I'm missing something...

[Greg Hermann]

At 7:29 PM 11/11/01, Bob Wooten wrote:
>Greg,
>
>I am missing something.  How can that NOT be a Mass flow device.  If an
>engine was running @ 20% throttle position on a flat road (constant load) @
>sea level it would be pulling X amount of air though the flow meter.  @ sea
>level there are going to be X' amount of molecules in the air & the flapper
>is going to move to some value.
>
>@ 5000 feet using the same 20% throttle position on the "same" flat road is
>going to simulate the "same" load (ignoring the air resistance issue, say 20
>mph, so it is negligible), the air is only going to be 83.3% as dense
>(24.92/29.92), so it is going to have 83.3% fewer molecules in the air to
>"push" the flow meter open.
>
>would this not "compensate" for the altitude change?  I would expect that
>depending on how "smart" the ECM was it would compensate right or wrong, but
>it ought to be doing some compensation.
>
>what am I missing?
>
>BW

You were beating around the right bush talking about the air resistance
issue at altitude. At the flapper valve, the force of air against the
flapper decreases directly with the air  density, but it increases with the
square of the velocity.

The difference between the direct function and the square function means
that the higher velocity, but less dense air going into the engine to get
the same number of O2 molecules into the engine (to make the same power)
deflects the flapper further because the velocity has gone up while the
density has gone down. Lower density makes for less force, but the higher
velocity makes for significantly more force on the flapper.

In a carby, the main jet diameter required to give a particular mixture
varies with the fourth root of the air density ratio. Similar deal here.

Greg
>
>
>
>
>
>In a word, you were told wrong. They are a volume flow device.
>
>Greg
>
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