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Why to Not Swap Airflow Meters

Why bother with MAP sensor and airflow meter swaps?

by Julian Edgar

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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 meter.

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

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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 airflow.

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 in-between points.

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

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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 swap.

Bigger Hotwire

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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.

Huh?

Now if we seem somewhat less than enthusiastic about all these swaps it’s because a few truths seem to be seldom recognised.

  • How Restrictive?

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 major restriction.

  • Vane Meters Unfairly Maligned

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.

  • MAP Sensor Swap Costs

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?

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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 passage.

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 connected.

Conclusion

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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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