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blowing boost through the afm

Old 03-10-2008, 03:10 PM
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Default blowing boost through the afm

Hey guys, for all of u who actually still retain the afm have any of you tried to blow boost throught it? What would happen? The reason im asking is if the afm reads boost as just regular incoming air wouldnt the ecu control my larger injectors better since im running 330cc on stock ecu.
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Old 03-10-2008, 03:23 PM
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AFM cant account for the new air density.
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Old 03-10-2008, 03:27 PM
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blowing boost through the afm-when i read this i though "hmm, your miata is sick"

but Sam nails it, your AFM can't read it accurately
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Old 03-10-2008, 07:41 PM
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I ran this way on my P5.

The AFM maybe more responsive, but if your on your stock ecu then it's still looking to obtain 14.7:1, not exactly an ideal boost situation. The mistake I made was to have my valve breather pre-turbo. This introduced oil to my AFM sensor and inaccurate reading all together, and several CELS..
I was able to clean up the sensor, and I moved the AFM TO PRE-TURBO as well.

If you are going to run Blow though, make sure you VTA your Valve cover breather (it only takes a very small amount of oil to contaminant the AFM sensor). Another source of oil is through the seals in the turbo itself, so make sure you have a really good rebuild or a brand new one.
IIRC a few years ago there was someone running with the AFM just before the TB on a Miata, it was in the Charge pipe between IC and TB. It Looked cool. Also IIRC the stock MAf will start throwing CELS ~15psi and higher.
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Old 03-10-2008, 08:51 PM
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Originally Posted by Jefe View Post
I ran this way on my P5.
This introduced oil to my AFM sensor and inaccurate reading all together, and several CELS..
I'm not sure what a P5 is, but it sounds as if you're describing a hotwire MAF sensor, not a flapper-style AFM.

The MAF would technically work in this scenario, so long as its temperature compensation was sufficiently fast and wide-ranging. With a mechanical AFM however, it absolutely will not work.

AFMs measure only the volume of air passing through them. The ECU then references its on-board barometer and the IAT sensor to convert the AFMs output to a true mass reading. The assumption is that the air passing through the AFM is at the same pressure (roughly) as the pressure being measured directly at the ECU.

If you double the pressure of the air (without, hypothetically, raising its temperature) then the mass of air (the number of mols) per unit volume will also double. You'll now be passing twice as much air through the AFM as the ECU thinks you are, because it doesn't know about the pressure increase.

Put another way, 1 cubic foot of air at atmo (1 bar) and 1 cubic foot of air at 14.5 PSI of boost (2 bar) are not the same amount of air. But the AFM cannot tell the difference between them. As far as it's concerned, one cubic foot per second is one cubic foot per second.
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Old 03-10-2008, 09:03 PM
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My bad..too many beers.... yeah I was describing a MAF.... (Mazda Protege5) uses the same MAF as a MX5

Funny thing I think when I originally typed out MAF, re-read the thread and said **** I miss spelled it, correcting it for AFM.. But looking out in the garage my only AFM car is my MX3...
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Old 03-11-2008, 11:44 AM
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I believe he means Mazda Protege 5 by P5.

As in the '01-'03 Mazda Protege 5 door wagon.
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