Fuel Injection High Speed Bypass for NA Engines
The high speed bypass is often used in a fuel injection setup.
NORMALLY ASPIRATED ENGINES
The high speed leans out the engine as the RPM go up and the volumetric efficiency goes down.
The VE is often near 100% at the torque peak.
The air flow per revolution is max.
A typical norm asp engine cammed up for racing may see a torque peak around 5,000 RPM.
Tuning should be with the main bypass for max power up to the torque peak or just beyond.
We often recommend an AFR of 4.8 to 1 for methanol fuel for the torque peak.
As the engine speed goes beyond the torque peak, the VE goes down. The air flow per revolution starts to drop off. That reduction is tunes with a high speed bypass. We often recommend an AFR of 5.2 to1 for methanol fuel for the horsepower peak.
That is often down with a high speed bypass, usually 0.070 inch dia, It is opened with a pressure controlled poppet or diaphragm valve. We ofetn recommend setting that high speed valve to open about 800 RPM above the torque peak RPM. That is then around 5,800 RPM. The specific pressure depends on the fuel pressure from the nozzle and main bypass combination. Common, moderate pressure systems may operate below 100 psi. Opening pressures for a high speed may be around 60 psi. This is quite common in drag racing engines, 305 & 360 sprint car engines, as examples.
Common, high pressure systems may operate above 100 psi. Opening pressures for a high speed may be around 110 psi. This later is common in max output 410 sprint car engines, for example.
When the main bypass jet is changed for weather changes, the high speed poppet needs to be changed as well. That is necessary to keep the fuel curve shape around the 5,800 RPM opening point.
Our mficalc can provide both the main bypass and high speed opening pressure for different weather. Determination of these revised values can be done by hitting about 5 buttons on a cell phone set up with mficalc.
