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MG MGB GT V8 Factory Originals Technical - What size fuel line? Peter Sherman?

I've decided to set up my EFi the same way that Peter Sherman did with an LP Pierburg feeder pump, a swirl pot and an HP Bosch pump. I have all the bits, but what size metal fuel line is needed from the HP pump to the fuel rail. Will the standard MGB pipe be adequate?

Also, should the in-line filter be between the tank and the LP pump, or between the pot and the Bosch pump? Does it matter?
Mike Howlett

MIke, use the original pipe for the return. Install a pipe with the same diam. as the HP pump outlet for the fuel rail feed. The Bosch HP pump is the expensive part of the kit, so that needs the most protection. Put a filter on the input side & an HP filter before the fuel rail in the engine bay. Barrie.E
Barrie Egerton

I'm not actually using a dedicated swirl pot.
I am using the fuel filter as a swirl pot. Any large efi filter will do. At the time I set it up swirl pots were expensive, and I figured that I could always go buy one later if the filter didn't work. I notice now that swirl pots have become quite cheap (china etc), however I'm not going to change since the filter works perfectly as a swirl pot and separate swirl pots take up space and need extra plumbing.


The sequence is
fuel tank-->big filter--> good suction/LP pump -->HP pump->engine-->back to tank.

Make sure there are no crimps in the lines.
The existing charcoal canister anti pollution set up takes care of any pressure build up.
Since i had to plumb a return fuel line in anyway I replaced the old line at the same time. There was also a dent/crimp in it that I was concerned about.
What size? The same size as the outlet on the HP pump.Or maybe a little larger, 6mm. (it's what they had in the shop) If memory serves, the MG line is a bit smaller. I can see no reason why you could not use the MG line I seriously doubt that this would cause a significant pressure drop between pump and engine(unless it is crimped of course). If you are thinking of using the charcoal canister line as a return line then that may pose problems as this is the bleed for the pressure relief on the fuel rail and is a good deal smaller. Might work, might not.

For fuel line I used "bundy tubing".Fuel line/brake line stuff. This is a alloy that may be bent into shape by hand if necessary and is cheaper than rubber fuel line. I measured the lengths I wanted and got them from a hydrolyic fitting place, ENZD. I got them to put a little bump on each end since I don't have a "bump making tool". I doubled up on the hose clamps for the high pressure line to be sure.
The inlet of the HP pump is about half an inch, the out let of the LP pump is much less. I used brass size changing fittings to make a U shape so as to get the pumps next to each other.
I only put the HP pump close to the LP pump for wiring convenience, but really the HP pump can go anywhere since it's got a pumped supply.
Peter Sherman

Swirl pot

http://www.bostream.nu/lindblad/sida5.htm

http://hem.passagen.se/larsragnar/
page 5

Ingemar Lindblad

Thanks guys. I actually have the swirl pot already. I just needed to sort out the plumbing.
Mike Howlett

Mike - 12.5mm up and 8mm back.
I have a fuel filter coming out of the tank and another just before the fuel rail. Maybe a bit OTT, but at least I will not get clogged injectors if nothing else!
H Adams

If you want to use your swirl pot in the usual manner then locate the HP pump close under it. The many small roller vanes of HP pumps develop good pressure but have very poor suction so they have a big inlet. Half inch line from the bottom of the swirl pot to the HP pump half inch inlet. The return line from the engine goes to the middle (if there is a middle) or one of the top inlets/outlets of the swirl pot.
To keep the swirl pot filled the LP pump (fewer,bigger roller vanes so good suction but poor pressure) pumps petrol in at the bottom of the swirl pot and excess petrol comes out the topmost outlet on the swirl pot and goes back to the tank.

Make sure all of this plumbing is clean and free of debris and have a filter between the tank and the low pressure pump.

When the fuel returns from the engine it is hot and full of little bubbles. Petrol is a blend of refinery products. Various boiling points, often quite low. That is why the return line from the engine goes at the middle/upper end of the swirl pot and any bubbles will be swept back to the main tank when excess petrol exits the topmost outlet.

Obviously people are getting away with it, but do keep in mind that the pressure relief valve on the fuel rail is set to keep the injectors supplied with petrol at a specific pressure and should be relesing into a line with minimal flow resistance. ie no back pressure. If the line too small and has too much flow resistance the rail will be at a slightly higher pressure than it should and you will be running rich. Unless of course you are using oxygen sensors in the exhaust, in which case the computer will probably adjust.
I'm not using these, just the aprox' 6mm (inside diameter) tubing and the inline filter/swirlpot. Goe like a rocket :)
You may want to give this approach a go, its so easy. Initially I just threw it in the boot. Not sure if it would work. Later I moved it to a box just behind the rear right hand wheel arch. That space up under the rear corner of the boot. Not particularly pretty, just a filter and two pumps joined together with brass fitting and hose clamps then crammed into my old glove box with a bit of foam wrapped around them. I change the filter about once a year.
Peter Sherman

Thanks again Hal & Peter. Loads of info. I've bought one of these http://www.sm-engineering.co.uk/alloy_fuel_surge_tank.php

Do you think it's safe to assume that it's clean inside, or should I fit a filter between it and the high pressure pump?
Mike Howlett

I would put a filter between the swirl pot & the HP pump. As Peter says, mount the filter & HP pump lower than the swirl pot to let gravity help prime the pump. Barrie E
Barrie Egerton

I will defer to the swirl pot guys (ie Barrie) on this. I don't have one so have no practical experience with them.
I would guess that so long as the HP pump doesn't cavitate the more filters the better.
The pump(s) should make a humming noise. If it makes a loud buzzing, it's cavitating and pulling air or a mix of air and vapor bubbles. The first time you switch on the pumps they will be pulling air. They will quiet down when the filters, lines and swirl pot fills up.
From your link, I would guess the bottom outlet is for the HP pump,
The second one up is for the LP pump to fill the pot.
The third one up is for the return line from the engine.
The top one is for the return to the main tank.

Petrol has a high vapor pressure (around 70KPa at 100 deg F) compared to water (about zero). Lots of light fractions in petrol ie butane is often 5% of the mix.So under suction bubbles will form. When the petrol gets hot the problem is worse. This leads to a noisy pump thats not delivering the pressure it should (possibly vapor locking), is wearing out fast and is very annoying.
Good luck with it.
Peter Sherman

Here's a subsidiary question. What size/type of fitting is it that screws onto the feed elbow on my shiny new tank? Is it a standard brake type fitting? Is the one on the tank sender unit the same? (I'm at work and can't look right now).

Yes, Peter, I think you have the pipe connections right for my swirl pot. I have to say it's a lovely piece of work - too good to put under a car. I tried both pumps to make sure they worked, with some flexible pipe into a can of fuel, through a new filter to the pumps. By Heck, they can shift some fuel! And as you say, once they are full of fuel, they run really quietly.
Mike Howlett

This thread was discussed between 13/12/2007 and 17/12/2007

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