Well i have been thinking about posting my thoughts on my upcoming fuel system and just wanted some feedback.


OK here we go, most numbers are close but still appox

Starting from the top, I am plaining on upgrading my fuel system to support my huge ported motor with the GT42 turbo on it. Right now i am still on stock fuel, just running like 5psi of boost, witch isnt anything really.

For the fuel pump(s) i am going to be using 2 Walbro Intank fuel pumps

There are two types for the 255lph intank

There is a Normal or High Pressure, for the rest of the thread i am using

I am hoping that everone understands as the pressure increases the fuel flow decreses ( Because the pump is having to work harder to jump the pressure up )

42HP for the 255lph high pressure
17Nor for the 255lph Normal Pressure
These are the last 2 numbers on the part number

Here is what i have come up with

42HP- 255lph (67.3gallons) @ 0psi
17Nor - 255lph (67.3gallons) @ 0psi

Most people think that these pumps flow 255 @ 40psi and this is incorrect

Now at 40psi

42HP- 219lph @ 40psi
17Nor - 219lph @ 40psi

Remember this is at normal operating pressure ( IE NO BOOST )

When you start adding boost to the intake you are lowering the fuel able to enter the lower pressure area ( IE Intake )

So at 10psi of boost you will need 50psi of fuel pressure to equal 40psi of normal pressure to get what ever your injectors will put out ( Like 850cc @ 40psi )

Now at 20psi of boost yoy need 60psi of fuel pressure

42HP- 196lph @ 60psi
17Nor - 174lph @ 60psi

Now i understand that there are very few people that are driving street cars that run higher than 30psi of boost, so this is where i will stop at 30psi, so you need 30psi more in fuel pressure

Here is where you can tell the Normal from the High Pressure

42HP- 181lph @ 70psi
17Nor - 113lph @ 70psi

Here you have a almost 70lph change in your fuel flow into you tiking time bomb


Now there is one thing that i havent calulated is the flow that the stock hard lines will flow ( If someone wants to do this for me please do )

So for a recap @ 60psi (20psi of boost), 2 42HP fuel pumps will move 392lph of fuel at 60psi, or 392,000cc/h or 6533cc/m

OK now we know how much fuel that we have to work with, lets see how much we can put into the motor.

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Now for fuel injectors

I am going to be running 2x850cc and 2x1600cc pretty normal now in the FD age

2x850cc/m @ 40psi (It maybe 42psi)
2x1600cc/m @ 40psi

This is a total of 4900cc/m @ 40 psi

Just sitting here writing this i think i forgot about something, i was going to calulate what the fuel flow @ 60psi would be but if the intake psi is 20 then it should still only flow 40psi of fuel, so all of the other work I did was a waste.

But i will share this though

We know that we need at least 4900cc/m of fuel flow and we know that we have 6533cc/m of fuel pump

So we have 1633cc/m of fuel left over, and this is if we only want 40psi of fuel, witch most people want more for safety

So lets say that we want 50psi of base fuel pressure, at least this is what i want

2x850cc/m @ 50psi
2x1600cc/m @ 50psi

This is a total of 6125cc/m @ 40 psi

Now remember that we only have 6533cc/m of fuel and 6125cc/m of demand so we are right on the edge of what the pump will put out

I do understand that this is all at 100% Duty Cycle so you will have agian 4900cc/min of fuel

You just dropped your DutyCycle by 20% just by increasing your fuel pressure by 10psi.


I just wanted to spend some time really explaning what you have to think about when you are plaining a new fuel system.

If you want to add please do.

 

Flow is not proportional to the pressure across the injectors. IOW, 25% more base pressure will not give as much flow as 25% bigger injectors.