Good Fuel Pressure with a New MFI System

Setting up a new MFI fuel system:

  • main bypass adjustment to control the air to fuel ratio
  • adjustment of the combination of the main bypass and nozzle sizes to maintain an optimum fuel pressure range from the lowest to highest engine RPM.

Changing the main bypass jet for tuning affects the air to fuel ratio and fuel pressure.  As long as the jetting combination is maintained for a fuel pressure operating range and the air to fuel ratio, the engine should run very well.  The fuel pressure operating range should be over 50 psi on the low end and not to exceed the fuel pump pressure rating at the high end.  Many tuners determine all of that with trial-and-error testing.  Our writings show how to do that numerically as a quicker way to the best performance.

Simple (neural network) Math for Fuel Pressure vs Flow

fuel pressure in psi = multiplier x [flow in GPM / jet area in sq. in.]2

Multipliers from some popular fuel systems

fuel system examples                                                             multiplier

Kinsler (radiused nozzles & bypass jets)                                 0.00055
Hilborn (flared nozzles & bypass jets)                                     0.00065
Enderle (square edge nozzles & bypass jets)                          0.00075
nostalgia nitro (16 misc nozzles)                                             0.00120
professional top fuel (misc nozzles @ real high pressure)       0.00140

Example*:

flow = 12.53 GPM

jet area = 0.029 square inches from our simple blown alcohol fuel system

  • hat jets: 8 x 0.053 in. dia.
  • MBP: 0.115 in. dia.
  • HS: 0.040 in. dia.

Enderle multiplier = 0.00075

fuel pressure in psi = 0.00075 x [12.53 GPM / 0.029 square inches]2

fuel pressure = 140 psi

Throughout our books & ProCalc MFI calculator, this simple multiplier is called volume area (VA) factor.  It is very easy to determine for a specific fuel system or jet or nozzle, and it remains a stable value for different tuning changes.  With it, tuning is easier!

Note: The actual mathematics for pressure vs. flow is very complex involving calculus.  Most racing engine tuners who try to get into that mathematics alternative become overwhelmed.

*Corrected 8 PM PT, 12/05/2018; if you read or downloaded this example prior to that, please read & replace that copy with this corrected version; in the previous copy, an error was noted by Earl Miller, Kinsler Fuel Injection; we appreciate Mr. Miller’s edit & correction.

More Tech Info Involving Fuel Pressure on Racecarbook Website

Solutions to Idle Lope in Mechanical Fuel Injection