People love the idea of swapping airflow meters. A
car has a vane airflow meter and that car’s online community of owners spends
all their time trying to work out how to convert the car to MAP sensor
operation, or how to swap the vane airflow meter for a hotwire. Or, if the car
already has a hotwire, the effort is adopting a MAP sensor or a larger airflow
In cars with increased power, there are two
potential reasons for changing the factory airflow meter. One is that the
airflow meter poses a flow restriction – that intake airflow is being restricted
by the size of the factory airflow meter.
The second is that the airflow meter output signal
maxes out well before peak power – even though airflow continues to rise, the
output signal stops increasing.
By using a MAP sensor, the airflow meter can be
deleted. By using larger airflow meter, the flow restriction at peak power is
reduced and the signal no longer reaches an output limit and then flat-lines.
Replacement with a MAP Sensor
Replacing the airflow meter with a MAP sensor
looks very attractive. MAP sensors are available off the shelf (the pictured GM
unit can be bought in a variety of pressure ranges) and it’s very easy to plumb
a MAP sensor to the intake manifold.
But there are problems.
Firstly, a MAP sensor and an airflow meter are not
directly swappable. Basically, what they measure is not the same.
An airflow meter outputs a signal that is
proportional to the air being breathed by the engine. A MAP sensor measures
engine manifold pressure. To derive the engine airflow from a MAP sensor signal
you need to combine this signal with RPM. It is the combination of revs and
manifold pressure that shows how much air is flowing into the engine. This
is easy to understand – manifold pressure might be high, but if revs are low,
airflow will be low. If manifold pressure is high and revs are high, airflow
will be high. You can see that manifold pressure alone doesn’t represent
So the new signal that replaces the airflow
meter’s signal needs to combine both MAP and RPM. That’s why commercial systems
that replace an airflow meter with a MAP sensor have a separate RPM input, or
the change to a MAP sensor is done with accompanying ECU software revisions.
For example, if you were using a sophisticated
interceptor, you could create a map that relates manifold pressure to rpm and
then outputs a voltage signal. This voltage signal would behave in much the same
way as an airflow meter output, with a low voltage output at low airflows and a
high voltage at high airflows – and of course, the right relationship at all the
Some ECUs that have their software fully cracked
can have the change from a MAP sensor to an airflow meter enacted in the
software, so that the ECU ‘knows’ what the MAP sensor signal actually shows.
Finally, if you replace the whole engine
management system with an aftermarket design, the airflow meter will normally
disappear in favour of a MAP sensor.
So while a change from an airflow meter to a MAP
sensor can be done, it’s just not as simple as replacing one with the other.
Vane to Hotwire
Now, what about swapping from a vane airflow meter
to a hotwire? Again this looks great on paper but the reality is much harder.
Firstly, vane airflow meters measure airflow
volume, not mass. To calculate airflow mass, the vane airflow meter uses a
separate temperature sensor protruding into the airflow. So the vane airflow
meter has effectively two outputs – air volume and air temperature.
On the other hand, a hotwire airflow meter
measures the mass of air being drawn into the engine. The temperature
compensation is, if you like, built into the design. So the hotwire airflow
meter has just one output – air mass. However, a hotwire has an extra input –
the one that causes the hotwire to be heated to red-hot after you switch off the
engine, so causing any contaminants on the wire to be burnt off. (Some film-type
meters don’t use this approach.)
So if swapping from a vane airflow meter to a
hot-wire, the original intake air temperature input from the vane airflow meter
will normally be discarded, or replaced with a fixed value (replicated by the
use of a resistor). Furthermore, when doing this swap, a new, timed output is
needed to trigger the hot-wire burn-off.
In addition, you’ll need to have the pin-outs of
both the original and new airflow meters, something that’s a lot harder to find
than it first appears.
Finally, the output curve of the hotwire is
unlikely to be the same as the output of the vane airflow meter, so an
interceptor will need to be used to tweak the shape of the curve to give the
correct air/fuel ratios.
As can be seen, this is also not a straightforward
Going to a bigger hotwire is probably the easiest
swap of those covered so far. The signal type (mass airflow) is the same and the
burn-off signal (if required) will be provided. The difficulties are working out
the compatible wiring connections and then tweaking the output curve to provide
the correct air/fuel ratios. Oh yes, and of course the cost of buying the big
hot-wire – often, they’re very expensive.
Now if we seem somewhat less than enthusiastic
about all these swaps it’s because a few truths seem to be seldom
The actual restriction the airflow meter imposes
is often not as high as people believe. That statement of course depends on just
how much extra power is being developed, but a maximum power pressure drop
across the airflow meter of less than (say) 5 inches of water is pretty minor.
And it’s dead easy to directly measure the restriction being posed by the
airflow meter – see
Negative Boost Revisited, Part 2.
There’s not much point in changing the airflow meter unless it actually poses a
For its cross-sectional area, a vane airflow meter
has very low restriction at full power. To put this another way, if a vane
airflow meter and a hotwire have the same internal cross-sectional area, the
vane meter has less restriction than the hotwire.
This makes sense – when the vane airflow meter is
fully open, nothing blocks the airflow. However, a hotwire airflow meter always
has at least something hanging in the way of the intake air.
Again, measuring the actual full-load pressure
drop across the airflow meter is vital if rational decisions are to be made.
Unless it can be done with a relatively simple ECU
software change, by the time you add up the costs of a MAP sensor swap, it’s
often not an economic proposition. Not only do you have the cost of the MAP
sensor and wiring plug, you also have the cost of the sophisticated interceptor
and the dyno time for a full tune (not just a tweaking). For another project we
recently researched some costs and stopped when it exceeded AUD$1500.
Bypass and Simple Interceptor
So what approach is easiest and cheapest?
If your car runs a hotwire (or hot film, etc)
system, the answer is straightforward. All that you do is add a bypass passage
around the factory airflow meter, so that air passing into the engine passes
both through the factory airflow meter and also through the bypass
passage. Normally the bypass passage is made smaller than the main airflow meter
Taking this approach means that less air passes
through the airflow meter. The result is that it poses less full-load
restriction and its output signal doesn’t rise as high.
People often think that this will mean the car is
untuneable, but that isn’t so - the more air flowing into the engine, the more
air that will always flow through the meter. Because in a bypass system the
interceptor needs only to output a voltage on the basis of the input voltage, a
simple one-dimensional interceptor such as
AutoSpeed Shop can be used to tweak
the mixtures. Of course, pretty well any other interceptor is also suitable.
For more details on a working bypass system see
Airflow Meter Bypass, Part 1 and Airflow Meter Bypass, Part 2.
Note: this approach has been shown not to work
with vane airflow meters, as at low loads, all the air bypasses the vane airflow
meter and so the vane is not deflected open. However, it seems to us that if
another vane airflow meter is placed in the bypass passage, this problem
will be avoided. The bypass vane airflow meter wouldn’t need to be electrically
MAP sensor and airflow meter swaps have
complexities and costs that few initially realise. But by retaining the original
airflow meter and using a bypass and simple interceptor, you have full air/fuel
tuning ability, overcome problems of airflow meter restriction and output signal
flat-lining – and do it all at vastly reduced cost.
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