Understanding Fuel Injection

by GhostMachine


Fuel injection is a powerful fuel management tool. Many enthusiasts are afraid of or intimidated by the wiring and ‘black box’ of fuel injection systems. This article will help take away the veil of mystery and help you to understand how the system works and how you can use it!What is fuel injection? In the simplest terms it is a direct replacement for a carburetor. It meters fuel and air into the engine to make power. Much like the carb there are parts you can change or tune to increase power and/or economy. First off let’s take a look at what makes up a fuel injection system.

All systems regardless of who makes them have these basic components

1. A throttle body to meter in air.
2. A throttle position sensor.
3. Some type of RPM sensor.
4. Fuel injectors
5. And of course the ECM or computer.

These are the basic components.Every system has a Throttle position sensor. It’s a potentiometer just like the volume knob on a stereo. As you turn the knob the volume increases or decreases. The computer sees the increase or decrease of the throttle and knows that the ‘volume’ of air is increasing or decreasing.

Every system measures engine RPM. Most systems use the tachometer signal from the distributor to accomplish this but in some cases they will add one or both of the following sensors; Crankshaft position, or camshaft position. Either sensor will give the RPM of the engine but will also provide other useful information as well. The Crankshaft or camshaft sensors will tell the computer where the engine is in the rotation cycle. This means the computer will be aware of which cylinder is firing and which cylinder is taking in fuel or exhausting burnt gasses.

Every system has fuel injectors. An injector is a solenoid and is like a switch. When it’s on it delivers fuel and when it’s off it doesn’t. Injectors are rated to deliver a certain amount of fuel in a certain amount of time at a certain pressure. Most fuel injectors are rated in pounds per hour @ 43.5 PSI. By increasing the pressure, there is a limit, you increase the amount of fuel the injector can deliver. Another thing to be aware of is the type of injectors. There are high and low impedance injectors. High impedance injectors are about 16 ohms and low impedance injectors are typically 2 ohms. Stock ECM’s run high impedance injectors and can’t handle low impedance injectors without modifications or they will burn up pretty quick. 50 lb/hr is typically the changeover point from high to low.

Of course there has to be a computer! This device holds large tables of information. It uses these tables to determine how much fuel to deliver at any given engine speed or load. It determines this from the inputs it gets from all the sensors and the output is the pulse of the fuel injectors.

Now that the basics are covered lets talk a little bit about the different systems out there. There are 3 major categories of fuel injection systems. First is the Throttle body or Central point injection. This system consists of one or more injectors that spray fuel just above or below the throttle blades. The fuel and air mix travels through the intake runners into the heads and into the cylinders just like a carburetor does. This is the least efficient system but is still better than a carburetor. The second system is a multi port or point system. In this system there is an injector for each cylinder and it is located at the end of the intake runner and sprays fuel into the cylinder heads intake port. This system is more efficient since the air doesn’t have to carry the fuel through the entire intake system. The air/fuel mixture is more even and burns in a more uniform manner making more power from less fuel. These systems typically use the ‘batch fire’ method meaning that injectors are made into groups and all injectors in that group fire at the same time even if the intake valve is closed when they fire. The last system is sequential port or point injection. This system uses the crank and/or cam position sensor to determine which cylinder is taking in air and injects fuel only into that cylinder at just the right time. This obviously is far more efficient since no fuel is being sprayed at closed valves.

Now that we know what sensors are required and what kind of systems are out there lets talk about the other sensors that are usually added to injection systems.

Virtually every system out there has an O2 sensor. The O2 sensor measures the temperature of the exhaust at a particular point in the exhaust stream. The exhaust temp is directly related to how rich or lean the engine is running. Hotter exhaust indicates lean and colder indicates rich. The O2 sensor operates by generating a voltage that varies by temp. This change in voltage is then interpreted by the computer as how rich or lean the engine is running and can be used to increase or decrease the amount of fuel being injected to maintain the most economy or power. Usually there is just one but there can be as many as 4 of them on one engine.

The M.A.P. or M.A.F. sensor is usually used to determine the load on the engine. The M.A.P. stands for Manifold Absolute Pressure and measures the amount of vacuum the engine is generating. A map sensor will know if you are towing a load or not by the vacuum level of the engine since the more load the lower the vacuum at any given throttle position and engine speed. All systems that use map sensors are called Speed Density. The M.A.F. stands for Mass Air Flow and measures the total volume of air being consumed by the engine. Any engine with a load will consume more air at any given throttle position and engine RPM than an unloaded engine.

Another sensor that is very common on system is the coolant temp sensor. This sensor measures the temperature of the engine coolant and can adjust the amount of fuel used and even the engine idle speed.

There is also a MAT or manifold air temp sensor that measures the temperature of the air coming into the engine and can change the amount of fuel to account for a colder denser or thinner hotter atmosphere.

In systems where ignition timing and advance is controlled by the ECM, there are Knock sensors that listen for certain tones generated by spark knock and can retard ignition timing until the knock goes away. Some of these systems use an add on ESC or electronic spark control module and other systems handle this strictly with the computer.

Now that we have covered the basics of the systems let’s get into how the system really operates! Below is a very simplified example of a fuel and spark table.

If RPM = 3500 and TPS = 22% and O2 = .700 Mv and map = 12.6″ and CTS = 185 degrees and MAT = 86 degrees then Fuel injector pulse = 5.6 Ms and Timing = 33 degrees.

This would be a hypothetical computer sequence. It looks at all the information coming in and uses it to produce a ‘pulse width’ and ignition timing amount. These tables are actually huge and cover every RPM range from the lowest the engine can idle to the highest it can rev, every throttle position from wide open to fully closed, every O2 voltage level from way too rich to way too lean and so on and so forth. It takes in these inputs for all the sensors and produces an output for each cylinder in every rotation of the engine. Obviously this happens several thousand times a second at higher RPM’s.

Let’s look at the output. in the example the output was 5.6 ms and 33 degrees of timing. First let’s look at the 5.6 ms. this is the amount of time the injector is opened or ‘pulsed’. There are certain things that the computer has to assume. The computer is programmed to use a value for the amount of fuel per Mili second the injector can deliver. This value is based on the ratings of the injectors themselves. If you change the injector or increase or decrease the fuel pressure without telling the computer it will keep using the same old programmed value and then start to compensate when the O2 sensor shows a rich or lean reading. After a set amount of time the computer will, if so equipped, will turn on a maintenance or service engine light to indicate a problem. Let’s say for this example the injector is rated at 19 lb/hr. This means that the injector can deliver 19 lb of fuel @ 43.5 PSI in 60 min. 1 of these injectors at maximum flow can supply enough fuel for one cylinder generating 30 HP. In an 8 cylinder engine with 8 injectors this will make a maximum of 240 HP. In order to make a 350 HP engine you have to have 28 lb/hr injectors with 43.5 PSI fuel pressure. Also changing fuel pressure up or down will increase or decrease the fuel delivery of the injector. For example the 19 lb/hr injectors rated @ 43.5 PSI will deliver 20.37 lb/hr @ 50 PSI. To figure flow based on pressure changes use the following method. Divide the new fuel pressure by the old pressure (50/43.5 =1.1494). Then find the square root of that number (SQRT = 1.0721) and multiply the square root by the original flow of the injector (1.0721 * 19 = 20.3699). This will give the new flow at the new pressure.

Fuel Injector Size Calculator

Number of (same size) injectors to be used
System fuel pressure @ fuel rail PSIG
Brake Specific Fuel Consumption
(* See note below)
Injector Duty Cycle
(Note: In practical use duty cycle should not be more than 80%.)
Actual Desired Crankshaft Horsepower B.H.P.

This is the size injector you will need when rated @ 43.5 PSI.

Lb/Hr    cc/min

* Brake specific should be .45 to .50 for naturally aspirated engines,
.55 TO .60 for supercharged engines, and .60 to .65 for turbocharged engines.


Second let’s look at the timing output. The timing of 33 degrees is how much before TDC the ignition should fire. Much like the old mechanical advance distributors the advance will increase with engine RPM. There is no vacuum advance in computer controlled ignition systems so the computer must add timing based on engine RPM and vacuum levels and throttle position. It must know all three in order to give the correct amount of timing to make best power or economy! For instance it will add more timing at very low throttle openings and steady mid range RPM with high vacuum levels to get the most economy and will not add any timing other than RPM advance if the throttle position is high and vacuum low.

Some systems also employ an ESC or electronic spark control module. This module and the knock sensor ‘listens’ to the engine and if it detects spark knock it will retard the timing a preprogrammed amount based on RPM, throttle position, vacuum level and knock intensity. This is done to prevent engine damage that detonation can cause.

Now that we have covered most of the injection system lets look at how the systems can be installed. For obvious reasons the factory systems are the easiest to retrofit into any project car. Mainly because you can run down to almost any local parts store and obtain replacement sensors or parts and there are literally thousands of cars in salvage yards that you can obtain complete systems from wrecked cars. There are also millions of aftermarket parts out there designed to upgrade existing stock systems for more performance. One of the most popular systems to retrofit is the GM TPI or tuned port injection system. Since this system was available on 85-92 camaros and corvettes there are plenty of systems out there.

If you want to install a fuel injection system on an older car that was not available with fuel injection you will need to keep in mind a few items. Older carburetors used a fuel pressure of around 5-7 PSI from a mechanical pump. Throttle body injection systems need 9-14 PSI and multi port systems typically require 35-45 PSI but can require upward of 60 PSI to operate. With this in mind you need to find a fuel pump that can generate this kind of pressure. Since most injection systems mount the fuel pump inside the gas tank this may pose a problem for some older cars that you can’t modify the tank to accept one. There are however inline fuel injection pumps that can be had by the aftermarket or even from other stock vehicles. For example the 1988-92 ford F-150 (I know, I know it’s the F word!!) with the 300 strait six fuel injected engine uses an inline fuel pump that can generate about 60 PSI at around 40 gallons an hour. This is sufficient to power an engine making up to around 325 or so HP. If you need a bigger pump you will have to go aftermarket. Holley and others sell inline pumps that can get as much as 225 Liters per hour or enough to feed a 500+ HP engine.

Another factor is the wiring. Unless you have a lot of time and patience the factory wiring harnesses are not a good choice. These harnesses typically have a lot of extra wiring in them and they are very interconnected with other non relevant systems in the cars. This is where the aftermarket makes our lives easier! Painless wiring and others sell custom, and very simple, wiring harnesses that are color coded and use all factory weather pack style connectors that are labeled! These harnesses typically only require you to hook up about 3 wires and plug in the connectors to the individual sensors. Since all sensors in most GM systems have a unique plug you can’t plug the wrong connector into the wrong sensor.

The last factor is the type of injection system you choose. While everybody would love to have a sequential port injection system, it is the most costly and complicated of all the systems. Most people typically used the multi port system like a TPI or the simplest system the TBI. The TPI is the most commonly used aftermarket system. Even here there are 2 different systems to consider, mass air or speed density system. They both have there advantages and disadvantages. The Mass air system doesn’t care how large the cam you stab in it is. All it cares about is the amount of air flowing into the engine. This has the advantage of unlimited cam choice but the disadvantage of the more complicated air stream plumbing, the expensive mass sensor itself and the restrictiveness of the sensor and related plumbing. Since the mass has to be in the air stream before the throttle plates these cars typically have a 90 degree elbow right in front of the throttle blades. The speed density system uses engine vacuum so it greatly simplifies the plumbing issue. You can stick a filter directly on the front of the throttle body if you want. The disadvantage is that since it does work on vacuum your more limited on cam choice. The engine must either generate 17″ of vacuum or you have to have custom adjustable map sensor or custom programming to work with your specific camshaft and engine combo. This can get expensive and time consuming since every time you change the cam you have to reprogram the chip. Most stock chips can ‘learn’ there way around typical bolt on items and will work to the 300+ HP range.

There are aftermarket programmable computer systems on the market that fit a very wide range of applications. Any engine making more than 550 or so HP on injection will typically need an aftermarket programmable computer and any engine that makes enough power to require larger than 50 lb/hr injectors will pretty much demand one. The programmable computer will be much more flexible and typically can be used for anything from NOS to superchargers and turbo applications and they can be obtained with injector drivers to handle the low resistance of injectors larger than 50 lb/hr. These systems will require some basic programming skills and a lot of trial and error. The advantage of programmable systems is you can reprogram as often as you want or need to without having to send out your chip and be without your ride!

So a fuel injection system can be your friend! With it you can have a very powerful car that is fun to drive, easy to start and much more streetable than most carbureted cars out there. So don’t be afraid of these systems! They are your friend in the ever-advancing electronic age!


Chevy Mania!

Facebook Activity


Chevymania! Sponsors Chevymania! Sponsors Chevymania! Sponsors Chevymania! Sponsors Chevymania! Sponsors

Copyright © 2017 Chevymania!