MAP Sensors

ahhhhh thats better!
Tinker, vacuum u said? Can u elaborate what the importance of that is for a high revving NA engine?

Quote:

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Originally Posted by pornstar

Can u elaborate what the importance of that is for a high revving NA engine?

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as the pistons move down on the filling stroke the create a vacuum that draws air into the cylinder ready for the compression stroke.

the more drawing power this has the more A/F mixture that fills the cylinder and therefore the greater the combustion energy.

so vacuum is important as it determines power.

the MAP sensor compares the air pressure compared to the absolute pressure (i.e. outside air at 1 bar) and determines how much air is getting drawn into the engine and thus calculating the pulse width of the injectors to regulate the A/F ratio...

so to answer your question, it might be better to relate to a non-NA car that has a bloody big pump forcing air into the cylinders - so you see why vacuum is important for NA cars - as it is the only thing that fills the cylinders with A/F mixture...

some Hondas have BARO or PA sensors to measure the outside air pressure that tinkerbell mentioned above for high altitudes

The MAP pressure sensor has the same operation throughout the rev range in that its a sensor which measures pressure and "Vacuum" to some extent SO in terms of High revving NA, it is as important as low revving NA.

The job of the map sensor is to tell teh ECU how much fuel/ignition is required at a particular MAP vs RPM setting. How do i know this, well ive decompiled the bin file (ala hondata) and read the code.

The MAP sensor does NOT compare the outside pressure AT ALL. It has certain boundaries, a min/max value as a check for sensor validity. If the voltage exceeds these tolerances, a code (CEL) is returned.

tinkerbell is incorrect by stating that vacuum determines power, this is NOT the case. The ECU will only see vacuum from the MAP sensor under idling, or when you back off the gas pedal.
Power is determined by the quality of the burn in the combustion chamber (a/f) and the ignition (adv/ret) combined with many other factors such as engine load, fuel quality etc.

Cylinder filling on the other hand, is a product of piston velocity and also a process commonly known as scavenging. This is the process by which cam overlap causes a low pressure condition in the cylinder head, hence mixture is drawn from high pressure to low pressure, increasing the amount of fuel at any given combustion process.

Manifold pressure is not directly related to the combustion process. Take for example a forced induced application.
You have probably heard people talking about B16's as being a torquless motor, this is due to the low piston speeds (long rods) at low rpm.

On the other hand, the D-series motors have more torque and are "nicer" to drive at low rpm's (smaller rod -> faster piston speeds)

This is somewhat of a side-track, but hopefully an insightful one.

ProEcu

thats an excellent explanation proECU. The map sensor is a common tricky sensor many get confused by. From what i have laernt, it is even less complicated than that, it merely is a temperature sensor in very very basic terms!

so why is it called a Manifold Absolute Pressure sensor?

because if u think about what the amount of pressure in the manifold is, the air is a particular volume but that volume is not the only factor, desnity si the factor to consider.

density is a function of heat :) thats what i said about the sensor in VERY VERY VERY BASIC terms is :)

edited post.

its an absolute sensor because it is relative to atmosphetic pressure

ie, stock MAP sensor reads 1000mbar = 1Bar is still not boost.
anything over this is boost. That because the first 1000mbar is atmosphetic.

Great info ProECU