Borrowed from Bob Szabo – Air Density online
Racing mechanical fuel injection jetting is made up of nozzles to the engine and one or more bypass pathways back to the fuel supply. The positive displacement fuel pump with the jetting layout makes a linear ‘fuel curve’ with engine RPM. That is, the fuel to the engine per revolution is essentially constant. As RPM goes up, fuel goes up. Typical fuel pumps are usually bigger than necessary. Tuning is done by changing one of the bypass circuits. That is done to reduce or increases the fuel bypass for one of several different purposes.
The main bypass is the most common one used for tuning. Main Jet, aka pill
>> A bigger main bypass is used to reduce engine enrichment (pill)
>> A smaller main bypass is used to increase engine enrichment (pill)
Idle bypass (unblown usually 1.5 to 2.0 lbs pressure opening)
Most fuel injection systems use an idle bypass. It is enabled at low throttle positions. It bypasses extra fuel for the idle needs where the throttle valve is closed down. Only a small amount of air is going into the engine. Only a small amount of fuel is needed for idle. The idle circuit shuts down as the throttle is opened. That is to deliver the full amount of fuel to the engine at full throttle.
High speed bypass aka Hi Speed lean out (from 50 to 100 psi with a restrictor jet, pill to control fuel curve)
A high-speed is used to provide an added bypass for various functions. For a normally (naturally aspirated) racing engine, it is commonly used to open at high engine RPM. That reduces fuel per revolution. That is used to match the engine fuel need. In a normally aspirated engine, that usually goes down at higher engine RPM because of a reduction in volumetric efficiency at higher RPM.
For a blown engine, older blowers would lay down at high speeds. The high-speed bypass was used in the same manner. It would open at higher RPM to reduce fuel per revolution. That would match the declining fuel need with the blower that would pump less air at higher speeds.
Pump relief bypass
Some FI barrel valves are equipped with an added bypass circuit for pump pressure relief. That is usually only enabled at low to mid throttle positions. When the throttle is partially closed at high RPM, the fuel pump is delivering a lot of fuel. However, the barrel valve is closed down. Without pump relief, the fuel pressure can go too high. This circuit is disabled at mid to full throttle.