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Thread: AFM location

  
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    AFM location

    Why is the AFM situated next to the air filter box?
    Surely if is supposed to measure engine air intake it should be fitted in the boost pipe as anything fitted before the turbo will be subject to the vagaries of the dump valve and waste gate, thereby potentially sending incorrect data to the ecu.

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    From the start the piston draws air in by using vacuum i.e the downward movement sucks air in, as the revs go up and more exhaust gas is created it spins the turbine wheel on the turbo which spins the compressor wheel as they are on a shaft so joined together, this draws air into the engine even quicker and it's compressed via the turbo..

    Because the AFM measures the air mass taken in then the turbo won't make MORE air - just compress it, then as it's compressed the MAP sensor which is on top of the standard intercooler measure the pressure (psi or BAR) also the temperature by this time the engine already knows how much petrol to put it as it knows the mass of air and the pressure it's under and also how much extra petrol to put it to help cool the engine down if the air temp is quite high..

    Dump valve only releases pressure off when the car is under vacuum (i.e turbo isn't compressing air any more) so that's when there's no accelerator in use i.e changing gear or letting off the loud pedal

    Wastegate has nothing to do with the intake as such mate it's the part in the manifold which when the turbo creates air pressure it closes off the exhaust and diverts the gases to the turbo instead, the more pressure you get the more the actuator moves and shuts off the wastegate and so the more the compressor wheel spins and draws air in whilst compressing it and putting under more pressure.. (this is why you should mess with the actuator because you can shoot the boost presure through the roof and kill the engine and turbo)

    I hope that's pretty clear and try looking it up on somewhere like wikipedia or here >> http://www.astraownersclub.com/vb/sh...ad.php?t=86465
    FORMERLY WAVEYBLUEWATER

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    mania's Avatar
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    I think because when the wastegate/dump valve are active the airflow doesn't need to be critically monitored, and only account for a percent or so of driving time. Whereas placing it after those it'd have to deal with much higher temperatures from the turbo which would affect the monitoring more.. that's my guess.

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    Thanks for the replies guy's, it's the situation where you lift off the throttle that doesn't seem right to me. From a technical point of view surely you would get a much more accurate reading of actual air intake if this reading was taken just before the plenum chamber (with adjustments made for boost by the map sensor) rather than taking the reading at the air filter. A considerable part of everyday driving involves throttle lift off and this must mean a momentary disparity between the information sent from the afm to the ecu and the actual engine requirements because of dump valve activity?

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