Idle & Low End Air/Fuel Ratio Tuning Tool, Flared/Radiused Jet Orientation

We are now developing techniques using our jetting calculator to set up idle air to fuel ratio for really good MFI racing engine tuning.  Idle settings in many MFI racing engines are key to superior performance from engine temperature and off idle response. That is done by adjusting the combination of idle vacuum (throttle blade opening), barrel valve leak down setting, and idle fuel amount. Our calculator can be used to determine an idle air to fuel ratio so that an optimum idle characteristic can be controlled.  One or more of these settings can be adjusted to produce an ideal idle air to fuel ratio that produces a consistent engine temperature for different air densities.  A combination can be developed for lope free idle that can be reproduced for a different engine combination such as a different altitude or, for blown engines, different blower speeds.

Note: Here are notes on how we are studying idle air/fuel ratio:

  • Idle vacuum of 10 inches of mercury, for example, is subtracted from barometric pressure. The temperature of the inlet from vaporization cooling is measured with a heat gun. Those values are combined with the humidity and engine or blower size information in ProCalc to determine the weight of air at idle.
  • The idle leak down of 25% for a normally aspirated engine, for example, is multiplied by the idle fuel volume determined from ProCalc to determine the weight of fuel at idle.
  • ProCalc is then used to determine the idle air/fuel ratio for the idle setup that is entered.

Low End air/fuel ratio from Part or Full Throttle
We are also looking at air to fuel ratio tuning analysis for part throttle or full throttle for drag and circle racing.

Flared or Radiused Jet Flow Direction
Restrictor jets from some manufacturers use a flared or radiused opening on one side of the jet.  That design provides better flow than the flow from a square edge jet opening.  The bypass jet (main, high speed, or pump sizer) is often added to a fuel line with a tee fitting.  It is normally held with a pill can or jet holder fitting.  Various setups put that bypass jet in different orientations.  In some plumbing layouts, the jet is mounted with fuel feeding the flare opening.  That is the correct layout.  In others, we saw the jet that was mounted with the fuel feeding the square side of the hole.  The flare faced the outlet.  That was an incorrect layout.  Attention should be paid to orient the jet with the flare or radiused hole entry facing the flow source.  The flare or radiused hole located on the opposite side does not help the flow.  This is a good example of the following:

Fuel injection tuning is a maze without an understanding of fuel injection science.