Placement Of Afm

[Color_Of_Green]

I've always been curious as to why an air flow meter is positioned so far away from the intake manifold?

Edited August 21, 2006 by Color_Of_Green

[LTHLRB]

its placed where the intake is. The amount of air the turbo is sucking in is sensored by the afm, which the data is given to the ecu. As long as the afm has air passing through it, your car will run fine.

Correct me someone if im rong.. A MAP sensor is placed inside or where the inlet manifold is. Manifold Apsolute Pressure.

[xRHETTx]

They talked about this (at least putting in the intercooler cold piping) in a recent issue of HPI magazine.

Seems like a good idea, as long as the AFM can hold up to the increased pressure (which I'd assume it can, we are talking in almost every case of less than 2 bar)

[LTHLRB]

90% of vl turbos have the afm in the intercooler piping. If your fussed about the size of your afm.. Q45 or Z32 is the way.. Alongside a PFC

[Gav]

AFMs work best with an even flow of air through the body. If the AFM is too close to points of sudden air pressure and volume changes (eg throttle bodies, blow off valves, inlet manifold etc) then you get "reversion" flow through the AFM body. This is perhaps most noticeable in an uneven idle or in near stalling on sudden deceleration. One of the functions of the mesh in AFMs is to ensure a smooth flow of air through the body.

[Color_Of_Green]

hmm good responses.

What if it was positioned between two synchronized throttle bodies - would protect the AFM from reverse air flow.

Anyone know the latency of the Power FC input scan?

[Busky2k]

I like the idea of the AFM on the cold side of the intercooler. Better throttle response (due to less air volume between the TB & AFM) and the fact that atmo BOVs don't affect it (if you place the BOV before the AFM) are reasons why I'd like to give it a go...

[mad082]

adding to what busky2k said, it would give a reading that takes into account the change in air density when cooled by a fmic on cold nights.

[Mark-R33]

90% of vl turbos have the afm in the intercooler piping

heheheh....guess thats y they dose there ass' off?

[R31Nismoid]

90% of vl turbos have the afm in the intercooler piping. If your fussed about the size of your afm.. Q45 or Z32 is the way.. Alongside a PFC

thats a pretty bold statement.

coming from a VL years ago, its definately not the case

[Craved]

and the fact that atmo BOVs don't affect it (if you place the BOV before the AFM) are reasons why I'd like to give it a go...

want to explain how this works?

my thoughts are : regardless of where you put the AFM it reads whats going through it.. so an atmo BOV stills lets air out, the turbo still forces air through the piping to this opening... hence still having the AFM read air flow. so it would be exactly the same

if anything it just makes it harder to have a recirc style BOV..

[Sydneykid]

the fact that atmo BOVs don't affect it (if you place the BOV before the AFM) are reasons why I'd like to give it a go...

Hmmm, I reckon no matter where you place the AFM an atmospheric (vented) BOV will affect its readings.

If the BOV is after the AFM, the AFM will measure air that doesn't end up in the engine. ie, whatever the BOV vents to atmosphere

If the BOV is before the AFM, then the air that has already gone past the AFM will reverse direction and get measured twice.

Either way = short term rich running (maybe stalling) after the BOV vents.

cheers

[Busky2k]

Well my theory is a bit different to you guys about the BOV.

Lets say the AFM is 3" from the throttle body. The BOV is about 5" away from the TB, ie placed just before the AFM.

On full boost we close the TB, and then we get a slight about of reversion which the AFM reads. But the AFM is is pretty close to the TB so we could say 3" x 3" of piping is not really a significant volume which contradicts SK's theory somewhat.

So.. the pressurised air between the turbo and BOV (ie FMIC etc) is all vented out through the BOV. But the key is that the AFM had never measured the vented air in the first place, so thus it won't go overrich.

Does my theory make sense?

[Craved]

Well my theory is a bit different to you guys about the BOV.

Lets say the AFM is 3" from the throttle body. The BOV is about 5" away from the TB, ie placed just before the AFM.

On full boost we close the TB, and then we get a slight about of reversion which the AFM reads. But the AFM is is pretty close to the TB so we could say 3" x 3" of piping is not really a significant volume which contradicts SK's theory somewhat.

So.. the pressurised air between the turbo and BOV (ie FMIC etc) is all vented out through the BOV. But the key is that the AFM had never measured the vented air in the first place, so thus it won't go overrich.

Does my theory make sense?

so your assuming that the air between the TB and the BOV doesnt move at all

and any air coming its way routes through the BOV without affecting the air within the AFM ..

i see your point but is that true?

[The Dan]

I have been running the AFM after the IC for a couple of years now and the results are much nicer than you would think. Throttle response improved and no more rich spot when the BOV let go. I ran over 20psi through mine and it had no adverse effect on the AFM at all.

The whole thing with the BOV not effecting it is because the air between the air filter and the turbo acts like a giant spring. This is what causes the AFM to go psycho and cause the engine to stall. When the throttle snaps shut, the BOV opens and the air passes out. While it is doing this, the turbo is still pushing some air. This is still measured by the AFM but the air doesn't bounce around like a spring as it would before the turbo. Then when the BOV shuts the throttle is open again and the air continues to move.

[Busky2k]

so your assuming that the air between the TB and the BOV doesnt move at all

and any air coming its way routes through the BOV without affecting the air within the AFM ..

i see your point but is that true?

Well the air between the TB and BOV is still insignificant at 5" of piping as in my theory. Its certianly faaaarrr less than the capacity of FMIC and its associated piping if the AFM in its factory position which causes it to run rich.

Also in reference to your second point, the TB is shut. No air is going to pass through the AFM when the BOV vents. I mean how would it? Its a dead end.

I guess 3lit3 32's experience reinforces my theory. I also noted that my car ran rich when backing off on full boost, even with a recirculating BOV.

I'm real tempted to try this on my car. But I'm just to pansy to hacksaw the filter flange of my Z32 AFM lol.

[TTT]

On my next skyline I want to play around a little with this.

Upon pulling apart the afm on my rb20 to re-solder it, I found it’s nothing more than a circuit board with wires and sensors etc.

And offcourse the hot wire below.

I plan to get a spare afm and cut it apart and make something up so that I can just have the hot wire sensor sit in the intercooler piping.

Not sure how I will do this just yet as I don’t have a spare lying around.

But I’ll give it a go eventually.

I don’t want to actually have the hole AFM unit… just the element and circuitry above it.

[Craved]

good way to do it GTST ..

then again, all you need to make is a flange to bolt on a 3" or whatever piece of piping to the std bolt holes ..

then just use a sealant between the AFM and flangee, then silicon hoses as normal

as the to Dead end .. your saying that the air stops dead and doesn't move when you start forcing air into that dead end .... it would have to move .. especially if it was after a bend where the air would be force more to the external side of the bend..

[aphid]

hah, i was always under the impression vl's didn't have factory fitted intercoolers, hence the low 7.0:1 (or there abouts) comp ratio.. you learn something new every day

90% of vl turbos have the afm in the intercooler piping. If your fussed about the size of your afm.. Q45 or Z32 is the way.. Alongside a PFC

[Craved]

hah, i was always under the impression vl's didn't have factory fitted intercoolers, hence the low 7.0:1 (or there abouts) comp ratio.. you learn something new every day

the intercoolers are aftermarket

as per

http://www.turboclub.com/turbocars/gm/GMH/gmh_vl.html