While mechanical fuel injection is frequently used by many motorsports racers, several are missing out on the fine control provided by this type of engine. One such control utilizes a high-speed bypass to divert fuel to the engine and maintain control over the engine’s power.
Both normally (naturally) aspirated and forced induction engines with mechanical fuel injection (MFI) typically use fuel pumps that are bigger than necessary for their purposes. They can then benefit by controlling the amount of fuel to the engine using one or more bypass jets to return excess fuel to fuel supply.
MAIN BYPASS JET: Racing mechanical fuel injection jetting is made up of nozzles to the engine and one or more bypass pathways back to the fuel supply. While individual nozzles control fuel flow to each cylinder, the main bypass jet determines overall fuel flow to the engine.
- A bigger main bypass is used to reduce engine enrichment
- A smaller main bypass is used to increase engine enrichment.
The current enrichment is needed for best performance.
A positive displacement fuel pump with a standard jetting layout makes a linear ‘fuel curve’ corresponding with engine RPM. That is, the fuel to the engine-per-revolution is essentially constant. As RPM goes up, fuel goes up.
HIGH-SPEED BYPASS: A high-speed bypass is used to shape the fuel curve. That shape depends on whether the engine is normally (naturally) aspirated or uses forced induction.
Normally aspirated: Torque Peak vs Horsepower Peak
For a normally (naturally) aspirated engine, the fuel to the engine-per-revolution varies between the torque peak and the horsepower peak. The torque peak provides the highest volumetric efficiency meaning more fuel-per-revolution is needed. It usually occurs at a moderate RPM. The horsepower peak occurs at higher RPM and has less volumetric efficiency causing less torque. More fuel-per-second is needed for the higher horsepower, but less fuel-per-revolution is needed because of the reduced volumetric efficiency.
The horsepower peak does more work than the torque peak even though the torque at the horsepower peak is lower. That is because there are more pulses of power per second. Hence, more work is done up to the point where torque at the high RPM is so low that the greater frequency of power pulses cannot overcome the torque reduction.
A HIGH-SPEED BYPASS: In order to maintain consistency between these two fuel consumption scenarios, a high-speed bypass can be used to tailor different fuel amounts for both of these engine speeds.
A common & best practice for both conditions:
- torque peak with high-speed bypass closed
- horsepower peak with high-speed bypass open.
A valve is used in the high-speed bypass circuit to control the opening point. It is usually one of the following:
- pressure-controlled device that automatically tips in with greater RPM
- solenoid device that is off-or-on
- manual valve that can be off-or-on, tipped in, or partially opened by the driver
- variable flow slide valve controlled by an automated system.
Pressure-controlled high-speed valve
A pressure-controlled device is most common to automatically control the opening point of the high-speed bypass. A poppet or diaphragm valve can be used. The pressure-controlled valve is typically set to open at about 600 RPM over the torque peak.
One example of utilizing a pressure-controlled valve is called flat lining. The torque peak of a typical drag or sprint car 2-valve racing V-8 engine is often around 5,200 RPM. The high-speed pressure valve would be set to tip open around 5,800 RPM. As engine speed goes up beyond that, the pressure valve continues to tip-in further. That reduces the fuel to the engine. That reduces the fuel-per-engine-revolution to match the reduction in volumetric efficiency as the engine RPM increases.
Some fuel injection combinations with the high-speed function are set up using only a high-speed bypass valve without a jet. The valve alone provides some fuel restriction for high RPM lean-out. The amount of fuel bypass flow restriction is provided from the open valve by itself. In a typical methanol drag or sprint car V-8 engine, it is approximately equivalent to a 0.070 to 0.075-inch diameter jet size by itself. If that bypass flow from only the open valve by itself is too much, it causes a curve that is too flat. The top end would be too lean. To compensate, a restrictor jet is added to further reduce the high-speed bypass flow when the valve opens.
The combination of valve flow restriction and restriction from a high-speed jet determines the fuel curve reduction as engine speed climbs to the horsepower peak. Again, for drag or sprint engine setups, 7,600 RPM is a typical horsepower peak engine speed. The high-speed bypass flow is from a combination of the valve and jet restriction. A high-speed jet size around 0.070 inches diameter is common. Ref. High Horsepower Tuning for Mechanical Fuel Injection for Normally Aspirated Racing Engines.
A high-speed bypass for top end leaning is not always needed
If the operating range of a racing engine is less than about 1,500 RPM, a high-speed bypass is usually not needed. The main bypass jet size can be adjusted alone for best power. Examples are top speed events or banked oval pavement circle races at 1/3 mile around or bigger that run flat out. Spark plug readings after a run are most often the best tuning indicator of a proper combination whether a high-speed bypass is used or not.
Other types of high-speed bypass controls
SOLENOID CONTROLLED HIGH-SPEED BYPASS: Some tuners use an electric solenoid valve to control the opening point of a high-speed bypass valve. Many are set with a timer to open at a specific interval after the start of a race. Others use a pressure transducer control as well that operate off of various racing vehicle functions. Racing category examples are drag racing and tractor pulling.
MANUAL VALVE CONTROLLED HIGH-SPEED BYPASS: Some tuners set up a manual valve for driver operation. The driver opens the high speed for a surge in power or at a repeated time in the race for best high-end power.
VARIABLE FLOW SLIDE VALVE: A variable flow slide valve such as a BDK, AFT, or AJ (Pete Jackson) valve is used in Top Fuel drag racing as well as other very high horsepower motorsports categories. It is a very high flowing valve commonly used with large fuel pumps. It is controlled by an automatic system that is usually timer based. Further info is beyond the scope of this article.
Forced induction with a high-speed bypass
The roots blower is a common supercharger in forced induction engines where the blower flattens out the torque over quite a wide RPM range. Because there is not a wide difference in volumetric efficiency between torque peak and horsepower peak, a high-speed bypass is not usually necessary for high-speed lean out. Adequate high-end tuning can usually be done with only a main bypass adjustment.
Steve Woods, a very well-known drag racing AAGS nostalgia class record holder and master tuner in the USA, did not use any high-speed bypass with his best power setup for his numerous record runs.
High speed for low end enrichment
For some drag racing classes, a high-speed bypass can be added for launch enrichment as a safety factor. The high-speed bypass is closed at the lower launch RPM. It opens after the launch.
For example, in our blown altered funny car, we had good experience with a 0.040-inch diameter high-speed jet for this purpose while using methanol fuel. We also had good experience with a high-speed poppet opening at just beyond our flash RPM at the start. That was about 4,500 RPM at the hit or flash, with a high-speed bypass opening about 5,200 RPM. The transition from the launch to full power mid-range and beyond was seamless. I could not feel any surge in power. The power was good for the duration of the run. For larger high-speed jets, I could feel a surge in power when the high-speed bypass opened. That indicated a loss in power on the launch.
Note that with a larger high-speed jet, a smaller main bypass jet is needed to maintain the best fuel mixture at the high end. It is a combination of flow from both pathways.
Launch enrichment for power reduction
Some fuel injection tuners, especially those who race on marginal drag strip pavement, use the high-speed bypass setup to intentionally reduce power on the hit. This is done with extra launch enrichment.
THE TORQUE CONVERTER: One case where that is frequently needed is when a vehicle is using a torque converter. The extra torque multiplication of the stator can make the roots blown engine strike the tires on the hit. A simple overly rich fuel mixture set up for the launch can clip the power. The racing vehicle with a blower becomes more repeatable.
The high-speed bypass is ‘OFF’ during the launch. As fuel pressure increases, the poppet or diaphragm valve opens. A good combination of oversize high-speed jet with a smaller main bypass achieves the best power air/fuel ratio (AFR) for the high end.
Increasing the high-speed bypass size and reducing the main bypass in correct combination can further reduce launch power. That correct combination can maintain high end power. A high-speed bypass poppet set up to open at a fuel pressure that occurs about 60 feet into the run is a good combination. Here are some jetting examples from a high compression ratio engine using a Roots blower on alcohol fuel:
The same top end power level is reached with different bottom end power levels. That is illustrated by the horsepower reduction estimates from various launch enrichment AFR’s. MFI tuners can reduce launch power on high HP drag cars who compete on poorly prepped racing surfaces. Note that an overly rich high-end air/fuel ratio of 3.8 to 1 is needed with a high compression ratio, high boost alky setup.
For example, we worked with a racecar tuner who intentionally decreases their drag race 60-foot times by 0.1 seconds with power reduction from launch enrichment. That represents a launch power level more capable of ‘hooking-up the tires’ in poorly prepped drag race starting lines. They set up an MFI jetting combination using a high-speed bypass that is OFF during the launch. That richens the engine. Then the high-speed bypass opens to lean out the engine after the launch. We worked with them to determine jetting for a launch AFR that is 0.6 AFR values richer than their best power air/fuel ratio. This combination provided good performance on poor-to-moderate track surfaces that were common for nearby racetracks.
Ram air without a high-speed bypass at the high end
Prior to the late 80’s, roots blower designs were less efficient and would flatten out at high blower speeds. Many racing tuners used a high-speed bypass to reduce high-end fuel just like in the previous normally aspirated description. That was to chase the air reduction from the blower air flow going flat.
Later blowers are better on the high end, and a high-speed bypass at the high end is less effective or unnecessary. For racing vehicles that get to higher velocities, ram air occurs at the high end occurs from any forward-facing air scoop. That needs more fuel with speed. Unless the blower is stalling from excessive RPM, more fuel is needed at the high end due to ram air. A richer air/fuel ratio provides good low-end response or punch. More fuel is needed for the high end. We are finding that in many applications, NO high-speed bypass may be better. With the proper AFR at launch, the engine may go a bit rich in the mid-range. That is usually good for increased cooling, especially in high boost applications. Then it leans out back to a good AFR at the high end. Tuning is easy. Only adjustment to the main bypass is needed for air density tuning. With the proper AFRs, spark plug readings remain consistent for best power.
Summary of Fuel System Delivery
The high-speed bypass used in MFI racing engines can enable finer control over the fuel curve throughout the course of the run. At points of higher RPM or fuel pressure, a high-speed bypass can reduce the amount of fuel to the engine. At launch, it can be utilized as a means to provide more enrichment for better starts.
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