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Author: Subject: Surge tank design, how not to do it
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posted on September 1st, 2003 at 06:26 PM
Surge tank design, how not to do it


I just updated my website, and I wrote some interesting things about my experience with my surge tank. For all you EFI guys out there... :thumb

http://www.offroadvw.net/bajawes/V6_baja/September03.html




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posted on September 2nd, 2003 at 01:50 AM


hey Wes I was just checking out your article and I think you have over engineered the system. Do you think its possiable that your pum was sucking harder than your gravity feed line could flow? I really don't see the poin of a lift pump in this application. The way I would make one would be to have three ports in the can. The first, just above 3/4 way would act as a return to the tank to allow for vapour and fuel return. The second at 3/4 would be a -10 or so fitting from the tank itself. The last line would be at the base of the surge tank to the rail. The return would be pumped to the actual fuel tank. I can see no problesm with my thinking and ask as this is what I have planned and am obviously keen to avoid problems so feedback is appreciated :) hey are you doing to take me for a spin on action day? what if I ask extra nice :bounce
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posted on September 2nd, 2003 at 02:49 AM
Look out for trapped air.


Did this fix all the problems Wes?
I have done a few efi conversions for turbo rotarys into old Mazdas and something to keep in mind is air likes to stay at the top so you need to be sure there is a good vent from the surge tank to the main tank, if the vent has loops or kinks in it when the tank goes in the you may be fighting an air pocket that keeps topping up each time you uncover the drain in the main tank, IE hard corners, braking or accellarating.
this is a really simple idea my friend ran with great success and I adopted it as my setup.

A Decent size hose with no loops and a vertical sort of angle so avoid trapping air from the Main tank to the surge tank of 1-2 litres or get a fuel trap welded to the bottom of the Main tank so it catches a litre of fuel and wont allow it to slop out.
then down thru the pump to the injectors slowing at the regulator and then returning to the top of the Main tank so it cools as it sprays into the Main tank.
It doesent need to be any more complicated than that, new cars run a similar setup with no extra tanks etc.
The more simple the setup the less hassle it will be.
Of course this wont help you much now you have finnished your setup, but its a good setup to duplicate,I know it feeds 400 hp with no dramas so it should suit anything we are doing.
Cheers Tony

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posted on September 2nd, 2003 at 07:16 AM
Ok it is time ..


I have to ask the obvious question here, well obvious to me anyway.

WTF are you doing with crap hanging off it everywhere ? It seems you have taken a simple concept and turned it upside down.

Is there any reason you can't simply plumb the return line back into the main fuel line ?
The HP pump has a one way valve and is use to maintaining line pressure, the returned fuel will be slightly hotter than thetandard stuff but the decrease in volume should mean no real increase in pressure.

Seems like a simple idea and was what I planned to do on mine.

Or do you need to allow your fuel system a chnace to breath ? I can't see why you would as I can't really see it getting that hot as to casue problems.

So have I lost the ploy ??? :(




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posted on September 2nd, 2003 at 08:59 AM


ok.

The first design did not have any air bubble problems. The line from the tank to the surge tank was basically straight down to the top of the surge tank. It was a short 10mm fuel line. It was not a restriction, as the car ran absolutely perfect for up to 1 hr. After that the heat from the return line heated up the surge tank. So the high pressure pump was not sucking harder than the gravity feed would flow.

Herbie, yes it has fixed all the problems. The only problem I had was the fuel in the surge tank getting too hot. Your design seems ok, but you would need multiple large lines going from the main tank to the surge tank. I did a lot of reading about this. Basically Volvo ran a similar set-up to you, thinking it would work. They ran into 2 problems.

One being that the hot return fuel going straight to the main tank caused the fuel to vapourise in the main tank, and a lot of fuel vapour was lost out of the tank breather. Modern cars avoid this by having a surge pot in the main tank. The return fuel goes back into this surge pot where it is basically trapped and cannot vaporise away as easily.

The second problem was the High Pressure pumps seemed to die relatively quickly (talking 1-2 years or so of daily use). It seems if they have to suck at all then it strains them. The high pressure pumps have ridiculously high flows, and gravity feed will not keep up unless you have really really really big lines going from the surge tank to the main tank.

They then changed their set-up to something similar to what I am running, with a low pressure feed pump. Their problem went away.

My set-up isn't really that complicated. The low pressure pump is really a recirculation pump, and doesn't need to be that big.




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posted on September 2nd, 2003 at 10:36 AM
So ??


Why can't you just plumb your fuel return into your out line and let the fuel keep going round and round till it is used ?



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posted on September 2nd, 2003 at 11:45 AM


That's essentially what I had, but that causes the fuel in the surge tank to slowly heat up and cause the problems I had. Weren't you paying attention?

If you have your return line going into your main tank, your low pressure feed pump or your gravity feed line (depending on setup) needs to flow more than what the HP pump flows. This will usually be around 4 liters per minute, depending on the pump. That's a lot of fuel. So that's a pretty big feed line if you want gravity fed.

The way I have and had it set-up, my low pressure pump / gravity feed only had to keep up with what the engine was actually using, which is a much smaller flow of fuel.




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posted on September 2nd, 2003 at 12:15 PM


I was wondering. In the set up you have would it be better to plumb the outlet of the Lp pump into the bottom of the surg tank so it was more likely to supply the cooller fuel from the main tank to the inlet of the hp pump???
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posted on September 2nd, 2003 at 12:26 PM


Possibly, but I already has a spare fitting in the top of the surge tank :)

I don't think it would make much of a difference in the end.




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posted on September 2nd, 2003 at 01:11 PM


I hadn't thought about the vapour loss, that makes lot of sense, and I guess it would encourage a smelly system.
does the fuel after the regulator flow at 4 litres/min, I suppose that sounds reasonable,

Buggy Brad,
Hot fuel is similar to hot air it has poorer efficiency.
and you can eventually boil the fuel as fuel boils at a lower temp than water, the you get vapours making your system fail as you replace liquid with gas and it doesnt do the job.
I actually have a little radiator that was an oil cooler from a motorbike and it has the right size piping for decent fuel line,
I am aching to be able to plumb it into a system someday, but have gone prehistoric in my car collection.

Cheers Tony,
PS Wes, As long as you got your problem sorted then its all good.




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posted on September 2nd, 2003 at 01:36 PM


MmmmmÖ.. Iím far from an EFI expert, but what you have is basically what Herbie suggested, and will heat up your tank as you described anyway. The only difference is you replace a large gravity feed hose with a small hose pumped under pressure, which only adds to heat.
As the LP pump most likely puts out more fuel than the motor uses, it will circulate the hot fuel into the main tank to keep the temp down as it seems to be doing.
As I see it, Herbieís suggestion is simpler, but will require modifying you main tank and header for a larger connection (or rely on a number of smaller hoses). Too overcome the fuel vapor problem, run a breather line back to your inlet some how?? Maybe through a small filter or condenser to trap some fuel before it escapes
In the stock Kombi FI system, the return from the regulator went into the bottom of the tank, not the top. This might help reduce vapor from the tank also.
Better still, fix the problem at the source. I assume the heat generator is the pressure regulator. Run a finned copper line at itís outlet before it enters the tunnel at the back. If it gets a bit of air flow, it should help the problem.
Oh, and on your alternative suggestion, wouldnít pressurizing the header tank affect the output pressure of the HP pump?? Will this affect the regulator? It will also again add to heat.
My 2 cents worth.

Do I get a spin at action day also!! Maybe you should sell rides to people.

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posted on September 2nd, 2003 at 01:48 PM


Tony, too cold fuel can also be a problem. I tried to fix a problem of vapor lock on a small motor bike by cooling the carby (admittedly it will be different for FI). Fixed the problem, but the fuel economy became appalling and the plugs began to fowl. I decided to live with the occasional vapor lock rather than try to re-jet etc.
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posted on September 2nd, 2003 at 02:21 PM
Well


Quote:

That's essentially what I had, but that causes the fuel in the surge tank to slowly heat up and cause the problems I had. Weren't you paying attention?



I am suggesting you plumb the fuel return in down stream of your pump, ie not into the surge tank but into the line. This will mix the returned / warm fuel with the fresh / cold fuel and send it back into the rail.
I don't see how it can cause vapour lock if it is kept in the line under constant pressure.

I have actually done it thisway and it worked fine. Daniel also did this on the EJ20 in his kombi with no issues.

[Edited on 2-9-2003 by Buggy Brad]




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posted on September 2nd, 2003 at 02:32 PM


Andy,

The source of heat is the fuel rail itself. The heat from the pumps and the regulator is nothing really. The fuel rail gets quite hot to touch after the car has been running for a while. Fuel vapourises at a relatively low temperature. When you back off the throttle, the injectors don't open at all, so all the fuel goes through the hot rail and back to the surge tank. This is when the fuel really gets hot. The more economic the car, the worse the prob (cos more fuel gets returned).

If the hotter fuel goes straight back to the main tank, it vapourises a lot more easily. This flows through the vent line and overloads the carbon cannister. Excess fuel coming out of the carbon cannister overflow has been known to cause engine fires.

That is why car manufacturers return the hotter fuel back into the surge tank area (although it is internal in a regular car), and not return into the main tank area. The hotter fuel will mix with other fuel and cool down before it reaches the tank vent line. This is what car manufacturers do, so I assume they know what they are doing.

The low pressure pump would raise the pressure in the fuel rail slightly, but not much. A low pressure pump will churn out 3-7psi. A HP pump puts out about 90psi, and the regulated reduced that to 45psi. And extra 3-7psi will make the engine run a little richer. If you installed a regulator on the LP pump, or replaced the EFI regulator with an adjustable unit then you wouldn't have the problem anymore. But I wouldn't and did use this option anyway. Some race cars use this option because the pressurised surge tank raises the vapour pressure of the fuel and prevents any probs with the HP pump cavitating under racing conditions.




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posted on September 2nd, 2003 at 02:38 PM


Brad,

Are you talking about the fuel return going into the HP side of the HP pump? This will not work, and then the supply and return from the fuel rail will be at the same high pressure?

If you are talking about returning it into the discharge of the low pressure pump, then that is really no different to what I am running now.

I don't understand what your saying?




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posted on September 2nd, 2003 at 03:00 PM


Where do the air bubbles come from when you boil water??
they are a physical change resulting from increased energy input.,
and the heat comes from the close proximity of the fuel rail to the Hot engine,
it needs to be close to minimse the time needed for the fuel to quickly jump through the injector/s the less mass of fuel trapped near the injector then the less fuel heating up and it can move quickly to where its needed, otherwise the pulse of the injector has expired quicker than the fuel could move.
If you loop the line back into itself then its not much better than just blocking off the line and your fuel pressure would go bananas, as there is no escape for the fuel once it passes through the pump, the regulator cant do anything as it has equal pressure on both sides.
A regulator should run around 40 psi and then match the rising rate of your turbo boost, if you are turbo, and depending on the source of the regulator and the fuel pump will run maybe 120 psi if it has no escape, the pump will not appreciate the locked system and you must be lucky not to have troubles.
As far as I understand, a regulator is governed by a spring and a diaphram so any extra presure from a low pressure pump would be negated by the regulator reaching max presure earlier and thus just letting go quicker,
And the pressure from any low pressure fuel pump would escape through the surge tank breather anyway, as once the surge tank fills it will overflow back to the main tank
The best Idea is to copy a production cars specs as a car company would have paid a lot of money to be sure their cars did not have trouble.

Cheers Tony

[Edited on 2-9-2003 by Herbie]

[Edited on 2-9-2003 by Herbie]




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posted on September 2nd, 2003 at 03:13 PM
mmm


Won't the fuel go into the engine thus reducing the pressure in the rail ?



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posted on September 2nd, 2003 at 03:14 PM


only at the rate that the motor can use it, but its being supplied at about 60 or more psi than needed for safety to avoid a leanout.



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posted on September 2nd, 2003 at 03:21 PM


Wes,
Run the return line back under the car and let it cool as you go through the puddles :D:D
Seriously, cooling it on the return run not an option?
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posted on September 2nd, 2003 at 04:11 PM


Andy,

I run my return line in the tunnel where it is safe, but won't get cool. You could run it through a small radiator, but why bother when a LP pump is cheap.

Brad,

EFI pumps like to be flowing near their full flow the whole time. They are completely different to a low pressure pump. If you have them pumping to no-where, they will crap themselves. An EFI regulator is also different to a low pressure regulator, and need to bleed back to something with no pressure, otherwise they won't work. Dan must've had his kombi set-up differently to what your thinking.

An EFI pump will run near it's max flow nearly all the time. That's why if it is entirely fed by the LP pump, the LP pump will have a hard time keeping up. Having the return into the surge tank reduced the duty of the LP pump to only the flow the engine needs.

Even naturally aspirated EFI engines have regulators which vary according to manifold pressure. A little line runs from my manifold to the pressure regulator. Although it is much more important on a forced induction motor.




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posted on September 2nd, 2003 at 04:59 PM


I'm almost frightened to enter this discussion hehe...
Simply because my set-up is about as complicated as a thickshake!
I don't use a surge tank firstly. I use one hp pump and pipe the excess back to the tank....I haven't had any problems what so ever on the freeway and as for cutting out going round corners...well from experience, the tank has to be extremely low, I'm talking, running on vapours... and the lack of power is momentary before it's back to 100%. I have no intention of adding a surge tank thats for sure...tho I understand why your using one...4wd-ing and the likes!
So what are your highway temps nowadays? we'll have to tee up a few races...on private property or course hehe
could be fun?
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posted on September 2nd, 2003 at 05:10 PM


Dan,
Kombiís had FI from ~72/73. Even if you donít have a specific FI tank (they had a return port next to the outlet), they were designed for it. The fuel actually collects in a recess with like a wall around it inside the tank (they do vary through the years though). Similar to what Wes says modern EFI cars have.
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posted on September 2nd, 2003 at 05:11 PM


Hi Dan, good to see you still visit these parts. So your HP pump is not returning to itself like Brad thought.

My friend runs no surge tank in an old falcon, and it cuts around corners and up hills sometimes. But you've got a kombi so it probably doesn't see the same g forces :)

Mine is running fine temp wise. Gets up to the half way mark then stays there. I think I need a bigger expansion tank to suit my bigger water capacity though. When it warms up it sometimes overflows out the overflow bottle, then when it's cold it emptys the overflow bottle. Then when it warms up from there it fills the bottle again. Not enough capacity for the volume of expansion I am getting.

As for racing, you realise you don't stand a chance don't you :D




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posted on September 2nd, 2003 at 05:24 PM


im about to put a V6 into my kombi.
can use explain to me in english (not this double dutch stuff use are talking) the basics of HP and LP pumps and the size that the surge tank has to be etc?
cheers
rhys
p.s. do all u guys just run standard tanks with more outputs/inputs put into it?




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posted on September 2nd, 2003 at 05:34 PM


alright wes...first too byron with 4 mals & 3 shorties & 7 mates wins :)

thanks andy... it suddenly all makes sense now hehe...
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posted on September 3rd, 2003 at 10:00 AM


Rhys,

Go out and buy this book
http://www.autospeed.com.au/cms/article.html?&A=0452

Then read it, read it, and read it again. It is probably the best car book I have ever bought. All the VW ones are full of opinions and old VW myths. This book is full of facts and extremely useful information. I recommend it to everyone, even if you don't have an EFI car yet.

Basically EFI needs a very high pressure pump to operate. It also needs a surge tank / swirl pot to ensure the pump is continually fed with fuel. Air bubbles in the high pressure fuel line is a bad thing. Also, a carb can have it's fuel flow interupted and not car because you keep running off the fuel in the float bowl. EFI does not have a float bowl, as soon as the HP pump sucks air, the injectors lose fuel pressure and the engine cuts out.

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posted on September 3rd, 2003 at 11:41 AM


Dan,
Make that 8 mates (leagally) and I'm in on the race :-)
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posted on September 4th, 2003 at 12:59 AM


Yeah, Wes I agree,
thats a great book, my brother had a copy at one stage.

I wouldn't mind a copy of my own.


Cheers Tony




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posted on September 5th, 2003 at 04:26 PM


do u guys know of any fuel tanks with injectyion that had a similar patter to a Kombi???? or do u guys reacon getting a commodore fuel tank chopping it open and get out the surge tank???
cheers
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posted on November 4th, 2003 at 06:54 PM


get yourself a tank from an efi kombi.

this is my old surge setup. I just welded some ends onto a short length of stainless steel pipe for the pot. Both pumps were EFI kombi. My theory being that the one pumping no pressure would outflow the one pumping hp. pump 1 sucked from a 10mm fitting brazed into the bottom of the tank. All the EFi pumps I have seen have 10mm inlets. when the surge pot filled it just overflowed back to the main tank.

It worked. But the injectors were maxed out and kombi pumps didnt flow enough in the end, so I bought a VL turbo pump and bigger injectors. Could only afford one pump so I removed the pot and decided I would just run with 1/2 a tank of fuel minimum. And that worked too until valla where the bloody thing surged with a near full tank.

If I ever finish uni I'm going to get a 100 litre alloy boat tank and mount that under the back seat of my kombi. Then I can use all that space currently wasted by a fuel tank for turbos and intercoolers. Perhaps the alloy tank would be a better thing for any kombi EFI setup.

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Chris.... kombi pilot, oval dreamer... finisher #26971 2005 city to surf
http://members.ozemail.com.au/~amazer39/vwsigline.jpg
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