Last week in Building a New Airflow Meter, Part 1 we showed you how to build a larger than standard airflow meter
that still takes the factory sensing element. It’s good for reducing intake flow
restriction and increasing the headroom before the meter reaches maximum output.
(And it also allows you to mount in-line a meter that was previously integrated
into the airbox.) However, an upsized airflow meter will give an altered signal
voltage output, resulting in a change in the mixtures.
In this story, we show you how to correct the air/fuel ratios, and as a
bonus, tune the open-loop mixtures as well.
As the body of an airflow meter is increased in size, the voltage output from
the sensor will be reduced. This is because the air speed through the meter is
lower and so the sensing element sees less air going past. So if you upsize the
airflow meter but don’t make any electronic changes to its output, the air/fuel
ratio will be lean as the ECU will think that there’s less air going into the
engine than there really is.
However, there’s a cheap and elegant solution in the shape of the Silicon Chip Digital Fuel Adjuster. This
electronic kit (it’s also available fully built and tested) allows the output of
the airflow meter to be adjusted up or down at up to 128 points across its load
range. The design interpolates between the points (ie smooths the curve of the
adjustments) and gives factory driveability. The programming of the DFA is via a
hand-controller that uses a backlit LCD. For more on the DFA, see
The Digital Fuel Adjuster, Part 1. In kit form the DFA costs
just AUD$80 and the hand controller AUD$60.
It is possible to upsize the airflow meter and then correct the output
voltage of the new design with the DFA without ever measuring air/fuel ratios.
Taking this approach has the major advantage that an accurate air/fuel ratio
meter is not needed. So how’s it done?
The first step is to use a multimeter to carefully measure the standard
airflow meter’s output voltage at a variety of engine loads during road testing.
(Obviously, use an assistant to read the meter or drive the car!) For example,
the output voltage can be measured at idle, full power (ie redline at full
throttle), and at perhaps eight points between these extremes. These ‘in
between’ load points can be set using a manifold vacuum gauge (or vacuum/boost
in a forced aspirated car) and the tacho – so for example, one of these points
might be 1 psi of boost at 4000 rpm, 3rd gear, another might be at 7
psi of boost at the same revs and gear, and so on.
Then, when the new airflow meter is installed and the DFA wired into place,
the output signal voltage of the DFA is measured and the hand controller used to
adjust the output voltage until it matches the original airflow meter at all the
previously measured load points. When the output voltage is correct at these
load points the rest of the curve can be set up using these points as the
reference (ie basically join the dots). If doing it in this way, gradually work
your way up to higher loads, setting the DFA correction ahead of your current
load point so the car never runs lean under power.
Once you’ve done all of that, the air/fuel ratios should be fine.
- With an Air/Fuel Ratio Meter
If you have access to a professional air/fuel ratio meter, just use the DFA
to set the mixtures at all load points. Note that when the car is operating in
closed loop, you should bring the mixtures back to standard - outside of closed
loop you can set them to whatever you want.
Alternatively, any tuning shop should be able to dyno-tune the Digital Fuel
Adjuster to suit the revised airflow meter – just take along the Hand Controller
instruction sheet that shows the function of each button.
Or finally, if you’re really careful with how you go about it, you may well
be able to get away with tuning via the output of a conventional oxygen sensor –
Real World Air/Fuel Ratio Tuning
In any case, don’t forget to operate the ‘lock’ button on the DFA when the
tuning is done.
the airflow meter upsize isn’t great, the DFA should be set to fine mode (and
bench-calibrated in that mode) and this will allow ultra-fine tuning of the
mixtures. If the airflow meter upsize is great, or absolute accuracy in mixtures
isn’t needed, the normal coarse mode is fine.
In the case of the NHW10 Toyota Prius on which the upsized airflow meter was
fitted, a MoTeC air/fuel ratio meter was used to monitor the outcome of tuning
changes being made with the DFA. However, even before the car would start and
happily idle, the airflow meter’s output level needed to be boosted.
With the DFA set to coarse mode and calibrated to work over the 0-5V range
that suits this airflow meter, a +10 correction was put in at all load sites
around idle. In other words, the voltage output of the new airflow meter was
increased by about 0.4V. The +10 value was arrived at by trial and error – at
this value the car idled happily and the ECU worked in closed loop, as indicated
by an air/fuel ratio of about 14.7:1.
But why do you need to set any correction at all if the car is in closed
loop? Won’t it just learn its way to the new mixtures? The answer in most cases
is ‘no’. This is because the ECU doesn’t have unlimited ability to change the
mixtures in response to the oxygen sensor feedback. In the case of the Toyota
being covered here, the decreased voltage output of the larger airflow meter
resulted in mixtures too lean for the ECU to learn around – a 0.4V boost at idle
was needed so that the car would run in closed loop. This is an important but
oft-overlooked aspect of the DFA – returning signal levels to close enough to
standard that the ECU self-learning will work.
With the +10 value working well at idle, this value was put in at all higher
load sites – right up to #128. This car runs in closed loop all the time and
adding this value across the whole load range made it driveable at all loads.
However, some fine tuning of the DFA was then needed as the car could be seen to
be still learning – that is, when a certain load site was first reached, the
air/fuel ratio was leaner or richer than standard before it quickly learnt its
way back to 14.7:1. By making further minor corrections to the DFA map, it was
possible to reduce the amount of learning the ECU needed to do and so mixtures
stayed closer to 14.7:1 all the way through the load range.
If your car uses a detachable airflow sensing element, making a new larger
airflow meter to house it is easy. Almost as straightforward is the adjustment
of its output with the Digital Fuel Adjuster.
Digital Fuel Adjuster