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Fuel Injection Bypass Circuits

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.

Main bypass

The main bypass is the most common one used for tuning.

>> A bigger main bypass is used to reduce engine enrichment

>> A smaller main bypass is used to increase engine enrichment.

Idle bypass

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

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.

The high-speed bypass can also be used for extra fuel enrichment at the low end to reduce power.  Drag racing with a torque converter may benefit from that.  Power reduction on the hit can help to avoid tire spin on the launch.

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.

6,000 HP on Methanol

Our 5000 Horsepower on Methanol tech manual features specs on some very high output methanol racing engines.  A 6,000 horsepower combination is detailed on p. 132. It is based on a top fuel drag racing V-8 long block with two stage compressors.  A minimum fuel pomp size is 32 GPM at 4000 pump RPM.  Boost is 7 atmospheres.  At a mechanical compression ratio of 10 to 1, approximately 0.23 cubic inches of fuel volume occupies about 4% of the combustion chamber volume at top dead center.  Going up in boost with more fuel pump can yield 12,000 horsepower also featured in this section.

Several 5000 horsepower combinations are described in Chapter 28 including Gene Sharber’s tractor pulling two stage turbo methanol engine combo gets 110 psi manifold boost.

Other high output engine combos are featured in appendix 29.  highlights are the GM Ecotec drag racing 4 cylinder at 1,450 HP and Smokey Yunick’s 209 ci turbo small block pulling 1,200 horsepower for the Indy 500  We thank GM Corp and Carbon press for insider details of these combinations.


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.