How To Set Up Your Fuel System

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Diagram shows how to convert your previously carby car for EFI conversion, or also for a rising rate carby turbo fuel set up.

Fuelflow.jpg

As mentioned on the diagram, The return line from the engine bay can go directly back into the tank, Or back into the surge tank.

For the carby turbo setup, Pretty much any EFI pressure pump will do the job (except in-tank pumps*). However all of the pressure and flow they make is not required for the carby. This can be successfully halved by running a ballast resistor (ceramic box next to coil) to drop the voltage to 6v.

Most in tank pumps have a relief valve that will let fuel exit through the body of the pump. If you used one of these setup in the above diagram, it would spray fuel all through your boot!!!


Draw-through CARBY>TURBO/SC>MOTOR


Provided the carby and its components (jets, needle and seat etc) are actually up to the job of flowing enough fuel, the fuel setup required is just the same as any other NA carby car. This is because the carby is not under pressure. You also cannot use an intercooler with this set, as the "air" passing through it is already mixed with fuel...perfect for combustion! If you were to have a back fire through the cooler (As there is no throttle body between the engine and cooler) Then it would most likely explode taking the other parts from the front of your car with it.


Blow-through TURBO>COOLER(OPTIONAL)>CARBY>MOTOR


Post-3856-1183177744 thumb.jpg


The problem: When you boost a carby in blow through, you also pressurise all of your fuel system - float bowl, fuel lines, pumps, filter even the tank can feel the effect of the boost. The only way to combat this is to have higher fuel pressure than boost pressure, Whenever the car is actually on boost. You always need to keep a buffer of at least 2-3psi more fuel pressure. This is only ON BOOST, When the car is running in vacuum it only needs the fuel supply of a normal NA carby. There's 2 way I've successfully done this:


1, I used a carter black fuel pump which flows 100gph @ 14-16psi. This is a big pump and would typically be used on a V8 etc, But all you need it for is the pressure. I had 12psi at the engine bay with this pump and did not use any reg. But that was ONLY because the needle and seat in the weber I was using could hold that pressure at idle. You would not be able you use this setup on say a stock Corolla carby, As the needle and seat would be forced open and cause it to flood.

Keeping a 2-3psi buffer of fuel pressure, 12psi will allow you to run around 10psi of boost. To use a pump like this with a carby that has a lower needle a seat tolerance see below.


2, Many brands make rising rate fuel pressure regulators for carby cars. These work exactly the same as an EFI RR reg, Except they run much lower base fuel pressure. The cheapest and most available seems to be a Malpassi (like EFI ones), and can be had on eBay occasionally or bought new for around $100.

Heres a cross section of the malpassi...


Large fuel reg.png


The inlets in the top left and right are parallel and can be used in a couple of different ways.

a, used inline before the carby like a normal NA reg.

b, If your carby has a return line, Run the line from the pump straight into the carby and then run the return of the carby into the inlet of the reg. Then block the other inlet/outlet off, Or you can use this port for a gauge.


Both methods will hold the selected base pressure in the fuel lines and carby, Then raise it on boost. These regs can be used with both high pressure carby pumps >20psi, Or a convention EFI surge tank set up as pictured above, But in both cases a return line must be used. When it comes to running high boost pressure, the surge tank setup is the only way to go as an EFI pump can make more pressure to keep your fuel pressure buffer. As I mentioned above, if high fuel pressure (20+psi) is not needed, but if you would like to run the surge tank setup with an EFI pump, you can run the power source for the EFI pump through a ballast resistor. This will drop the voltage to 6v and halve the output of the pump if it is too high.

A rising rate regulator works by using a vacuum/boost line off the manifold. When the boost pressure comes up in the reg, It raises the fuel pressure in relation to boost. You can get them in ratios of 1:1, 1.5:1, 1.7:1, 2:1.....12:1 etc, But most carby ones are 1:1, With a base pressure of 4-5psi (which most carby will hold) and a rising rate index of 1:1 no matter how much boost you run, that 4-5psi will always be your buffer.

Think that about covers it


See also



Article by Taz_Rx



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