Fuel Injection Fundamentals: Understanding The Three Different Fueling Strategies

10 min read

Fuel Injection Fundamentals: Understanding The Three Different Fueling Strategies

10 min read

Although it has been offered for sale in commercial applications since 1957 (in mechanical form), many enthusiasts consider the concept of tuning fuel injection a black art. Over the last decade, the advent of self-tuning systems, and highly user-friendly software interfaces) has made it much more affordable and attractive to install a high-performance aftermarket fuel injection system on an enthusiast vehicle. The various types of injection systems use different parameters to determine appropriate fueling depending upon the system’s capabilities and the application. The goal of this article will be to provide a high-level overview of each type to help you understand how each works and which will deliver the best performance for your vehicle and needs.

In the interest of providing accurate information from the engineering and tuning side, we enlisted Doug Flynn, Engineer at Holley EFI, to assist us in breaking down the details. Doug’s background in EFI tuning and development is extensive and expansive to say the least.

Three Methods of Measurement

Although each type of system uses a different strategy to arrive at its method of fueling, the end goal is to maximize performance while minimizing the potential for fueling errors. Different types of engine combinations thrive with varying methods of fueling. For example, what is suitable for an enthusiast’s engine may not be appropriate for an all-out race car.

Flynn explains, “When you have a fuel-injected engine, there has to be some sort of calculation or methodology where every time the fuel injector fires, it asks, ‘how much fuel do I put in?”’

Sniper EFI timing

In addition to fuel injection, Sniper EFI systems offers a custom timing control feature to allow the user to build a custom timing table using a laptop. There’s an excellent video describing this process on Holley’s website.

“Closed-loop operation is where the oxygen sensor corrects by adding or subtracting fuel that it’s calculated through the feedback from the oxygen sensor. Open-loop operation solely relies upon the calculated value. Just to be clear, every engine first uses the open-loop value, then takes the feedback from the oxygen sensor to alter the value. The closed-loop is a secondary function after the open-loop calculation.”

Every engine needs some sensor inputs to calculate fuel amounts for injection, and each of the three methods — Speed Density, Mass Airflow, and Alpha-N — uses RPM as one of the measurement factors.

Mass Air Flow

This type of engine management system uses a mass air flow sensor (MAF) to measure the amount of air volume entering the car’s engine. The sensor is located before the throttle body. Several different types exist, from a hot-wire sensor to a frequency-based sensor, but the main task it performs is to measure the airflow that passes over its surface. The sensor then sends its values to the ECU alongside the measurement from the intake air temperature sensor. These combined values are then used by the ECU to determine air density and corrections required by the fuel injection system.

“Assuming everything is calibrated, the mass air flow sensor will tell the engine how much airflow is coming in. The mass air flow sensor gives you half the calculation. The downside for a mass air flow sensor is that it’s very particular to how and where that sensor is installed. Basically, it needs straight air going into it. If you have an aftermarket air tube or stick it somewhere random, that sensor isn’t going to read correctly, and your calculations are going to be a mess, which is one of two reasons the aftermarket doesn’t like using them,” says Flynn.

In a production vehicle, the mass air flow sensor is installed in a known location (for each model/engine configuration) and there is no variance on the location of the MAF.

MAF sensor

In a mass air flow system, the sensor measures the amount of air that passes over the sensor. In conjunction with this measurement and the information from the intake air temperature sensor, the ECU can calculate air density and required fuel adjustments to meet the target air/fuel ratio.

The other concern with using a mass air flow sensor shows its face concerning camshaft dimensions and overall engine operation; specifically, how the airflow movement occurs when the camshaft is on the large side, with the potential for reversion into the intake tube. Reversion is when the exhaust gas flows back up into the intake tract. It is an issue that gets worse as RPM levels rise and then fall again as the RPM maxes out and would appear like a bell curve if it were graphed. Most importantly, this reverse flow affects air/fuel ratios and can contaminate the fresh charge while hurting combustion efficiency. At low airspeed and low RPM, reversion will confuse the mass air sensor and present a multitude of issues with tuning the engine.

“If you have a factory engine with a factory air tube, mass air is likely a nice thing. But once you get into modifying the engine, you lose it,” says Flynn.

Alpha-N Tuning

The Alpha-N style of engine tuning looks at the throttle position sensor and measures RPM to arrive at its calculations, instead of looking at the intake manifold pressure like Speed Density does.

“You only use Alpha-N on a naturally aspirated engine; you never use it on a boosted motor of any type,” says Flynn.

Alpha-N works on the pounds-per-hour concept and references RPM versus the percentage of throttle opening as measured by the TPS. Typically, Alpha-N is used in a situation where the user has a big camshaft that makes terrible or fluctuating vacuum, so the engine management system instead uses the throttle angle and makes fueling adjustments based on that parameter.

Pro Stock

The Holley HP engine management system is powerful enough to be the spec unit for every car in the NHRA’s Pro Stock class.

“It won’t care if the MAP sensor is pulsing all over the place. But the bad thing about Alpha-N is that since it’s looking at throttle angle, if you change the throttle body on the engine, you lose your entire tune-up. At 50 percent throttle angle with a four-hole throttle body compared with 50 percent throttle angle on a huge single-hole throttle body, you might have double the airflow. The idle tune-up and wide-open-throttle tune-up may not change, but everything in between will,” says Flynn.

Flynn also explains that Alpha-N can’t sense a change in load — for example, the shift from Neutral to Drive where load changes. The tune-up needs to be slightly rich in Neutral so that it is correct when in Drive.

“Alpha-N is best used on a drag race-only engine that needs to idle, go up on the chip, and go down the track. It’s not the ideal choice for a street driven car,” he explains.

Speed Density and VE-Based Tuning

Many aftermarket engine management systems used by enthusiasts, especially in the racing world, use what is called the Speed Density style of calculations, which measures the engine’s speed and the engine’s air density to help the tuner arrive at the proper calibration. Included in this type of tuning is VE-Based tuning, which instead uses the engine’s volumetric efficiency to develop a fuel map. Neither is right or wrong, instead Flynn says they are different tools to achieve the same result — a properly tuned engine.

Let’s start with a discussion of VE-based tuning, which is used in Holley’s Sniper and Terminator X products as the base calibration style. Note that these systems also permit the use of speed density tuning through the use of a laptop.

Holley EFI VE table

Volumetric efficiency calculations are the key to the Sniper system’s performance. These calculations utilize the engine’s size and the manifold air temperature to help calculate the amount of fuel required to reach the targeted air/fuel ratio for a given situation.

“For VE-Based models, the fuel table units are calculated in the engine’s Volumetric Efficiency. One of the pros about tuning in Volumetric Efficiency is how much air is packed in at a given RPM and throttle angle compared to the engine’s cubic inches. A good explanation of this is that if you have 100 percent volumetric efficiency on a 400 cubic-inch engine, it means you’re probably wide open and the throttle body is not restricting air the engine needs. You’re getting 400 cubic inches of air each time all eight cylinders are filled. Basically, 100 percent volumetric efficiency equals wide-open throttle. The highest VE value also indicates where peak torque occurs,” says Flynn.

There are some variations to this theory, concerning super- and turbocharged engines specifically as they pack in more than the engine’s theoretical naturally aspirated capability. Still, NHRA Pro Stock engines also exceed the 100 percent mark due to camshaft and cylinder head optimization strategies.

According to Flynn, “VE table values for boosted engines in the fuel table are still around 100 percent when the engine is at WOT, even under boost. The calculation takes into account pressure over atmospheric, so if the MAP sensor is reading 14.7 psi of boost, the amount of fuel injected would be double compared to no boost at WOT.’ As boost goes up, the VE drops due to losses so you’ll usually be at 70-90 percent. The engine’s ‘real VE’ may be 200 percent, but the table value is still around 100 percent as the calculations take the boost into account.”

VE calculations look at the number in the fuel table, then look at how many cubic inches the engine is, the manifold air temperature, and finally, the target air/fuel ratio. For example, a 400 cubic-inch engine has eight cylinders for 50 cubic inches each. At wide-open throttle, the engine is at 100 percent volumetric efficiency, and the system says there is full airflow, 50 cubic inches to fill, and 13.0:1 commanded air/fuel ratio. The system knows via these calculations how much fuel needs to be injected to satisfy these requirements properly.

“The beauty of this method is that once the engine is tuned and the VE table is correct, you tune it by changing air/fuel ratio and not the VE table if you are trying to lean out the cruising air/fuel ratio from 14.0:1 to 15.0:1,” explains Flynn.

Sniper cubic inch selection

One of the advantages of the Sniper system is its ability to provide accurate fueling based on thousands of test vehicles in Holley’s database. The user answers several questions about the engine, and then the system tunes itself based on these inputs.

For the Sniper product specifically, when using the handheld tuner, the end-user is asked a series of questions during the setup process, one of which is to specify the engine’s displacement. All of the information that the Holley EFI team has compiled from tuning thousands of vehicles with many different types of engine combinations plays into this process, as the answers provided by the end-user allow the system to optimize itself based on the responses received. This information makes the self-tuning process simple, and many users will never need to mess with it again. These calibrations are done both on engine dynos and in real-world vehicles.

“The guy with the Sniper wants to type in that he’s got a 400 cubic-inch engine and he wants a wide-open throttle air/fuel ratio of 12.5, and all the magic happens using the VE method. That’s what he cares about,” says Flynn.

Additionally, laptop software is available for the Sniper systems for those users who want to control fueling and timing themselves.

MAP sensor

The MAP sensor delivers instant information to the ECU concerning the engine’s manifold pressure when used in conjunction with the IAT sensor and engine speed, and this information helps the system to calculate the air density to provide accurate fueling.

Conversely, Holley’s software for the HP and Dominator systems do not have a handheld tuning unit and require laptop tuning. These can provide base map calibrations in both VE-style and pounds-per-hour fueling. Flynn says that these systems are pointed squarely at the customer who is out at the racetrack regularly, trying to squeeze every last bit of performance from their vehicle.

These engine management systems use a Manifold Absolute Pressure (MAP) sensor in conjunction with an Intake Air Temperature sensor and a monitor for engine RPM to utilize the Speed Density style of tuning. The MAP sensor measures engine vacuum and provides instant feedback to the ECU, which enables it to then use the IAT sensor to determine air density along with the engine’s airflow. From those calculations, it determines the amount of fuel required to achieve a specific air/fuel ratio to support that flow effectively.

“We use two different strategies using the MAP sensor to calculate fuel flow. What you have is a lookup table with the X-axis showing RPM and the Y-axis showing engine vacuum from the MAP sensor. The more you apply the throttle, the less vacuum there is into the engine as you get toward wide-open throttle, and as you get to wide-open throttle, it’s pulling none to just a little bit of vacuum. So the MAP sensor is a good indicator of engine load; the more vacuum you have, the less load you have going on with the engine,” explains Flynn.

PPH Fuel Table

The fueling table in the Dominator’s software uses Pounds Per Hour fuel flow calculations to achieve the specified air/fuel ratio. This method of fueling is typically utilized by hardcore racers who have measurable data regarding the engine combination and can make educated decisions regarding potential changes.

With Speed Density calibrations in the Holley engine management software, fuel flow is measured in pounds per hour. It is determined by what the user knows about the engine in question: is it supercharged or turbocharged, how much horsepower it makes, and many other factors that affect the required fuel demands. For engine builders, knowledge regarding the engine’s Brake Specific Fuel Consumption will provide a guide to base tuning parameters.

With the Dominator, the user can toggle from the fuel flow method to the VE style of calibrations. Flynn says that both choices are available because there are benefits to each for the advanced tuner, such as the Stevie Jackson-tuned Musi engine in Marcus Birt’s Radial Vs. The World nitrous-injected machine.

Ultimately, selecting the proper type of engine management system for your project means that you need to determine your needs effectively and which kind of tuning solution will be best for your situation. Holley’s broad line of engine management systems can provide precisely what you need, regardless of the type of performance engine you have.

Marcus Birt's RvW Corvette

The image tells the story: Holley’s Dominator is docile enough to run a street car, and power the most advanced nitrous-injected Radial Vs. The World car on the planet.


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