To understand how a MAP sensor works and what advantages it has over the standard air flow meters (AFM) that came on our cars, or even upgraded mass airflow meters (MAF), we have to examine how these devices generate a volume-air-flow signal to the Bosch Motronic computer (DME).

 

AIRFLOW METERS (AFM)

AFM - air flow meters, are the crudest form of sensing devices. It uses a mechanical flapper barn-door that is physically pushed aside by the intake air stream. The volume of this air-flow then determines how much the door opens. The door then is mechanically attached to a variable-resistor assembly that then sends a variable-voltage signal to the computer that roughly correlates to the volume of air flowing past it. However, there are some disadvantages to this method.

 

MANIFOLD ABSOLUTE PRESSURE (MAP)

MAP - manifold absolute pressure (also known as speed-density), measurements combine simplicity in sensor design with the power of digital microprocessors to compute a simulated volume-air-flow signal that is sent to the stock computer. You can completely replace the entire stock AFM sensor (or upgraded MAF sensor) and their associated wiring with a simple vacuum hose. As far as the stock computer's concerned, it's seeing the signal from an actual stock air flow meter. Thus the computer will inject the appropriate fuel volume to produce the highest power possible. This MAP sensor upgrade kit doesn't suffer from any of the drawbacks of AFM or MAF sensors and has some unique benefits as well.

 

AIRFLOW MAPS

There is a lot of misunderstanding about flow maps. A flow map is there to assist turbocharger designers in choosing the best compressor and turbine wheels for a given application. The flow map graphs themselves are produced in laboratory type, controlled conditions with little or no variances in altitude, temperatures, barometric pressures etc. Conditions in 'real-life' or 'in-car' can be radically different. 

As such, in our opinion, a flow map can at best be used as a guide. We have heard of experts being able to map a car just from a flow map, remarkable!

The corrected airflow is shown on the X-axis, normally in Lb/min but sometimes in CFM or Kg/sec. The Pressure Ratio is shown on the Y-axis. Plotted on the graph are the 'efficiency islands', 'speed lines', the 'surge line or area' and the 'choke line or area'. The basic idea is to calculate various pressure ratio and airflow figures for different engine speeds and the plot these onto the graph, choosing one that offers the best efficiency.

It must be noted however that the flow map graphs are produced with a compressor wheel running in a particular A/R compressor housing. If we have produced a Hybrid turbocharger using a different air ratio compressor housing than that used on the standard unit, this in itself radically changes the map.

So looking at the flow map for a particular compressor wheel does not guarantee you are looking at the results of the complete turbocharger. Similarly, modifications to the turbine wheel, compressor housing inlet, compression face clearance and angle and ported shroud conversions all affect the flow characteristics.

This brings us back to the map simply being a guide. There is no substitute for experience. We have designed Hybrid turbochargers in the past, where the map looks fantastic but on the vehicle in real-life situations, it has been a disaster. Likewise, the other way around, where the map looks all wrong but perfect on the car.

In any case, the performance of the turbocharger is so radically affected by the engine and manifold designs; that unless the turbocharger designers and engine builders 'talk', usually a disaster is just around the corner. The most common mistake is over camshaft selection and cylinder head modifications.

 

 

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