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Nitrous Fuel Injection Guide
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Fuel Injection Fundamentals
When you combine the precision control of fuel injection with the latest high-flowing top end
components and nitrous oxide, you have the potential to make your fuel-injected engine a monster
killer. Anytime you increase airflow, which is essentially what a nitrous system does, you also have
to introduce more fuel to keep the air/fuel ratio at optimum levels. This means you have to do two
things: tune the fuel curve by programming the computer and install a set of injectors that will
deliver the fuel required by the engine. Deciding how to select the appropriate size and style of
injector is the focus of this story. But first a little background on electronic port fuel injection.
Port fuel injection, as the name implies, injects fuel directly into each port just upstream of the
intake valve. This type of injection uses at least one injector per cylinder. One of the main
advantages is that fuel can be introduced near the valve, leaving most of the intake manifold dry.
This allows near-perfect cylinder-to-cylinder fuel distribution. A dry-flow intake manifold is much
easier to design since fuel distribution isn’t a problem. Port injection also promotes superior fuel
atomization and subsequently more efficient combustion because fuel is injected at high pressure
through a small hole directly in the high-speed airflow.
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Two different types of airflow calibration are used with port fuel injection: speed density and mass
flow. It’s further characterized by two types of injection: batch, or group fire, and sequential fire.
Sequential injection means that the injection of fuel is timed to coincide with the valve opening.
Group fire triggers a bank of injectors with each ignition cycle. Sequential injection is the current
state-of-the-art in electronic fuel management.
Speed density fuel injection uses the speed of the engine and the density of the air, along with a
sensor to measure manifold vacuum, to calculate engine airflow. Most after market EFI systems
also use speed density.
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Mass flow fuel-injection systems use a Mass Air Sensor (MAS) to measure the mass of the air being
inducted into the engine. Intake air is ducted past the MAS, which measures total airflow in one of
several different ways depending on the type of MAS. The most prevalent type is the hot wire
sensor pioneered by Bosch. The hot wire sensor routes airflow past a heated wire(hot wire). This
wire is part of an electronic circuit that measure electrical current in milliamps. Current flowing
through the wire heats it to a temperature that’s always above the inlet air temperature by a fixed
amount.
Air flowing across the wire draws away some of the heat, so an increase in current flow is required
for it to maintain its fixed temperature. When air flow is low (idle), little current is required to heat the
wire to temperature. At high airflow (WOT), it takes a lot of current to heat the wire because heat is
being removed from it more quickly. The current necessary to heat the wire is proportional to the
mass of air flowing across the wire. A temperature sensor in the MAS provides a correction for
intake air temperature so that the output signal is not affected by it. A circuit in the MAS converts
the current reading into a voltage signal for the Electronic Control Module (ECM), which converts it
to grams per second (gps). The output of this sensor is not linear with respect to airflow; it’s
sensitive to low air flow and less sensitive at high air flows. Idle speed air flow is typically about 4 to
7 gps, increasing with rpm. The hot wire is made of platinum and is sensitive to contaminants or
deposits; therefore, it is super-heated after engine shutdown to burn off any deposits.
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Mass flow fuel systems measure the mass of the air directly, so there’s no need for the ECM to
correct for air density. Other inputs to the ECM include a throttle position sensor and an O2 sensor
for closed-loop air/fuel ratio control. Once the ECM knows the amount of air entering the engine, it
looks at the other sensors to determine the engine’s current state of operation idle, acceleration,
cruise, deceleration); then it refers to an electronic table or map to find the appropriate air/fuel ratio
and select the fuel injector pulse width required to match the input signals. Finally, the ECM
energizes the fuel injector for the appropriate number of milliseconds to inject the fuel.
A Mass Flow fuel system adapts easily to changes in the engine as well as hardware because
airflow is measured directly. In other words, a Mass flow system is self-compensating for most
reasonable changes to the engine and is extremely accurate under low-speed,part-throttle
operation. The downside is that the sensors are expensive and sometimes unreliable. Many MAS
also provide a considerable restriction to airflow in high-horsepower engines, limiting their power.
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A speed density system calculates the airflow of the engine since it has no sensor to measure it
directly. If you simplify the engine as an air pump, theoretically, it will move half of its displacement
in air for every rotation of the crankshaft (half because it’s a four-stroke engine). Thus the engine
itself is an air meter. Engines, however, rarely flow the theoretical airflow due to restrictions in the
inlet, the cylinder head, and the exhaust.
The volumetric efficiency (VE) of an engine is defined as the ratio of the actual mass airflow to the
theoretical mass airflow. If an engine flows its theoretical airflow, then the VE would be 100percent .
At WOT, high-performance engines can approach a VE value of 100 percent and racing engines
can exceed 100 percent within a specific rpm range because of more efficient inlet and exhaust
tuning. All engines will have low VE values at part throttle (except for engines equipped with a
turbocharger or supercharger, where the inlet manifold is often pressurized under part-throttle
conditions.). The volumetric efficiency of an engine changes for every throttle position and engine
speed. A large table or map of these values can be generated on an engine dynometer by
measuring the actual airflow at all the speed load points and calculating the VEs. This procedure is
called mapping an engine.
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Speed density systems use this map of engine volumetric efficiency to calculate the air flow of the
engine under any operating condition. These systems measure engine vacuum via a Manifold
Absolute Pressure (MAP) sensor. This sensor reads absolute pressure in KPA (Kilo pascals) and
supplies a voltage signal to the ECM proportional to manifold vacuum. All of the VE maps are
referenced by manifold vacuum and rpm; the computer reads engine speed (rpm) and manifold
vacuum (KPA) and looks in the reference table to find the volumetric efficiency at this speed load
point. Once the computer finds the VE value, it computes the airflow directly. As most racers know,
air density changes with temperature; therefore, the computer must then correct the calculated
airflow value based on a sensor reading of the air temperature in the manifold.
The computer’s calculations are all based on the map of VEs. Production variations and wear and
tear are not compensated for when a test engine is mapped. If the intake or exhaust manifolds were
changed, this would seriously affect the volumetric efficiency of the engine and throw the computer’
s calculations into error. A racing engine would be remapped to incorporate any changes, but this
is obviously not feasible for car manufacturers. Production cars compensate for wear and
production variation through the closed-loop control provided by the exhaust gas oxygen (EGO)
sensor. This sensor supports calculation of the air/fuelratio based on the oxygen content of the
exhaust. The ECM looks at the air/fuel ratio from the EGO sensor (also known as the O2 sensor)
and corrects fuel delivery for any errors. This works fine when the engine is in closed-loop control
mode (all part-throttle driving conditions),but when the engine is at WOT, it’s not under closed-loop
control and correction factors are not that accurate. Obviously, the ECM is doing a lot of number
crunching with a speed density system.
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N Alpha fuel systems are a simple design for engines that operate primarily at WOT and are thus
used extensively in racing. N Alpha uses only the speed of the engine (N) and the throttle angle
(alpha) to calculate the required amount of fuel delivery. These are simple speed-density systems
that use throttle angle to approximate load instead of a MAP sensor. This approach is logical for
racing engines with aggressive camshaft profiles that generate weak manifold vacuum signals and
spend little time at part-throttle. N Alpha systems are just as accurate as speed density systems at
WOT, but have much less accuracy at part throttle due to the reduced size of the engine map.
That’s the basic run down on how electronic fuel-injection systems work. The point for all this is to
give you some background so you can see just how delicately balanced your engine’s fuel
management system is. All the components have to work together to make power while retaining
good driveability. Selecting the proper fuel injectors is just one piece, though a critical one, of the
electronic fuel-injection puzzle.
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Choosing the proper injector is always a compromise, especially for a car that needs part-throttle
response. In other words ,if you still have to drive the car on the street or around the pit, you can’t
necessarily install the injector that will deliver the most fuel. Installing over-sized injectors is
somewhat analogous to over-carbureting an engine. And in extreme cases you can pump so much
fuel into the combustion chambers there isn't a chance in hades that it’ll fire or idle.
That said, here are some useful formulas to guide you in selecting the proper injector for your
combination. The two main criteria are injector size, or the amount of fuel it will deliver, and injector
compatibility with the electronics that control the injector.
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How To Calculate the Proper Size Injector for Your Combination
An injector consists of a solenoid that moves an internal plunger when the magnetic windings are
energized by the application of voltage. A sized orifice is opened when the plunger is activated,
allowing pressurized fuel to flow through the created opening. The critical element is the injector’s
ability to maintain linear fuel flow from narrow pulse widths to wide pulse widths, so that the dynamic
range of fuel delivery remains accurate for any given rpm and load requirement. The injector’s
metering orifice is designed to spray the fuel in a cone-shaped pattern of 15 to 30 degrees for
optimum fuel atomization.
Fuel flow is controlled by varying the pulse width or duty cycle of the injectors. Pulse width is the
time in milliseconds that the injector is open, while duty cycle is the injector’s overall percentage of
open time. A 70-percent duty cycle means that the injector is open 70 percent of the injector’s
maximum cycling time.
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Ultimately, to find the optimum injector size for a given application, you have to test it. You can map
it out on a dyno sizing the injector based on observed maximum brake horsepower (BHP) and brake
specific fuel consumption (BSFC) at peak power. You can also use a wide band oxygen sensor that
tells you the a/f ratio at the load points for which you’re tuning. The following formulas will get you
close to the correct size injector for WOT performance. Driveability and idle require a little more
finesse, which we’ll cover later.
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(BHP x BSFC)
______________________
Number of injectors x 0.8)
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= Injector size (flow rate)
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The scaler 0.8 adjusts the calculated injector size to produce the fuel necessary for peak power at
an 80-percent duty cycle. An accurate BHP figure is critical for proper injector sizing, but not all
dynometers have fuel flow instrumentation, so BSFC is often estimated at approximately 0.5
lb./bhp-hr. for normally aspirated engines.
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Take an engine with a known BSFC of 0.49 making 300 horsepower. Applying the formula, we
derive:
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(300 x 0.49)
___________
(4 x 0.8)
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= 45.9 lb./hr. (required injector flow rate)
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You also can calculate the maximum horsepower a given injector size can support by plugging a
known injector size into the formula using either the measured or estimated BSFC.
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Flow Rate x Number of Injectors x 0.8
__________________________ = HP
BSFC
or:
50 lb../hr. x 4 x 0.8
_________________ = 326.5 HP
0.49
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Running an engine on a dynometer to determine its performance statistics isn’t practical for most of
us, so BHP is often estimated by using quarter-mile performance and one of the performance slide
rules, or “dream wheels.” You also can calculate the CFM flow of the engine using assumptions
about volumetric efficiency. (See the section on sizing a carburetor for the formula to find the CFM
of your engine at max rpm.)
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Then use the following formula to convert CFM to estimated injector size.
CFM x 0.44298
_________________________ = estimated injector size
# of cylinders
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This formula only gives an estimated injector size and assumes one injector per cylinder on a
normally aspirated engine.
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Fuel Flow vs. Fuel Pressure
A fuel injector is a precision-calibrated orifice. All injectors are rated for flow at a specific fuel
pressure, typically 43.5 lb. (3 bar). The injector flow rate will change if the supply pressure is varied.
For Example: Calculate the static flow rate of a 24 lb./hr. injector when the fuel pressure is raised
from 30 to 40 psi.
F2= Sqrt of 40 psi/30 psi x 24 lb./hr.
F2=1.1547 x 24
F2=27.71lb./hr.
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Higher fuel pressure generally means better fuel atomization, but it also makes the injector works
harder when opening. Increasing fuel pressure also slows down an injector’s response time. Typical
response time is 1.5 to 2.0 milliseconds. When pressure is raised significantly—from 43.5 lb. (3 bar)
to 72.5 lb. (5 bar)—the injectors may have to work so hard that their useful life is drastically
shortened. Generally it’s safe to raise the fuel pressure no more than 10 to 15 percent. Raising the
fuel pressure of a stock injection system changes the specific fuel flow calibration of the injector.
The computer bases all its calculations on the known calibration of the injector. When the
calibration changes due to an increase in fuel pressure, the computer cannot know this without a
calibration change (PROM change). Since the injectors flow rate changes, all the computer’s
original calculations are in error and the fuel curve will experience a shift that may be harmful
across most of the engine’s operating range. On a turbocharged engine, or with the appropriate
NOS system installed, with a linear pressure regulator, extra high pressure exists under boost
conditions where fuel pressure rises in proportion to boost. This situation is different from trying to
run a normally aspirated engine with the idle pressure cranked up to 50-plus psi. If you do, you’re
idle air/fuel ratio will be way too rich and it will bog and stumble coming up off idle.
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Previous | Next
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This has been a sample page from
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How To Install and Use Nitrous Oxide
Injection Systems For Maximum Horsepower
by Joe Pettitt
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Includes information on nitrous basics and advance
nitrous theory. Written with the assistance of Nitrous
Oxide Systems
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Nitrous oxide injection is one of the potentially easiest, least
expensive, and fastest ways to substantially increase engine
horsepower. This new title, authored with the assistance of one of
the industry's largest manufacturer of nitrous equipment, provides
the latest technical information available regarding the proper
installation and use of this high performance, yet potentially
damaging equipment.
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Click below to view sample
pages from each chapter.
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"How to Install and Use Nitrous Oxide is filled with information
on nitrous, including the basics of advanced nitrous theory.
Photos, charts, and graphs accompany the text and illustrate
key points. Hands-on sections of the book cover how to plumb
a nitrous system and how to set up an engine to handle nitrous.
There's information on ignition timing, compression, wiring,
solenoids, octane, and fuel delivery." -- SPORT TRUCK, April
1999
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Chap. 1 - Introduction to Nitrous
Chap. 2 - How Nitrous Works
Chap. 3 - The Nitrous System
Chap. 4 - Installation Tech
Chap. 5 - Operating and Tuning
Chap. 6 - Basic Engine
Chap. 7 - Advanced Tuning
Chap. 8 - Nitrous Fuel Injection
Chap. 9 - Dyno Sessions
Chap. 10 - Real World Project
Chap. 11 - Chemical Reference
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8-3/8 X 10-7/8
128 pages
300 b/w photos
Item: SA50
Price: $18.95
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Click here to buy now!
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This is a great book that anyone using, or considering using a
nitrous oxide system will love!
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Other items you might be interested in
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How to select and install the correct turbo for big or small
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Sport Compact Turbos & Blowers
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How to Rebuild and Modify Carter/Edelbrock Carburetors
Author David Emanuel outlines carburetor types, gives a thorough look
at carb selection and carb function, and offers detailed information on
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Also features the history of Carter as well as the history of the AFB
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Price:
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