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page 21.)
New Jet= 0.057 (Calculated new main jet size)
Fuel Injection, High Speed Bypass Adjust.
This program will calculate a high speed bypass (lean out) orifice diameter change for mechanical
fuel injection systems equipped with a high speed bypass circuit. This circuit is often used to alter
the engine high RPM fuel curve to compensate for the loss in volumetric efficiency when operating
above peak torque RPM. Normally, the high speed bypass circuit has 2 working components; an
orifice to control the amount of engine fuel that can be bleed off and a spring loaded relief valve
(poppet) to control when the orifice can bleed off the engine fuel. The spring pressure is then
adjusted to correspond to the engine fuel manifold or rail pressure at which the bleed off will occur.
Important: This program assumes that a percentage of engine fuel can be bypassed back to the
fuel tank to make the high RPM air/fuel ratio leaner. The program will first calculate the current
HS bypass percent with the existing orifice. Refer to the example below where the
Old HS %= 16.21. Knowing the existing bypass percent, a new bypass percent can be requested.
In the example below, the New HS %? entry is 18.00. That change will make the engine almost
2% leaner at high RPM – bypassing almost 2% more engine fuel back to the tank. The program
will then calculate an orifice size for the new desired HS bypass percent. If less HS bypass is
desired to make the engine run richer, the New HS %? entry would be a smaller value like 15.00
or 14.00. Once the correct HS orifice is found for an engine setup, it normally won’t require
adjustment for air density changes.
Important: Suggested changes to the high speed relief valve (poppet) pressure setting are
presented in both the Fuel Injection Jets programs on the Rail Chg= program line. That line
suggests that an increase (positive number) or decrease (negative number) can be made to the
relief valve cracking pressure (PSI) setting to maintain the same engine RPM high speed bypass
operating point. This change is necessitated by the new main bypass jet and the resultant fuel rail
pressure change. For example: If the HS relief valve is currently set to open at 50 PSI and the
Fuel Injection Jets program indicates the Rail Chg= 2.0 (a positive value), adjusting the HS
relief valve cracking pressure to 52 PSI will enable the high speed bypass orifice to work at the
same RPM as before the main jet change.
The first time this program is used, all the engine fuel nozzle sizes must be entered in the setup.
When the setup is completed, the computer will remember the total nozzle area for future
calculations. This program and the Fuel Injection Jets, Area Method program use the same
total nozzle area number from computer memory.
Press TUNE-UP then Jetting Menu then select High Speed to start this program.
Setup Display Example:
== Calc Area == (Nozzle area setup routine)
# Orifices? 8 (Enter number of nozzles of this size)
Diameter? 0.028 (Enter nozzle diameter in inches)
More? No (Press NO to stop)
== Total Area == (Total nozzle area screen)
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