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Step 4: The Fuel System

In the last section you learned how to size your fuel injectors for the air/fuel requirements of your motor. Now you will learn about the fuel pump you will need and how to setup the fuel delivery system to feed your injectors.


The diagram below shows you the fuel path. Notice how the fuel flows across all 4 injectors, then through the fuel pressure regulator and back to the fuel tank. This is the design used on the vast majority of fuel injected vehicles because it  balances the pressure and flow to each injector.
High Pressure Fuel Pump ComparisonFuel Pump:  Up to now, all you needed in a fuel pump was something that would make 2 or 3 PSI and have enough flow to keep the float bowl full. On blow-through turbo applications, it had to stay 2 to 5 PSI above boost pressure, to keep the float full.

With a  "Sequential Double Fire" Fuel Injection system it is possible, under boost, that this system could end up with all the injectors on 100% of the time. This would require the fuel pump to match the flow of 4 injectors at once at max fuel rail pressure. If the pump does not keep up, the engine will lean out.

You can find the high output pumps on the German cars in the wrecking yard. This includes the Volvo's, BMW's, Audi's, Mercedes, and VW's. Most of these ran some version of the Bosch fuel injection. The electronic versions didn't run quite as high output of pump as the mechanical versions did. Why, I don't know. I do know if you find a VW Rabbit with a mechanical version fuel injection, it will have a high output pump!

Test the Fuel Pump: The only real way to know what you have is to do a flow test @ 45 PSI. This test includes everything from losses due to restrictive fittings to the voltage. Unless you do this test, everything else, charts, graphs, etc., everything these people tell you is 100% Pure Bogus! I can't stress this point enough. (Probably because with fuel injection, you have to be able to generate a fuel surge, where with a carb, you pulled it out of a reservoir of fuel).

The are many ways to convert the flow of an injector to something one can more easily understand. The unwritten industry standard is becoming CC's per minute at 45 PSI, or 3 Bar (three barometric pressures). So for a 2276 VW using four 45 Lb/hr injectors, each injector will flow about 16.5 Ounces per minute at 45 PSI. At worst case, the fuel pump will have to be able to flow 66 Ounces of fuel per minute at 45 PSI across all 4 injectors or the engine will go lean!

To test a pump for a turbo 2276 VW using four 45 Lb/hr (16.5 Ounces/min), Take the fuel pump and connect it to a regulator with a pressure gauge, set the regulator to my max rail pressure (static is usually 30 PSI and Boost compensation is 15 PSI max boost) which is 45 PSI. Next measure the amount of fuel that returns to the tank in one minute. The amount of fuel that returns to the tank is what the capacity of the pump is at 45 PSI. The pump either keeps up, or it doesn't. If it doesn't pass this test, you need another pump. Don't expect the "Little Girlie" pumps, the ones that are the size of a roll of quarters, to pass this test. The only ones that we have found to work best are the Bosch. There are some other options here, but none cheap.

Gas Tank Mod: The intake of the fuel pump is 12mm, or about 7/16. We have gotten away with using 3/8 ID hose if it is real short. The pump needs to be mounted under, or as close as possible to the tank. They are a roller vane style pump. They cannot suck very well, but they sure can push the fuel.

The output of the pump can be either 5/16 or 3/8, whatever is convenient. The input and output of the Bosch fuel pressure regulators is 5/16.

Most round aluminum gas tank have to be modified. First, the 1/4 pipe thread fitting, inside the fitting where the fuel comes out of the tank, if you look, it only has a 1/8 hole punched into the tank. This will have to be drilled out as big as possible without cutting into the threads on the fitting. Next, a second port will have to be added for the fuel return.

Note Some have added a 3/8 pipe port and use this as the intake to the pump and then use the 1/4 pipe port for the fuel return line.

Don't let the guys at the welding shop song and dance you to how dangerous this will be by adding the second port. Any competent shop knows that if the tank is filled with an inert gas, such as CO2 when welding, it cannot explode. You have to have fuel and oxygen to have an explosion. No oxygen, no problem. Some boat shops and truck stops do this all the time.


Turbo Fuel Pressure Regulator:
(aka Rising Rate Fuel Pressure Regulator)
For a Turbo and Fuel Injection application you need a fuel pressure regulator that understands boost pressure.  ie. The regulator needs to see the intake boost pressure not atmospheric pressure. The reason is simple: As you increase intake manifold pressure (boost), you need more fuel pressure to push the gas into the intake.

They can be bought from Summit Racing for about $61 (#MSD-2222) That unit is the Bosch regulator shown above but MSD has repackaged it with their own pressure sensing port in the adjustment screw. To save a few bucks pick one up at the junkyard for $5, or buy a new one ($40) and add your own pressure port.

To make a Turbo Fuel Pressure Regulator: Start with an adjustable fuel pressure regulator Bosch #0 280 160 001. It was used after 1968 on the Bosch D-Jetronic cars: VW Type III 68-73, Porsche 914 70-76 1.7L or 2L, Saab 69-74, Volvo, Renault 72-74.

The screw below is the modified adjustment screw.


Not for the faint of heart: One trick is to chuck the screw into the drill chuck and mount the drill bit on a small vise on the drill press table. Then drill the screw onto the bit. This will work if you mount the drill bit straight and with enough room between the vise and the screw for it to flex enough to accommodate any mis-centering.

You need to drill a hole across the bottom of the screw, then drill a hole down the length to meet that hole. The lengthwise hole is difficult to drill by hand so you will probably need a machinist to do it on a lathe. Once you get the hole drilled, solder a piece of tubing into the hole. The local hardware store should have a small piece of brass tubing.

To solder to steel, it has to be freshly ground, not wire wheeled, or sanded. You can use regular electrical solder, 60 / 40, rosin core. If the top of the bolt has just been kissed on a grinding wheel, solder will stick to it just like copper!

Remember: When you get done soldering the brass or steel line into the top of the bolt, make sure you can blow through it. (After it cools off that is)


Tim's Octane, AV Gas, and Snake Oil Tech:

Octane is a rating of how fast fuels burn under compression. The higher the compression, the faster fuel burns. The higher the octane, the slower fuel burns. Remember, it is a rating. It is impossible to have over 100% of something, so don't be confused by a rating of 110 octane! (Read the Gasoline FAQ)

There are lots of different fuels out there that claim all kinds of things. It is a matter of personal choice. On a turbo motor, you will have to use a good fuel, but just how good is another story. Is a good fuel measured by it's "Octane" equivalent number, or by price? Different fuels use anything from paraffin to ?? for octane boosters. I have tried just about every fuel known in the free world, all with different results. What I like about the Av-Gas is it is controlled by the FAA. Nobody wants to go to jail for messing with the quality. Peoples lives depend on it! It is designed to have low vapor pressure, doesn't sour, and not a whole lot of "Brother In-Law" additives. The price for the Bang-For-The-Buck is a lot cheaper than the "Other" premium fuels. (Note that a lot of the "Others" use Av-Gas as the base stock for their fuels. They just add their snake oil and mark the price up 300%. Then there are legitimate vendors. Just be aware).

Our buggy group has mixed and matched, added our own blend of snake oils, etc. and we have all come back to using Av-Gas and methanol in our rails. This is "Our" observation and testing and opinions. You are free to try whatever, but all I ask is do real measurements on performance when comparing and not by some wanna-be speed shops recommendation. Read about the makeup of fuels, how they burn, and educate yourself. 


Further Reading:

Fuel pumps and fuel injection


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