Ive been reading everything on the forum about how to get rid of the afm, it seemed to be a fairly big topic a while back. I have the s4 with the trap door style afm and decided to take a look at its function with the cone filter off under different throttle positions, and im a little confused to exactly how it works now. Heres why; from what i understand the door is suposed to measure the amount of air passing threw it by the degree wich the door is "sucked" open by the engines intake. From what i observed the afm door did not respond to the air suction but it percicely responded to every slightest move on the throttle cable. the slightest touch on the cable wich didnt even affect the engine speed moved the door. The door also closses on decel but no matter how slow or fast i let go of the throttle the door whould close at the exact same rate. This leads me to believe that the afm door is controled to let the right amount of air in according to throttle position, not calculate the amount of air entering the intake..... try it youll see what i mean, there is no way that the air mass entering the intake coresponds to every little movement of the throttle, you can actualy reach the cable and look in the afm at the same time. whooo that was long

 

AFM's job is to let the ecu know how much air is entering the engine. Add the temperature, and the ecu figures out the mass. That measures load on the engine (as opposed to MAP system that rely on manifold pressure to get engine load).

The AFM flapper is just a pot and a spring that gives voltage to ECU.

Hugues -

 

Yes i understand that is how it is suposed to work, i just cant get over the fact that the flapper door moves exactly with the throttle even before the engine has a chance to respond. Im so sure of it its obvious when you look at it... Under wot the door will open right away to 3/4 open and then as the engine catches up and increases rotation the door will only then be forced open more by the airflow.

 

Since you're into engine experiments, you should install a vacuum gauge and see the relation between throttle plate angle, vacuum reading (on NA), and AFM opening.

Hugues -

 

hmmm sounds like fun, cept for measuring the afm opening that seems kinda tricky. I dont see how that would prove or dis-prove my theory that the door is automaticaly controled until wot though....

 

The flap door is 100% most definately in my mind aytomaticaly comtroled until wot. I was just checking it out again and when i reved the car up real high and let it decel the door would close on decel of course but if i lightly tap the throttle while the engine is still under decel (lightly enough so the engine continues to decel) the door still moves according to the throttle. Can anyone else confirm this so I can maintain my sanity?

 

There is no motor or mechanical actuator on the afm door. When you touch the throtle, the throtle plates move. This changes the pressure in the intake piping - causing the afm door to move.

Rob

 

It is all about pressure differential like Rob said. If the car is sitting still and not under a load it will repond totaly different than if the car were under a load. If you could look at it when you were trying to accelerate at full speed you'd see a different scenario than what you have been observing. The afm also bypasses a small amount of air around the flapper door so it can be closed at times.

 

The flapper is moved by the force created by the air flow into the engine.The voltage from the potentiometer is related to that force.The ecu does a calculation to air mass,based on temperature and force.
Note there is a dampening chamber on the the left side (looking from the front)which provides a reverse torque to the plate when there is a sudden recuction in airflow ie rapid closure of throttle plates in order to prevent over response of the airflow meter.This also smooths out pulsations in air pipe hence giving a stable airflow signal.

 

i know someone who hooked a tps to the throttle pedal and then hooked the tps to the afm input, so the ecu would see the afm signal directly as a throttle input. you can get it to idle or wide open but it didnt work in between. if you drive around with a voltmeter on the afm it almost follows the throttle, but not quite

mike

 

Man...If you doubt how it works so much, just take the damn thing apart and try and find the motor that is controlling it. Youre gonna be looking for a while.

 

That makes sence that the volts out of the afm would not follow the throttle exactly under load because you can have different throttle positions at different rpms. its better now that i think of it under load, what i9fd3s says corresponds well with this and rotorygod... I would take my spare afm apart but all the screw are stuck and they strip out soo easy. Does anyone know where to find this so called hot wire kit to remove the afm ive heard about?

 

There is no such kit. YOu can do it but you have to make it yourself.

 

it wouldn't make much sense for the engine to be responding to the extra airflow, before the extra airflow arrived...

 

When you open the throttle, more air can flow past it. The revs don't need to increase for the airflow to increase. That's why the flap moves before the revs start to rise. Simple physics.

A good example. You come to a hill and you open the throttle a bit more so the car maintains a constant speed. The engine is now sucking more air and making more power, but the revs haven't changed.

 

Take a 3" piece of pipe and hold it up in front of you. There is no air flow through the pipe. Thats because the pressure is the same at both open ends. Now connect one end of the pipe to a vacuum pump and start the pump. You have air flow through the pipe. The open end of the pipe sees higher pressure than the other and air is flowing from the high pressure end to the low pressure end. The same situation exists in your car. When the throttle plates are closed the pressure at both ends of the intake duct are nearly equal and you have reduced air flow. When the plates are open one end of the duct now sees greatly reduced pressure from the pump that is your motor and you get higher air flow.

Rob

 

When you tap the throttle.. it opens the throttle plates and sucks in air, thus opening the afm.


........

 

The AFM door only cares about air volume.
The throttle only regulates air volume.
The difference in throttle movement & engine RPM acceleration is usually called throttle response.
The biggest factor in throttle response is the flywheel weight.
No mystery..