Placement Of Afm

[Sydneykid]

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?

Yep, that's right on an RB20/25. But an RB26 has the throttle bodies after the plenum, which holds a lot of "compressed" air. That air reverses direction and goes back through the AFM and out the BOV. Hence rich running. Certainly not as much as with a atmo BOV and the AFM before the compressor.

With the RB20/25 you need to be carefull not to put put the AFM too close to the throttle butterfly. Partially open butterflies disturb the approaching air flow quite a bit, and you want the AFM to be metering perfectly. Plus it needs to be before the idle bypass feed. Otherwise you will have non metered air going into the engine.

The usual place for the AFM is just after intercooler outlet, maybe halfway to the throttle body. There are plenty of cars with it in the location.

cheers

[Busky2k]

^^ Ah yes well I never thought of the RB26 but as you say, it'd still work better than the stock location with an atmo BOV.

Come to think of it there may be better improvements with fuel economy too? I notice that when I back off from a medium load, the AFM continues to read air for another 500ms or so and thus the PowerFC is still dumping in fuel instead of going to decel injector cut...even though the TB is shut (and no, the atmo BOV is not venting). I attributed this to the volume of the FMIC and the fact that the turbo is pulling some air etc. I guess with the AFM after the FMIC, we could say that due to the more precise metering of the air, better fuel economy may result?

[Craved]

With the RB20/25 you need to be carefull not to put put the AFM too close to the throttle butterfly. Partially open butterflies disturb the approaching air flow quite a bit, and you want the AFM to be metering perfectly. Plus it needs to be before the idle bypass feed. Otherwise you will have non metered air going into the engine.

The usual place for the AFM is just after intercooler outlet, maybe halfway to the throttle body. There are plenty of cars with it in the location.

cheers

doesnt that mean that the BOV needs to be turbo side of the AFM ???

[Color_Of_Green]

Good stuff.

And yes, my concern is the fact that a certain volume of air gets measured, fuel added, then BOV vents so running rich. I would really like to eliminate this and tell the PFC exactly how much air is going into the engine...

Re the scan time question before, I would not like to delay the AFM reading to the PFC if its input scan latency exceeds the time it takes for air to travel past the AFM and actually into the engine.

With the RB20/25 you need to be carefull not to put put the AFM too close to the throttle butterfly. Partially open butterflies disturb the approaching air flow quite a bit, and you want the AFM to be metering perfectly.

I have seen some throttle bodies where each half of the butterfly rotates back in the direction of the air flow and is supposed to reduce turbulence...

Also, all of this stuff assumes a single throttle body setup.

So, based on this discussion its seems like a worthwhile exercise.

Edited August 22, 2006 by Color_Of_Green