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There was talk of what power gain is realized by changing the AFM to some other less restrictive form of air measurment. First, lets talk about the AFM operation.

Obviously the AFM will restrict air when it's not fully open. But, once theres enough air to open the AFM past say 85%, it's restriction is very small. Especially using it the way it was intended. That is, once the car is at WOT, the computer defaults to a WOT map, and ignores it. This is because the AFM flapper is nearly wide open before peak power is reached, causing very little restriction. Again, the flapper is wide open(essentially) before peak power, so from say peak torque to peak power, the restriction is very small.

With this, I would say that the peak power loss from AFM removal is minimal, less than 10hp. The low to mid end power will be more significant. The biggest benefit from AFM removal is throttle responce. This will be much better with no AFM. Also, the biggest benefit from a MAF, is the way measures air flow. It's most linear(which makes for the most accurate measurement) at the mid to upper end of the power range. The AFM is only linear in the mid range of air flow. So the MAF will be much more accurate where you want it the most, mid and top end power range.

To be sisinc, I don't believe the top end power realized from AFM removal is 20hp!

RussC
(oppps, thanks for the correction, brain fart)


[Edit by russc on [TIME]1111707878[/TIME]]

ummmm, you have a double negative in your conclusion..... care to correct it so we can babeque your post???? :D:D:D (j/k!)

dasMafia, I'll translate it for you...

"To be sisinc, I don't believe the top end power realized from AFM removal is 20hp!"

"To be succinct, I believe removing the AFM provides gains that are much less than 20hp!"

I thought the biggest advantage to a MAF is that it measures air mass, not air flow like an AFM.

From looking at the way the afm is made inside, I thought it would be more of a restriction than that!

But yes, fitting a maf sensor to the M30 would greatly enhance throttle response! I swapped the stock mas ( Karman Vortex airflow type sensor) on the Talon for an '95 Chevy Impala SS 3" maf and boy did it make a differrence! Full boost happened about 400 rpm earlier and off boost driveability was astonishing!

But remember, that k vortex sensor is already less restrictive than the plunger type sensor on our Bmw's. I can't imagine what the gains would be!

[QUOTE="Shifty"]I thought the biggest advantage to a MAF is that it measures air mass, not air flow like an AFM.[/QUOTE]

They both measure air mass, just in different ways.

The AFM gets it's signal from variable geometry, with constant air temp. A MAF gets its signal from variable voltage needed to keep a wire a certain temperature, with constant geometry.

Right?

Once the AFM is removed, the next restriction would be the throttle body. If you were to go with a larger throttle body, and possibly a better plenum system, you may see significant gains on the top end.

This is why the alpha-N systems are so popular with the s14 engine, but, it does have the individual throttle bodies. The alpha-N system gets rid of the AFM and allows for a larger diameter intake and larger plenum. Basically it reduces any restrictions which cause pressure drop. Sweeney's concept is very similar.

Damon in STL

[QUOTE="booker535"]They both measure air mass, just in different ways.

The AFM gets it's signal from variable geometry, with constant air temp. A MAF gets its signal from variable voltage needed to keep a wire a certain temperature, with constant geometry.

Right?[/QUOTE]

You're right on how they measure air, but the AFM still measures only air flow, IIRC. This is why it doesn't work extremely well with a turbo: because the air flow has more mass than atmo air.

No, as booker535 said, the AFM, in conjuction with the temp sensor, does measure air mass, but the computer does calculations to convert the airflow and temp to mass. The MAF does it with one device, as the air temp and flow change the current characteristics of the thermistor.

Umm, I was talking about this issue the other night with a gent, he mentioned also that throttle responce will not significantly improve on the AFM cars. This is because the computer is programmed to be compatable with an AFM, which has a very fast and under damped responce to air mass changes. Since the computer is programmed to be very over damped to compensate for the overshoot and undershoot of the flapper, that just adding a MAF will have minimal change in throttle reponse.

Essentially, the AFM removal will increase how quickly the air flows in from a throttle change, but the computer will still change fuel slower than the air is comming in. So you get more air with less fuel. But that would be bad, so I'm not sure this is the case. Ill have to think on this on some more.

RussC

[QUOTE="booker535"]They both measure air mass, just in different ways.

The AFM gets it's signal from variable geometry, with constant air temp. A MAF gets its signal from variable voltage needed to keep a wire a certain temperature, with constant geometry.

Right?[/QUOTE]

[QUOTE="Shifty"]You're right on how they measure air, but the AFM still measures only air flow, IIRC. This is why it doesn't work extremely well with a turbo: because the air flow has more mass than atmo air.[/QUOTE]

Russ,

Which temp sensor are you talking about?

I have always been under the impression that the position of the AFM's flap is how the AFM knows how much air is flowing through it. This determines the amount of air flow through the engine.

I"m failing to understand how an AFM can measure air mass. This is one of the huge advantages to the MAF, I thought.

Shifty,

The temp sensor is in the AFM, in a little cage. On my M30 AFM, it's before the flap.

Not to be too picky about it, but neither an AFM nor a MAF truly "measures" mass flow -- they are "dumb" and only report back with resistances and/or voltages. It's the computer's interpretation of the sensor's signals that determines mass flow.

-Shawn

[QUOTE="Shifty"]You're right on how they measure air, but the AFM still measures only air flow, IIRC. This is why it doesn't work extremely well with a turbo: because the air flow has more mass than atmo air.[/QUOTE]

Since the AFM is in the intake plumbing before the turbo (thus before the air is compressed), it measures the air flow perferctly fine whether under boost or not.

Seeing as we're tightening down the spring in the AFM to compensate for larger injectors I guess the AFM is more of a restriction on our cars than on a naturally aspirated M30, but with Todd getting more than 300whp out of it, it's not very high on my list of things to upgrade.

Yup, Shawn has it right. With flow and air temp, you can calculate mass. You may or maynot be familiar with the equation PV=nRT. As gas rises in temp, theres less mass for a given volume, and vise versa.

Ya, the sensor is in the AFM itself, Im supprised you didn't know that?

RussC

[QUOTE="Shifty"]Russ,

Which temp sensor are you talking about?

I have always been under the impression that the position of the AFM's flap is how the AFM knows how much air is flowing through it. This determines the amount of air flow through the engine.

I"m failing to understand how an AFM can measure air mass. This is one of the huge advantages to the MAF, I thought.[/QUOTE]

Hmmm.

Maybe I should try to remember more Physics from college. The equation does ring a bell. That clears it up fine.

BTW, I knew the air temp sensor was in the AFM. It just didn't click at the time.

Wow, I'm off today. . .

Thanks guys!