MAF
Overview
The Mass Airflow Sensor (MAF) is a more modern version of the AFM. None were fitted to any E30 model, but they were installed on contemporary engines, starting with the M50. Rather then using moving parts, a wire or membrane monitors the airflow to provide the varying voltage signals to the ECU.
Because its design doesn't interrupt the airflow, it allows for improved air intake which translates to increased engine performance.
How it Works
To appreciate MAF, you have to understand the existing Air Flow Meter. The AFM fitted to all injected E30 engines is the same fundamental design, whereby a flap or "vane" sits over the air intake. As the engine naturally sucks air in, the change in pressure will pull the vane open. On the other side of the vane's hinges is a metal arm moving over a horseshoe of carbon - the simplest form of variable resistor, which looks a lot like the needle arm on a record player. The wider the vane opens, the farther the arm sweeps across the carbon, sending an increasing electrical signal to the ECU.
This gives us two problems. AFMs, by their design, restrict the airflow because of that flapping door. They also only measure one thing; the flow of air. They don't measure the mass. Instead, they measure air volume and air temperature, and send that to the ECU to try and calculate what the mass is. It works, but it's not the best solution.
It's important to measure mass rather than volume, because what the engine really cares about is how much oxygen it's receiving. The ECU will have a basic idea of how much oxygen one cubic metre of air will contain, but if the air temperature or our altitude or just the humidity changes, then there'll be a difference in the amount of oxygen in the air - effectively how much oxygen is squeezed into one cubic metre. Without that knowledge, the ECU won't be able to fuel accordingly, and we'll be down on power
MAF sensors do measure mass; the clue is in the name. And their solution is elegant in its simplicity.
Most MAFs use a hot-wire technology. If you look, all you'll see is one wire, like a light-bulb filament, stretched across the bore of the sensor. This wire is heated internally, and the ECU measures the temperature and the volts needed to maintain that temperature.
As air flows over the wire it has a cooling effect, just like a radiator. Based on tables within the ECU, it will measure how many volts are needed to warm it up again, and calculate the air's mass from that. Even better, these readings are done without disturbing the air in any way - there's no vane to block the air flow. This allows the MAF to return better airflow as well as more accurate readings.
Some MAFs use a diaphragm instead of a wire and are known as hot-film MAFs. These can return even more accurate results but are a very delicate technology, with the film needing replacement after a few years of usage. However, their operating principle is the same as hot-wire MAFs.
Potential Gains
There has to be a reason why people have been trying to do this modification on old engines like the M20.
Due to the fact that there is no flap inside the MAF like there is in the AFM there is nothing to restrict the airflow into the engine and more air into an engine generally means more performance. There is also the fact that the sensor inside the MAF is quicker to respond to changes meaning that the ECU will be able to make adjustments sooner and therefore the air/fuel mixture should be more precise for the engine's needs at any given time.
The MAF isn't likely to give any peak power gains on an engine as at WOT the AFM flap should be fully open so not restricting airflow. However, lower down in the rev range is where the difference has been noticed, both on the road and on dyno graphs. Dyno graphs have shown that peak power is reached much earlier and the power curve is smoother. Torque curves also show similar improvements, peaking earlier and being smoother. This helps improve drivability as the power is available at more usable RPMs and gives a feeling of increased throttle response.
Disadvantage
As with everything, there are pros & cons to a modification. Some people when fitting a MAF have found that the car hasn't run so well, with either poor idling or power loss. There are many theories on why it doesn't always provide the gains mentioned above, ranging from MAF not working well with Motronic to 20+ year-old engines falling outside the tolerances a generic chip was written to perform within, after years of abuse and lack of servicing have taken their toll.
MAFs are also more sensitive to dirt build-up on the sensor, as well as other electrical faults that may lead to a shortened lifespan compared to an AFM.
Application
You can't just go to a scrapyard, grab a MAF and bolt it to your M20. Well, you can, but your car will run terribly because the ECU has no way of understanding these new signals. MAFs really only make sense in conjunction with other upgrades around the engine.
They are considered an essential component in forced induction engines, or any other rebuild that uses non-standard engine management, such as Megasquirt. People have managed to get them working with the standard Motronic engine management system using reconfigured chips; off the shelf kits can be bought just like this.
A MAF is also used on the following engine swaps:
Versions
As mentioned above, Miller sell MAF Conversion kit which includes a reprogrammed chip you fit into your standard ECU to allow it to run. This is design to be a plug and play kit.
For those that want to take a more DIY approach MAFs from other vehicles have been identified over years of trial and error as being compatible with the M20. Regardless of which MAF you use, changes need to be made to the engine management, by either fitting a non standard one like Megasquirt, fitting a piggyback ECU or using a custom chip (chip fitting guide).
The following MAFs have been used successfully:
- Mk1 Ford Focus 2.0 MAF
- BOSCH - 0280218032 (VAG part)
