actually modified my fuel tank to fit a return pipe which went almost to the bottom of the tank (just to cut out the noise of returning fuel splashing back in to the tank!).
This is something else you could consider with your swirl pot if you would rather have the outlet at the top of the tank.

Mike

[Dane_Rescueman]

Re: Swirl Pot

Hi Pete. I eventually fitted a nice swirl pot supplied by Merlin Racing. The LP pump supplies fuel to near the top and the tank return is at the top. The fuel rail returns to the tank about two thirds of the way up and the HP pump is fed from the bottom of the tank. My local injection/tuning firm was very clear that I must have at least a 12mm, but preferably a 15mm feed to the HP pump from the swirl pot to prevent cavitation. I notice that yours is 8mm? These pumps put out a fair volume and need a good supply. I have mounted my pot on a rear bulkhead ahead of the rear axle; there is plenty of room in the Berli for this but I'm not sure about the Cabrio.

[dogoncrazy]

Re: Swirl Pot

Thinking on from comments made, the problem seems to be more to do with the level in the tank and less to do with the swirl pot .I think we just need to keep more fuel in the tank and/or make sure the fuel gauge is not telling porkies!

[greyV8pete]

Re: Swirl Pot

Thinking on from comments made, the problem seems to be more to do with the level in the tank and less to do with the swirl pot .I think we just need to keep more fuel in the tank and/or make sure the fuel gauge is not telling porkies!

Yes true although that's what I have been doing since I had the car, which I guess is why I never had the problem until running the tank down for the fuel gauge research. However it rather negates the benefit of a 12 gallon tank if you can only use half of it. Will pursue the swirl pot options for a while yet. Peter.

[greyV8pete]

Re: Swirl Pot

Hi Pete. I eventually fitted a nice swirl pot supplied by Merlin Racing. The LP pump supplies fuel to near the top and the tank return is at the top. The fuel rail returns to the tank about two thirds of the way up and the HP pump is fed from the bottom of the tank. My local injection/tuning firm was very clear that I must have at least a 12mm, but preferably a 15mm feed to the HP pump from the swirl pot to prevent cavitation. I notice that yours is 8mm? These pumps put out a fair volume and need a good supply. I have mounted my pot on a rear bulkhead ahead of the rear axle; there is plenty of room in the Berli for this but I'm not sure about the Cabrio.

Hi Dane. Thanks for the info. I have just done some measurements and the existing S/Pot is 1.7L capacity. The small Merlin one is only 1L and the 1.5 and 2.0L versions won't fit under my bonnet anyway.

My swirl pot pipes are 10mm O/D so 8mm bore. It’s a bit of a struggle to get the hoses on but at least there is 8mm bore through everything. I did notice that the larger Merlin S/Pots only come with 12mm outlets to the HP pump. However my HP pump has an 8mm hose connector screwed into it so not sure if it would take a 12mm one???

Where is your HP pump located? Under the bonnet or at the rear next to the swirl pot?

I already have the LP pump on the rear chassis panel in front of the O/Side drive shaft, so guess there will be room there for the other stuff too. Will look later.

For a quick fix meantime I am thinking to tilt my existing S/Pot up at the front, on an extra bracket, to create a bit more of a head of fuel at the HP pump outlet. Peter.

[CosworthCabrioLee]

Re: Swirl Pot

Hi Peter I know a man who make one for you to any design you want, and with any outlet/inlet size you want in alloy ? And it won't be a fortune.

[Mike]

Re: Swirl Pot

Hi Pete. I eventually fitted a nice swirl pot supplied by Merlin Racing. The LP pump supplies fuel to near the top and the tank return is at the top. The fuel rail returns to the tank about two thirds of the way up and the HP pump is fed from the bottom of the tank. My local injection/tuning firm was very clear that I must have at least a 12mm, but preferably a 15mm feed to the HP pump from the swirl pot to prevent cavitation. I notice that yours is 8mm? These pumps put out a fair volume and need a good supply. I have mounted my pot on a rear bulkhead ahead of the rear axle; there is plenty of room in the Berli for this but I'm not sure about the Cabrio.

Hi Dane

This is interesting, as I have always had a noisey HP pump, and the fuel rail sounds as though bubbles are 'rattling' through it. My HP pump is fed from a sump below my main tank so I thought lack of fuel could not be my issue.
However, when I sent my HP pump back to Sytec, asking them to have a look at why it was so noisey, they tested it, and returned it saying it was quite normal. They advised it has a flow rate of 350 litres per hour, so I would need a minimum of a 12mm fuel pipe from my sump to the pump to prevent it cavitating. When I checked the tank outlet supplied by Marlin, it had a 10mm O/D union, and only an 8mm I/D. I had only fitted a 10mm fuel pipe too.
I bought one of Sytec's own outlet flanges for a 12mm pipe, and a length of their 12mm fuel pipe to go with it.

I have yet to test if it makes any difference, but am hoping it will have cured the problem?

Mike

[danny_nelson]

Re: Swirl Pot

Hi Peter, and everyone who has posted on this subject. There are a lot of Cabrio/ Hunters on the road since around 1998 using this swirl pot with no problems reported.
I have not had any problem like you describe, and I have had several with this set up. So why not ask Mark what his thoughts were with this design? after all they have been quite successful with their racing set up. What pot are they using there? Peter you are beside them, I think I would talk over the design before I done anything.
Just my thoughts.
Danny.