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Thread: EFI Fuel Systems - How do they work? What's a surge tank do?

  1. #1
    MR 18RG Chief Engine Builder The Witzl's Avatar
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    Default EFI Fuel Systems - How do they work? What's a surge tank do?

    Now this is a question that's asked almost as much as "how do i fit a BOV to my 4A-GZE?"...... ok maybe not, and its probably not that stupid a question.

    So here it is, a breif pictoral guide to EFI fuel systems on previously carburetted cars, and explanation of surge tanks.

    We'll just go through it briefly, if you have any more indepth queries about a particular part of this FAQ, please PM me and i'll try to update with more details... or direct you to the search button!! hahaha

    So whats the difference between the carby system and an EFI fuel system?

    On a carbied car, fuel is pumped (sucked) out of the fuel tank by a mechanical fuel pump driven off the engine's camshaft, and pumped to the carburettor at around 4-6psi worth of pressure. These pumps have three fittings: IN, OUT, and RETURN.
    IN - from the fuel tank
    OUT - to the carby
    RETURN - back to the fuel tank.

    Of course a fuel pump has no idea about how much fuel an engine is supposed to be using, it just pumps fuel at a given rate continously. Your engine wont be using ALL of that fuel, and so any excess is returned to the fuel tank via the return fuel line. Hence there should be fuel flowing in both the supply and return lines at any given time, and thus a continous circulation of petrol.

    So.... how does EFI vary from this?

    Well for starters they use an electronic fuel pump, and the supply lines after the pump are pressurised to around 40-60psi. Thats a LOT!
    In most cars, the electric pump is mounted INSIDE the fuel tank and pumps fuel up the length of the car to the engine's fuel rail. Some cars, eg. VL commodore, JE Camira, use an external pump just next to the fuel tank to pump fuel up to the fuel rail.
    So this means that you have 40-60psi pressure on the fuel supply line from the pump, right up to and inside the fuel rail.
    The fuel rail is what holds your fuel injectors onto the intake side of the engine and houses a volume of fuel under pressure which can flow through the injectors and into the engine when the ECU tells the injectors to open.
    Pressure is maintained in the fuel rail by the fuel pressure regulator, which has a mechanically controlled diaphram that allows excess pressure (fuel) to escape, and in doing so maintaining the correct pressure within the fuel rail.
    So the unused fuel flows out of the fuel rail through the fuel pressure regulator, and back into the fuel tank.... and then the cycle continues.


    Now a little about EFI fuel pumps....

    There is a reason they are usually mounted inside the fuel tank - THEY DONT SUCK GOOD. Unlike a mechanical carby pump, they arent very good at drawing fuel out of the tank... they are useless at it. Fuel almost literally has to be poored into the inlet of the pump - you'll hear the term "gravity fed" used to explain this. By having the pump inside the tank, the inlet is submerged in the petrol right at the bottom of the tank and thus gravity is forcing the petrol into the pump.
    In a VL commodore, the pump is mounted next to the tank and the supply from the tank is relatively low on its side, keeping the pump at a lower potential than the fuel..... these VL and JE pumps are able to suck a little... but i wouldnt rely upon it

    So THAT'S why you need a surge tank!

    Well you dont need one, but they are often used to easily facilitate gravity feeding an EFI fuel pump. Basically they are a secondary fuel tank you can place somewhere suitable to gravity feed the EFI pump, because mounting the EFI pump under the fuel tank is a silly idea in most cars!!



    Of course somehow fuel needs to get from the fuel tank to the surge tank - this is where an electronic carby pump, or "lift pump" is used. These are a low pressure, high volume pump that CAN suck.



    The easiest way to explain how it all works is with pictures, and so I will:







    The configuration like above ensures that the surge tank is always kept full, and thus fuel supply to the EFI pump is kept constant.

    Now some notes:

    # MAKE SURE to use EFI fuel hose for any rubber lines after the EFI pump. Carby fuel lines cant handle the high pressure. Some places also have "EFI" hose clamps... these are just normal hose clamps with bevelled edges and solid clamping face that doesnt bite into the rubber hose at all.

    # You MUST use a carby fuel filter before the lift pump, and an EFI fuel filter after the EFI pump. For ease of installation, a 300zx style efi fuel filter works well, having barb fittings at either end.

    # You can get away with not using a surge tank. AE86 and AE71 are two prime examples... using a JE camira pump on these, mounted in a similar location to where it would be on the camira/VL, works fine and has never been a problem for me or the people who's cars i have done this to.

    # Simply chucking an in-tank EFI pump into your carby fuel tank wont always work.... in most EFI tanks there are baffles that keep the fuel around the pickup of the pump to avoid starvation when the fuel is low and you are cornering. Most people put baffles in themselves or make a "swirl pot" - a small lowered pool at the lowest point of the tank where the inlet of the pump is then put.

    # Doing this yourself is EASY. Just be careful, and make TRIPPLE SURE you know which line is which when you are attaching hoses!!
    Last edited by The Witzl; 20-07-2007 at 10:24 AM.
    ...... butt scratcher?!


  2. #2
    Toymods Vice President Chief Engine Builder TheToyman75's Avatar
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    Default Re: EFI Fuel Systems - How do they work? What's a surge tank do?

    Karl,

    Great introductory overview and here is a few more important details.

    The primary role of a surge tank is actually to reduce fuel surge. A carburetor has a float bowl which acts as an internal fuel storage system. As such a fuel tank intended for a carby application will have less internal baffles and the fuel is prone to slosh around inside the tank when enjoying tight corners or a trip to the drive inn's (Of course you should have the car turned off at the drive Inns so that doesn't matter )

    When the fuel "surges" inside the fuel tank and you have a light fuel load (less than say a 1/3 of a tank) the pick up line will frequently suck at a pocket of air as the fuel is concentrated at one end of the tank. With a carby this is no problem due ot the float bowel holding a fuel reserve but with EFI there is no factory fuel reserve and an moment of reduced fuel supply with cause the engine to fuel starve. Ie run lean and possibly break.

    A surge tank is the answer to this dilema. As such a surge tank is ideally narrow to redue internal surge and maintains a good head of fuel.

    On a seperate note it is a good idea to run a prefilter before your main feed pump. An EFI filter profiides to much restriction and a carby style paper element filter is of no use against the higher feed rate of an EFI pump. As such dedicated Pre Filters are available to screen out foreign matter without restricting flow.

    Pump selection, When choosing your lift and main pumps its a good idea to ensure your Lift pump will slightly outflow your main pump, keep in mind this is flow not pressure. Your lift pump needs ot be able to supply enough fuel all the time.

    Last but not least for now is Voltage, ensur eyou run a good stable powersupply to your Fuel pumps. A voltage drop equals a drop in the pumps efficiency and generaly a reduction in flow and/or fuel pressure.
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