by C. Asaravala
Understanding Mass Air Flow
Last month when we introduced you to our latest "project"
vehicle, appropriately titled the Big
& Ugly Work Truck, we indicated where we were headed
with the 1989 F150. We were pumped that it was a 5.0L, which
in our minds meant Mustang like modifications would ensue. However,
we knew the "speed density" fuel injection system
would need to be upgraded to a mass air flow system in order
for the engine to respond properly to induction upgrades. Well
here we are, and we're going to show you how to do it, and why.
Our 5.0L F150 has an identity crisis.
Speed Density was Ford's earliest method of electronic fuel
injection. Fuel injection of any type works by monitoring various
sensors to determine engine load and then adjusting the amount
of fuel delivered to maintain the desired air-fuel ratio. Speed
density systems are called so because load is calculated by
the speed of the engine (rpm) and the density of air within
the intake manifold, as measured by the MAP (manifold absolute
pressure) sensor. Intake air temperature is also measured and
used as a correction factor. Speed density systems are accurate
because the data within the control module (e.g. computer) is
based around the specific engine configuration. For instance,
the speed density computer in our F150 contains spark and fuel
tables specific to the F150 5.0L engine. Thus speed density
systems are not adaptable to significant changes in an engine's
volumetric efficiency. One you start adding cams, heads, etc.
the computer cannot compensate properly because it is operating
around a pre-set range of air volume which that particular engine
Mass air flow (MAF) systems evolved later and are highly adaptable
because they directly measure the volume of air going into the
engine. The mass air flow meter mounts between the air filter
and throttle body, and consists of a pair of small wires located
within a calibrated housing. One wire measures the temperature
of the ambient air, while the other wire is always heated to
200° greater than the ambient or "cold" wire.
The amount of air flow through the meter and the temperature
of the air results in cooling of the wires. The resultant voltage
output signal is sent to the computer to make fuel adjustments
based entirely on how much air is going into the engine. Add
heads, cams, intakes and superchargers, and the system is able
to compensate. While there are limits, they are not insurmountable
with some tuning.
The speed density EFI systems on trucks and Broncos also presents
another weak point, and that is in the way the injectors are
fired. In the speed density system the injectors are "bank
fired", meaning that four injectors for one bank of cylinders
is fired simultaneously, followed by the other bank. In fact,
in our introductory article to this project, we incorrectly
presented the trucks speed density system as being asequentially
fired system as they were on the speed density EFI Mustangs
in 1986 and 1987. This can be confirmed by looking at the wiring
configuration of the truck speed density computer harness (see
page 3) which indeed shows injectors 1,4,5 and 8 on one pin
while 2,3,6 and 7 are wired together on another.
Converting to mass air not only gives our engine more accurate
air-fuel ratio management and adaptability, but also rewards
us the throttle response and fuel economy by way of the sequential
electronic fuel injection (SEFI).
Planning the Conversion
There are several ways to go about making the conversion on
1987 to 1993 pickups or Broncos. However the preferred method
largely depends on the type of transmission you have. If your
truck has the E4OD automatic transmission (typical on the
351 and 460 equipped trucks and 93-95 Lightnings) then you
have to consider that this transmission is controlled via
the engine's computer. The most common mass air conversion
calls for using the 88-93 Mustang computer, which does not
have E4OD control. The options in this situation are to use
a 1995 or 1996 F-150 or Bronco computer, as by that year the
trucks were mass-air and all running E4OD's. Other E4OD owners
have creatively performed the mass-air conversion using the
Mustang computer to control all the engine functions, while
retaining the stock speed density computer to handle the electronic
transmission. Finally, many have taken the more drastic approach
and replaced the E40D with an AOD, C6 or manual gear box.
Either way, if you have an E4OD equipped F-150, Bronco or
Lightning and want mass air, you have some research to do.
Spend some time at the National Lightning Owners Club (http://www.nloc.net/)
or at Ford Truck Enthusiasts Forum (http://www.ford-trucks.com)
and you'll get plenty of ideas.
For the purposes of this article we are focussing on trucks
which came equipped with AOD or manual transmissions; neither
of which have any interaction with the computer. For these
vehicles the mass air swap options are quite simple. Which
option you select comes down to your patience and pocket book.
The most rudimentary of swaps would come from pilaging all
of the materials (injector and mass air harness, computer,
and mass air meter) from a donor vehicle, such as a post-87
5.0L Mustang. Getting the parts would be easy. You'd then
have the arduous task of cutting up your factory harness and
the Mustang harness and splicing it all back together in accordance
with the wiring diagram for the mass-air equipped Mustang.
Alternatively, you could get the entire EEC-IV harness from
the Mustang and replace the truck harness. Neither of these
tasks are for those who get queasy seeing their vehicle in
a major state of disarray. Sure, you'll save some money, but
at the expense of major heartache. If you're feeling like
you need the challenge at least use Bjorn
Bremb's documentation as a guide.
The other alternative, and most costly, is to purchase an
off-the-shelf mass-air conversion kit. Ford Racing's M-9000-T50
and T51 kits cost about $800 and include all the electronics
(wiring and mass air sensor) and hard parts (computer, air-box
lid, ducting.) If you have cash burning a hole in your pocket
this is by far the most convenient choice.
The final method, and the way we opted to roll, is a compromise
in price and convenience. Professional Mass Air Systems offers
a conversion harness at a third of the price of the full Ford
Racing conversion. The harness (see side bar) is better termed
a lay over harness, as it is intended to install along side
the factory harness. The wiring includes eight sequentially
fired injector connectors, an additional (left side exhaust)
heated oxygen sensor, mass air sensor connector, as well as
power and ground leads. Also included are wiring for the vehicle
speed sensor and fuel pump relay monitoring. The harness comes
with detailed instructions indicating which pins need to be
moved, replaced or spliced at the factory computer connector.
If you purchase this harness then what you need to bring to
the table to make the mass air conversion happen is the following:
||EEC-IV Computer (mass
||88-93 Mustang 5.0L
(e.g. A9L, A9M, A3M)
||Mass Air Sensor &
||88-93 Mustang 5.0L
||Narrow Band Heated Oxygen
||88-93 Mustang 5.0L
||18mm Oxygen Sensor Bung
|| Summit G-2990
body to mass air meter "Y" duct
We like the idea of sourcing the hard parts ourselves because
it allows considerable flexibility in pricing and the parts
we select. For instance, we in fact had several left over
stock mass air meter assemblies from our past Mustang project
cars. This helped keep the costs down. Furthermore, if you
are performing this swap in conjunction with performance enhancements
then you can just purchase the upgrades (e.g. bigger mass
air meter) from the get go. For instance, we able to adapt
our K&N cold air kit, as you'll see later on in this article.
Incidentally, perhaps the most aggravating part of this swap
is figuring out how to get the two throttle body tubes to
converge into the single mass air meter housing. The simplest
way would be to obtain or fabricate a "Y" connector,
such as what is used in the Ford Racing conversion kits. We've
heard you can purchase this separately from Ford, however
we did not look into this. We're sure anyone handy with a
welder could fab this up as well. The other option is to purchase
the factory bellows (see side bar) which is on '95 and up
4.9L 6cyl F-150's.
Performing the Conversion
The mass air conversion took us about 4 hours to complete.
In fact it probably would have taken less time if we had not
spent an hour preparing to remove the upper intake manifold
for easier access to injectors 1 thru 4. Ultimately we decided
not to remove the intake and just put up with the fuss of
wedging our hands under the plenum to disconnect the connectors.