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This article was first published in 2003.
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Every engine management system uses a load sensors - either an airflow meter or MAP sensor. But which is better to have when you're modifying an engine? In this article we'll look into some of the pros
and cons of each.
Types of Load Sensors
A load sensor is an essential part of any production car EFI system. The load
sensor serves to inform the ECU of the amount of air being consumed by the engine.
This enables the correct quantity of fuel to be injected and helps determine the
appropriate ignition timing for each situation.
There are two different types of load sensors - airflow meters and MAP
(Manifold Absolute Pressure) sensors. Leon Vincenzi of Adelaide's Awesome
Automotive suggests that today's car manufacturers favour airflow meters because
they offer greater tuning accuracy.
"This enables tighter control of emissions,"
he says.
There are three different types of airflow meters.
The most commonly used airflow meter in late-model EFI cars is the hot-wire
airflow meter. The hot-wire airflow meter incorporates a
thin platinum wire mounted in the intake system prior to the throttle. The mass
airflow into the engine is calculated by the amount of current required to heat
the wire to a predetermined temperature.
Many older EFI systems - such as the BMW Bosch system seen here - employ a
vane-type airflow meter. In this arrangement a pivoting flap is mounted in the
intake system prior to the throttle. This pivoting flap opens as engine airflow
increases; the ECU receives information on the flap angle. Mass airflow into
the engine is determined by the flap angle and the input from a separate intake
air temperature sensor.
The third type of airflow meter is the Karman Vortex, as used on many
Mitsubishi engines. The Karman Vortex meter operates by producing internal
vortices. An ultrasonic transducer and sender measure the frequency of these
vortices. Unlike most hot-wire and all vane airflow meters, a Karman Vortex meter sends a
frequency output to the ECU.
A MAP load sensor operates on a completely different principle to an airflow
meter. A MAP sensor is a pressure sensor that's connected via a hose to the intake manifold downstream of the throttle. In the case of a naturally aspirated
engine, the MAP sensor reads manifold vacuum only, while those fitted to forced
induction engines measure vacuum and boost. The output of the MAP sensor is fed
into the ECU where it is referenced against revs and intake air temperature. These inputs allow the ECU to calculate the engine's mass intake
flow, enabling it to provide the appropriate fuelling and ignition timing.
Pros And Cons of Both Types of Load Sensors
The airflow meter is widely regarded as the most accurate type of load
sensor. To achieve the optimal air-fuel ratio, it's the mass of air
entering the engine that it is critical to determine.
"The airflow meter gives a
grams of air per second measurement of intake flow, which is exactly what the
computer needs to deliver the right air-fuel ratio," says Leon Vincenzi. "It's
not like a MAP system where it arrives at an indirectly calculated value."
Hot-wire meters
provide superior transient response to vane-type meters. Vane-type airflow
meters are better damped and offer greater smoothness.
But Leon Vincenzi says the MAP sensor offers even more advantages in terms of transient
response.
"You can feel
that the earlier MAP-sensed V6 Commodores are a lot snappier than the late
airflow metered V6s," he says.
Further advantages of MAP sensors include compactness, no
requirement for maintenance and potentially greater reliability. Another important point is since the induction air doesn't have to flow through a MAP sensor (as it does
with an airflow meter), it poses no intake airflow restriction, allowing
optimal torque and power to be generated.
An Aftermarket Perspective
When modifying an EFI car (with extractors, an exhaust and air intake, for
example) the MAP load sensor arrangement typically gives the biggest power gain.
But this does come with some drivability trade-offs.
Leon Vincenzi uses the example of Holden Commodore V6 to illustrate the
characteristics of a MAP-sensed vehicle with breathing enhancements.
"You can do
extractors and exhaust on a late VS-onward Commodore [which uses an airflow
meter] and the management system will know what's going on and it'll maintain about
the standard air-fuel ratio.
"But the earlier cars [with MAP
sensors] will be running off the same base program - oblivious to the
effects of the exhaust changes. That means there'll be a mixture variation at high load - it goes leaner. This leaner mixture helps to make power in itself, but you'll
often end up with flat spots."
Note that, in addition to giving leaner air-fuel ratios at high load, the MAP
sensor allows the engine to breathe without restriction. In contrast, as the engine's airflow capacity
increases, so does the
restriction of the airflow meter.
According to David Alexander of Sydney's Silverwater Automotive, MAP sensors
also have advantages in high power turbo applications.
"Some modified turbo
engines that run airflow meters can suffer mass flow reading issues. When you've
fitted a big turbo you can get a lot of turbulence that affects the airflow
meter at idle and during low speed operation. This can be very tough to
fix if you aren't aware of exactly what's going on. When you make a
speed-density calculation [as you do with a MAP sensor arrangement] intake
turbulence isn't an issue," he says.
Yet another advantage of a MAP sensor induction system is its resistance to
engine backfires. Airflow meters, particularly vane-type airflow meters, are
very susceptible to backfire damage.
Various programmable management systems are available with external 1, 2 or 3
Bar MAP sensors while other aftermarket ECUs come with an in-built MAP sensor. In the latter
case, note that running a vacuum/boost hose from the engine bay through to the
cabin (where the ECU is typically mounted) is illegal in some areas. MAP sensors
in production cars are invariably mounted on the firewall or directly on the manifold.
Having said all this, the airflow meter has a few advantages up its
sleeve...
"An airflow meter system gives more modification flexibility before you have
to re-chip the ECU - it's measuring the actual airflow of the
engine," says David Alexander.
"If you make an exhaust change, for example, it
won't upset the ECU's operating characteristics. There may be other hidden
functions in the ECU that override that advantage, but that's another whole
issue...
"Also, in most cases, an airflow meter system is more accurate and
requires less computer brainpower - it doesn't have to make constant
calculations like a speed-density system. Airflow meter systems generally also
require a bit less tuning," he says.
Airflow meters are also more suitable in applications where hot cams are
used. Big cams can cause pressure fluctuations in the intake manifold at light
load. These fluctuations are known to confuse a MAP sensor.
"You find that
vacuum drops at low rpm and the engine will run too rich," says Leon Vincenzi.
One of the biggest benefits associated with a MAP sensor is that is poses
zero intake flow restriction. While this is a valid consideration, Leon Vincenzi
points out that many people ignore that availability of 80mm airflow meters that
flow extremely well - the Cobra Mustang SVT engine (pictured here) comes
standard with a 80mm airflow meter.
"I know a lot of people rip out the screens or honeycomb in airflow meters,"
says Leon Vincenzi. "All that does is stuff up their intake flow readings,
particularly in the case of airflow meters that have a thick internal honeycomb.
"Another thing people forget is that they can run twin airflow meters
if airflow restriction is a concern."
Final Words From the Experts
David Alexander suggests in most instances the decision to go for an airflow meter or MAP
sensor is not terribly important.
"Each approach does have some
subtle advantages but so much comes back to how well it's tuned," he says.
In contrast, Leon Vincenzi suggests a different approach for modified forced
induction and naturally aspirated engines.
"MAP sensors are easier to set up on
a forced induction engine - you can get a very good result quite easily. But I
tend to lean toward airflow meters for serious naturally aspirated engines," he
says.
"The best approach, though, is to rely on a MAP sensor for transient
response and an airflow meter for steady-state conditions. We've run a few
Commodores with that set-up and it works brilliantly."
Why chose?
Contacts:
SAS
Silverwater Automotive Services
02 9748 1300
info@silverwaterauto.com.au
www.dynotuning.com.au
Awesome Automotive
+61 8 8277 3927
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