​Map Sensor Constraints

A project log for Guri: Ignition Controller for Air Cooled Engines

Ignition controller for VW air cooled engines.

danjovicdanjovic 05/25/2016 at 19:160 Comments

The easiest way to measure the Manifold Air Pressure is using an off the shelf automotive MAP sensor which integrates all signal conditioning circuitry and provides a linear response to a vacuum signal.

The MAP sensors are specified in kPa (kilo Pascal), so before choosing a sensor we must convert the original vacuum capsule response range to kPa. Talking in numbers the engine vacuum capsule provides advance from 60mmHg to 270mmHg which corresponds from 8kPa up to 36Kpa.

Unfortunately the MAP sensor with the range most close to this that I could find was the PRT 03/04 which covers from 17 to 105Kpa. The problem with this sensor is that the measurement starts at 17kPa which will make us loose the measurement down to 8kPA (60mmHg).

However, a closer look at the vacuum x advance table shows us that the vacuum contribution to overall advance can be zero up to 110mmHg. Then if we add 0° of advance for vacuum signals below 127mmHg in practice we have only lost a small range of measurement (110mmHg to 127mmHg). Good enough.