Fuel Injection Jetting for Ram Air

For a mechanical fuel injection (MFI) engine with a forward facing air scoop, ram air into the scoop increases with speed.  At about 200 MPH, a 14% increase in air was determined in our drag racing car as an example.

A corresponding increase in fuel was the best way to maintain an optimum air to fuel ratio.  In our case, that was 3.4 to 1 for blown methanol.

For MFI, that increase can be done in several ways.


The high speed bypass can be shut off for ram air at high speed for the enrichment if it is the proper size.  The shut-off function can be done with a manual/valve, pressure/solenoid/valve, or timer/solenoid/valve.


For MFI fuel systems with a main bypass, an extra nozzle can be added.  It is off at low speed.  As the speed increases, the nozzle can be enabled with a manual/valve, pressure/solenoid/valve, or timer/solenoid/valve.

Note: Some MFI fuel systems are run without a main bypass.  Nostalgia Nitro drag racing is an example.  This method will not work in that case.  With the extra nozzle, only a drop in fuel pressure would occur.  No added fuel would be delivered with ram air increase.


The unique Engler stumble valve can be added to a bypass circuit.  It operates opposite to a high speed poppet.  As the fuel pressure increases, the stumble valve will shut down.  In the bypass circuit, that will richen the engine.  The stumble valve by itself will automatically initiate with fuel pressure.

Note 1: That would occur in low gear as well if the engine is revved high enough.  To disable that, a manual or timer controlled shut-off valve would be needed.

Note 2: The Engler stumble valve bypass requires an inline jet to restrict the fuel flow when the valve is open.  That jet restriction is part of the bypass circuit for tuning.


Jetting for proper enrichment for high speed ram air can be done with our ProCalc MFI jetting calculator.  Lining up the nozzle or bypass jet sizes can be done to maintain an optimum air to fuel ratio throughout a race.  Enabling pressures can be determined as well.  We can also provide this setup service and training with our ProTune & ProTune Advance services.


In a 225 MPH top dragster, a 0.060 in diameter high speed bypass was initiated during the low speed portion of the run.  At 200 MPH the high speed bypass was turned off to maintain a 3.5 to 1 air to fuel ratio above 200 MPH for the blown methanol engine.

Note: Without this function, the spark advance had to be reduced about 10 degrees as a band aid to the leaner running engine from ram air.  The car ran more consistently with fuel enrichment for ram air than with ignition retard for ram air.


Especially with low percentages of nitro for match racing, enrichment for ram air is better for tuning than an ignition retard alternative.  An optimum air to fuel ratio can be mapped & maintained throughout a run with proper enrichment for ram air.