A relatively simple design, N Alpha systems use only engine
speed and throttle angle to calculate the amount of fuel needed
by the engine. This system doesn't measure airflow directly;
instead, engine load is assumed based on throttle-angle versus
engine rpm. The various load-rpm points make up the computer's
lookup table, with the amount of fuel needed at each point
manually programmed by the tuner.
N Alpha systems work well on engines that operate primarily
at wide-open throttle—such as race cars—but are much less
accurate at part-throttle than more sophisticated systems
because of their relatively simple fuel map. They generally
do not have a closed-loop mode for air/fuel correction, resulting
in part-throttle calibration that is crude at best when compared
to other systems. This also makes them incompatible with modern
catalytic converters. Any significant engine change requires
Popular N Alpha systems for V-twin motorcycles include the
Magnetti Marelli EFI used by Harly-Davidson from 1995-2001,
Zippers Thundermax, Daytona Twin-Tec, S&S VFI.
Speed Density systems accept input from sensors that measure
engine speed (in rpm) and load (manifold vacuum in kPa), then
the computer calculates airflow requirements by referring
to a much larger (in comparison to an N Alpha system) preprogrammed
lookup table, a map of thousands of values that equates to
the engine's volumetric efficiency (VE) under varying conditions
of throttle position and engine speed. Engine rpm is provided
via a tach signal, while vacuum is transmitted via an intake
manifold-mounted Manifold Air Pressure (MAP) sensor. Since
air density changes with air temperature, an intake manifold-mounted
sensor is also used.
Production-based Speed Density computers also utilize an oxygen
(O2) sensor mounted in the exhaust tract. The computer looks
at the air/fuel ratio from the O2 sensor and corrects the
fuel delivery for any errors. This helps compensate for wear
and tear and production variables. Other sensors on a typical
Speed Density system usually include an idle-air control motor
to help regulate idle speed, a throttle-position sensor that
transmits the percentage of throttle opening, a coolant-temperature
sensor, and either a knock sensor or, in the case of a Harley,
Ion Sense as a final fail-safe that detects detonation
so the computer can retard timing as needed.
Because a Speed Density system still has no sensors that directly
measure engine airflow, all the fuel mapping points must be
preprogrammed, so any significant change to the engine that
alters its VE requires reprogramming the computer.
Harley-Davidson's Delphi EFI system is a Speed Density system
and has been used since 2001 to present.
By contrast, Mass Air Flow (MAF) systems use a sensor mounted
in front of the throttle body that directly measures the amount
of air inducted into the engine. The most common type of mass-flow
sensor is the hot wire design: Air flows past a heated wire
that's part of a circuit that measures electrical current.
Current flowing through the wire heats it to a temperature
that is always held 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 to maintain
its fixed temperature. The amount of current needed to heat
the wire is proportional to the mass of air flowing across
the wire. The mass-air meter also includes a temperature sensor
that provides a correction for intake air temperature so the
output signal is not affected by it.
The MAF sensor's circuitry converts the current reading into
a voltage signal for the computer, which in turn equates the
voltage value to mass flow. Typical MAF systems also use additional
sensors similar to those found in Speed Density systems. Once
the electronic control module (ECM) knows the amount of air
entering the engine, it looks at these other sensors to determine
the engine's current state of operation (idle, acceleration,
cruise, deceleration, operating temperature, and so on), then
refers to an electronic map to find the appropriate air/fuel
ratio and select the fuel-injector pulse width required to
match the input signals.
MAF systems are much more flexible in their ability to compensate
for engine changes since they actually measure airflow instead
of computing it based on preprogrammed assumptions. They are
self-compensating for most reasonable upgrades, as well as
extremely accurate under low-speed, part-throttle operation.
On the other hand, the MAF sensor is not used very commonly
on motorcycles. They require laminar flow across the sensor,
and with it's typically short intake tract, what is present
in a motorcycle is turbulent flow.
Which Is Best?
In a perfect world, virtually all street-performance engines
would use Mass Air, due to its superior accuracy and greater
tolerance for engine changes.
On a Harley-Davidson, it is
not practical to use MAF due to packaging or hardware constraints,
so the Delphi Speed Density system is the next best choice.
Because production-based Speed Density systems won't tolerate
major engine changes without computer reprogramming, they
usually require the services of an outside specialist; if
the reprogrammer isn't specifically familiar with your combo,
the end results may be less than satisfactory.
On radical engines (those with cam duration over 260 degrees
at 0.050 or less than 10 inches of idle vacuum), even user-programmable
Speed Density systems have difficulty due to an erratic or
insufficient manifold vacuum signal. If the application is
for a race vehicle operated primarily under full throttle,
N Alpha is the solution. If you intend to drive on the street,
a system that blends N Alpha with Speed Density—varying which
is in control per specific operating point and conditions—may
be the answer. The stock Delphi EFI found on the late model
Harley-Davidsons is such a system.
As electronic engine-management system usage becomes more
widespread in the motorcycle community, prices and ease of
use should become more user-friendly. Already, the latest
system has the ability to construct its own baseline fuel
curve for the Delphi Speed Density EFI, and the new user programming
interface is a full-fledged, Windows-compatible program.
While there are many products available to modify injector
pulse width and ignition timing for the Delphi EFI system,
none do it as appropriately, accurately, and thoroughly as
the TTS Mastertune.
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