Technical Information Fuel Pumps

Fuel Pressure

The relationship between changing the fuel pressure and the amount of fuel delivered is the square root of the ratio of fuel pressure. If fuel pressure is doubled then about 40% more fuel is delivered. To double the fuel delivery four times the fuel pressure needs to be used. But that is only possible if the pump is capable of delivering fuel at that pressure. You will see that the fuel volume drops as pressure is increased in the test below.

Approximate injector sizing for a boosted B16A
Target Flywheel HP Injectors needed at 40psi fuel pressure
170 240 (stock)
280 440
350 550
460 720

Other disadvantages of running high fuel pressure:

  • Fuel injectors require more current to open meaning they run hotter and are less reliable as a result.
  • Fuel injectors can take longer to open.
  • There is a greater tendency for the fuel to leak past the injector seals.
  • There is a greater chance of rupturing the diaphragm of the FPR (usually rated to 100 psi) dumping fuel into the intake.

Fuel Pump Dyno Test By RC Engineering

Pump Type: 2000 Civic Si Pump # AF195130-4113

Voltage: 13.8

Temperature: 74 F

Specific Gravity: .76

As the fuel pressure rises, the volume of fuel the pump can flow drops.

This means that the fuel pump can flow enough fuel for large amounts of power if the the fuel pressure is kept low. This means that large injectors will need to be used.

What is important to note is that this pump (and most others) has an internal bypass valve that opens at 75 psi.

So from this graph, Hondas running the stock pump, and a rising rate regulator top out at 75 psi and about 200hp. So, this is not a good solution.

But, if you run your stock fuel pressure (say 40 psi) and this stock fuel pump, 650cc injectors will easily provide you fuelling for low 300 wheel HP. Honda computers will easily idle a 1600cc engine on 720cc injectors.

At higher pressures the fuel pump works harder. At 75 psi the pump is generating 40% more heat than at 35 psi. This will obviously heat your fuel more than necessary.

For further information on fuel flow and aftermarket Walbro fuel pumps click here.